Method for detecting chemical oxygen demand in water

Abstract

The invention discloses a method for detecting chemical oxygen demand in water, and relates to the field of wastewater and surface water detection. The detection method comprises the following steps: taking a sample to be detected, adding a masking agent such as mercury sulfate powder and a sulfuric acid solution, adding an oxidizing agent and a catalyst after complete reaction, heating for complete reaction, adding an indicator, titrating with a reducing agent, recording the consumption of the reducing agent, and converting to obtain a chemical oxygen demand result of the sample. According to the detection method, mercury sulfate is added in a solid mode, interference on chloride ions can be improved, high-chlorine and low-chemical-oxygen-demand wastewater detection can be achieved without chlorine correction and sample dilution, high detection accuracy is achieved, the minimum chemical oxygen demand detection limit can reach about 3.5 mg / L, and the method is suitable for industrial production. The detection method is simple in steps, easy to operate and high in accuracy and precision of detection results.

Description

Technical Field

[0001] The present application relates to the field of wastewater and surface water detection, and in particular to a method for detecting chemical oxygen demand in water quality. Background Art

[0002] Water resources are one of the most precious natural resources on earth. They play an irreplaceable role in human survival and development. From drinking, agricultural irrigation to industrial production, they are indispensable elements. They not only maintain ecological balance, but also support the sustainable development of the economy and society. However, with the acceleration of population growth and industrialization, water resources are facing unprecedented pressure. Among them, water pollution is particularly prominent, and it has become a global environmental challenge. Water pollution mainly comes from industrial wastewater, agricultural emissions, urban sewage and plastic waste. These pollutants contain toxic and harmful substances. Once they enter the water body, they will have a serious impact on water quality, affect the survival of aquatic organisms, destroy the food chain, and then affect the balance of the entire ecosystem. In addition, water pollution will also have a negative impact on economic development, such as increasing water treatment costs, reducing the quality of agricultural products, and affecting the tourism industry. Therefore, the society attaches great importance to water resource protection and actively takes effective measures to prevent and control water pollution to ensure the sustainable use of water resources, reduce pollution at the source, and strengthen sewage treatment and supervision.

[0003] Wastewater contains various organic matter, nitrite, sulfide, ferrous salt and other reducing substances, mainly organic matter. The chemical oxygen demand in water is usually used to measure the degree of organic pollution in water. Chemical oxygen demand (COD) refers to the amount of reducing substances that need to be oxidized in water samples. It is usually used to measure the amount of organic matter in water. It indicates the amount of oxidant consumed when a strong oxidant is used to treat a water sample under certain conditions. In the study of river pollution and the properties of industrial wastewater and the operation and management of wastewater treatment plants, COD is an important and quickly measured organic pollution parameter. Therefore, the larger the COD value, the more serious the water pollution by organic matter. Due to the presence of high levels of chloride ions in wastewater from some special industries such as monosodium glutamate factories, bacon factories, ammonia and alkali factories, the chloride ion concentration above 1000mg / L is usually defined as high-chlorine wastewater. At present, according to the Class A standard of urban sewage discharge standard (GB 18918-2002) issued by the state, this standard is regarded as the basic criterion for wastewater reuse in urban sewage treatment plants. The standard only stipulates the chemical oxygen demand (COD) in the wastewater. Cr ). The COD Cr It is the chemical oxygen demand measured using potassium dichromate (K2Cr2O7) as an oxidant, also known as the dichromate index. CrWhen the oxidant is used to oxidize the reducing substances in the sample, the COD can be calculated using the consumed oxidant content. Cr However, the chloride ions in the sample will be oxidized by the oxidant at the same time, resulting in an increase in the consumption of the oxidant and a high measurement result. In addition, the chloride ions will react with the silver salt to form a precipitate, affecting the effect of the catalyst and further interfering with the COD Cr Determination of.

[0004] For samples with a concentration less than 1000 mg / L, the chemical oxygen demand test is currently carried out using the "Determination of Chemical Oxygen Demand in Water - Dichromate Method" (HJ 828-2017) standard to determine COD Cr . In order to meet the requirement of this method that the chloride ion concentration in the sample is less than 1000 mg / L, high-chloride samples need to be diluted. Experiments have shown that for high-chloride samples, the results of the chemical oxygen demand determination will be higher after dilution, resulting in positive interference. As the dilution factor increases, the error of the chemical oxygen demand determination result also increases, directly affecting the accuracy of the determination. At the same time, for samples with high chlorine and low chemical oxygen demand, dilution determination will bring large errors or cause the measured value after dilution to be lower than the detection limit, and the original sample concentration cannot be accurately determined. Therefore, this detection method is not suitable for samples with high chlorine and low chemical oxygen demand.

[0005] Secondly, for samples with chloride ion concentrations between 1000mg / L and 20000mg / L, the "Chlorine Correction Method for Determination of Chemical Oxygen Demand of High Chlorine Wastewater" (HJ / T 70-2001) can be used. However, the chemical oxygen demand (COD Cr ) concentration detection limit is 30mg / L, and the chemical oxygen demand (COD Cr ) concentration is lower than 30mg / L, and samples with chloride ion concentration greater than 20000mg / L need to be diluted, which affects the detection accuracy. The operation steps of the chlorine correction method are also relatively cumbersome.

[0006] In addition, the COD of the sample can be determined using the "Determination of Chemical Oxygen Demand of High Chloride Wastewater - Potassium Iodide Alkaline Potassium Permanganate Method" (HJ / T 132-2003). Mn , COD Mn Potassium permanganate (KMnO4) is used as an oxidant to measure the chemical oxygen demand of the main reducing substances in the wastewater, and then converted into COD through the conversion coefficient K CrIn the actual determination process, the K value needs to be obtained by using the potassium dichromate method and the potassium iodide alkaline potassium permanganate method to determine the oxygen demand of representative wastewater samples. Due to the complex composition of reducing substances in wastewater samples, there are certain difficulties in actual operation, and the reliability of the results cannot be guaranteed. It is difficult to obtain the conversion coefficient K according to the requirements of the method, so it is impossible to obtain the chemical oxygen demand (COD) through the conversion coefficient. Cr ). Summary of the invention

[0007] The present application provides a method for detecting chemical oxygen demand in water quality. The detection method of the present application can detect high-chlorine wastewater with a chloride ion concentration of 1000mg / L-40000mg / L or above, and is also suitable for wastewater detection below 1000mg / L. The minimum detection limit of the chemical oxygen demand of the sample reaches 3.5mg / L, which can meet the detection of wastewater with high chlorine and low chemical oxygen demand at the same time. The detection process does not require dilution of the sample, and does not require chlorine correction treatment like conventional detection methods. The detection results have high accuracy and precision.

[0008] In order to solve the above technical problems, the present application provides a method for detecting chemical oxygen demand in water quality, comprising the following steps:

[0009] (1) taking a sample to be tested, adding a masking agent and a sulfuric acid solution, and after the reaction is complete, adding an oxidant of known concentration and in excess according to the predicted chemical oxygen demand of the sample to be tested, and adding a catalyst at the same time, heating the reaction to complete, and obtaining a digestion solution;

[0010] (2) adding an indicator to the digestion solution, titrating the remaining oxidant in the reaction with a reducing agent of known concentration, recording the consumption of the reducing agent, and calculating the chemical oxygen demand result of the sample to be tested;

[0011] Wherein, the amount of sulfuric acid added to the sulfuric acid solution is R1, in mL, R1 value ≥ Y1×V1ˊ / 200, V1ˊ is the volume of the sample to be tested in step (1), in mL;

[0012] Y1=0.0017X1+0.4635, X1 is the chloride ion concentration of the sample to be tested, in mg / L;

[0013] Among them, the masking agent is solid mercuric sulfate powder or silver nitrate, and when the masking agent is solid mercuric sulfate powder, the mass ratio of the mercuric sulfate to the chloride ion content in the sample to be tested is greater than 9; when the masking agent is silver nitrate, the molar ratio of the silver nitrate to the chloride ion content in the sample to be tested is 1:1.

[0014] In the detection method of the present application, a known amount of oxidant is added to the sample to be tested. Under acidic conditions, the oxidant can oxidize the reducing substances in the sample, and the catalyst can also catalyze the effective oxidation of straight-chain aliphatic compounds. The chloride ions in the sample will consume the oxidant, especially when the chloride ion concentration in the water sample is high. The present application uses mercuric sulfate to form a complex with the chloride ions in the sample, thereby eliminating the effect of the chloride ions in the sample on the oxidant and improving the accuracy of the detection. After digestion, the unreduced oxidant in the water sample is titrated with a reducing agent, and the chemical oxygen demand result of the sample is calculated by the consumed reducing agent content.

[0015] At the same time, the inventors have found through experiments that in the detection method of the present application, the sulfuric acid solution added before digestion helps to strengthen the shielding effect of mercuric sulfate on chloride ions. Only when the dosage of sulfuric acid solution meets the above-mentioned limiting conditions can the detection accuracy of high-chlorine wastewater be effectively improved. At the same time, the inventors found that the dosage of mercuric sulfate and the solid or liquid state of mercuric sulfate when added will also affect its detection accuracy of high-chlorine wastewater. When the mass ratio of mercuric sulfate dosage to chloride ion content is greater than or equal to 9, the test results can obtain a higher accuracy, and at this time, mercuric sulfate has a good shielding effect on high-concentration chloride ions in the sample.

[0016] In the detection method of the present application, the accuracy of detection when mercuric sulfate is added in solid form is significantly higher than that when added in liquid form. It is speculated that this is because the chemical reaction occurs in the collision between the reactant molecules. In the solid reactant, the distance between molecules is relatively close, and the collision frequency between particles is relatively high, which makes the reaction rate faster. Sulfuric acid continuously washes and dissolves on the solid surface of mercuric sulfate, and mercuric sulfate reacts with chloride ions to form a stable mercuric chloride complex, thereby achieving effective interference. When mercuric sulfate is dissolved in sulfuric acid solution and added as a liquid, the molecular freedom of the liquid reactant is large, which makes the collision frequency lower. The reaction rate of mercuric sulfate and chloride ions to form a stable mercuric chloride complex is relatively slow, and mercuric sulfate cannot fully shield the interference of chloride ions, resulting in low detection accuracy.

[0017] As a preferred embodiment, the mass ratio of the solid mercuric sulfate to the chloride ion content in the sample to be tested is 10-20.

[0018] As a preferred embodiment, the mass ratio of the solid mercuric sulfate to the chloride ion content in the sample to be tested is within the range of one or any two of 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20.

[0019] As a preferred solution, in step (1), the chloride ion concentration of the sample to be tested needs to be tested in advance.

[0020] As a preferred solution, the difference between the R1 value and Y1×V1ˊ / 200 is in the range of 0-10.

[0021] As a preferred embodiment, the difference between the R1 value and Y1×V1ˊ / 200 is within the range of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any two thereof.

[0022] As a preferred solution, the concentration and dosage of the oxidant, catalyst and reductant can be added according to the standard of "Determination of Chemical Oxygen Demand of High Chlorine Wastewater - Chlorine Correction Method" (HJ / T 70-2001).

[0023] As a preferred embodiment, the oxidant is a potassium dichromate solution, and the concentration of potassium dichromate in the potassium dichromate solution is 0.025 mol / L-0.5 mol / L.

[0024] As a preferred embodiment, the total amount of potassium dichromate added to the sample to be tested is E, Wherein, E: the amount of potassium dichromate added, mol / L; ρ': the predicted chemical oxygen demand of the water sample to be tested in step (1), mg / L; a≥15, V2: the volume of the water sample to be tested in step (1), mL.

[0025] As a preferred solution, a≥30.

[0026] As a preferred solution, the ρ' value can be determined by the sense of smell or vision of the personnel in this field combined with past experience to make a preliminary judgment on the chemical oxygen demand of the sample to be tested, such as by sniffing the sample to be tested at close range to see if there is a pungent odor and combining it with past treatment experience to judge whether the chemical oxygen demand has reached a certain critical value; it can also be determined by the source of the sample to be tested or the process related to its production and combined with past experience to make a preliminary judgment, such as when the sample to be tested is classified as wastewater treated by a monosodium glutamate factory, a bacon factory, an electronics factory, an ammonia-alkali factory or a hospital, etc., a preliminary prediction can be made based on the chemical oxygen demand measured in the past treatment of wastewater from such related processes.

[0027] As a preferred embodiment, the concentration of the potassium dichromate solution is 0.025mol / L, 0.05mol / L, 0.075mol / L, 0.1mol / L, 0.125mol / L, 0.15mol / L, 0.175mol / L, 0.2mol / L, 0.225mol / L, 0.25mol / L, 0.275mol / L, 0.3mol / L, 0.325mol / L, 0.35mol / L, 0.375mol / L, 0.4mol / L, 0.425mol / L, 0.45mol / L, 0.475mol / L, 0.5mol / L or any two of them.

[0028] As a preferred embodiment, the volume ratio of the potassium dichromate solution to the sample to be tested is 1:(1-5).

[0029] As a preferred solution, the volume ratio of the potassium dichromate solution to the sample to be tested is 1:2; when the COD predicted by the water sample to be tested in step (1) Cr When the COD of the water sample to be tested is 90 mg / L or less, the concentration of the potassium dichromate solution is 0.025 mol / L-0.05 mol / L, preferably 0.025 mol / L; when the COD of the water sample to be tested is predicted in step (1), Cr When the concentration is 180 mg / L or less, the concentration of the potassium dichromate solution is 0.05 mg / L-0.075 mg / L, preferably 0.05 mol / L.

[0030] In the detection method of the present application, when the concentration of potassium dichromate standard solution is 0.025 mol / L, it is found that the detection method can only detect COD Cr The concentration of the sample below 90mg / L was accurately detected, and the concentration of the potassium dichromate standard solution was 0.05mol / L. It was found that the detection method can accurately detect COD Cr The samples with concentration below 180mg / L can be accurately tested. It can be seen that the potassium dichromate standard solution has a higher detection limit when the concentration is 0.05mg / L. Cr The concentration can reach 180 mg / L, and at the same time has a low detection limit of 3.5 mg / L, which can be used for the detection of wastewater samples with higher chemical oxygen demand.

[0031] In the detection method of the present application, the total amount and concentration of the potassium dichromate solution will affect the detection limit and the upper detection limit of the detection method. Therefore, in the actual detection process, the potassium dichromate concentration can be selected according to the predicted chemical oxygen demand in the sample to be tested. Further, in step (1), when the predicted chemical oxygen demand of the sample to be tested is 270 mg / L or less, the potassium dichromate concentration is selected from 0.075 mol / L to 0.1 mol / L, preferably 0.075 mol / L; when the predicted chemical oxygen demand of the sample to be tested is 360 mg / L or less, the potassium dichromate concentration is selected from 0.1 mol / L to 0.125 mol / L, preferably 0.1 mol / L; when the predicted chemical oxygen demand of the sample to be tested is 450 mg / L or less, the potassium dichromate concentration is selected from 0.125 mol / L to 0.15 mol / L, preferably 0.125 mol / L; when the predicted chemical oxygen demand of the sample to be tested is 540 mg / L or less, the potassium dichromate concentration is selected from 0.100 mol / L to 0.150 mol / L, preferably 0.100 mol / L. When the chemical oxygen demand of the sample to be tested is predicted to be 630mg / L or less, the potassium dichromate concentration is selected to be 0.175mol / L-0.2mol / L, preferably 0.175mol / L; when the chemical oxygen demand of the sample to be tested is predicted to be 720mg / L or less, the potassium dichromate concentration is selected to be 0.2mol / L-0.225mol / L, preferably 0.2mol / L; when the chemical oxygen demand of the sample to be tested is predicted to be 810mg / L or less, the potassium dichromate concentration is selected to be 0.225mol / L-0.25mol / L, preferably 0.225mol / L; when the chemical oxygen demand of the sample to be tested is predicted to be 900mg / L or less, the potassium dichromate concentration is selected to be 0.25mol / L-0.275mol / L, preferably 0.25mol / L.

[0032] As a preferred embodiment, in step (1), when the predicted chemical oxygen demand of the sample to be tested is between 90 mg / L and 180 mg / L, the concentration of potassium dichromate is selected to be 0.05 mol / L-0.075 mol / L, preferably 0.05 mol / L; when the predicted chemical oxygen demand of the sample to be tested is between 180 mg / L and 270 mg / L, the concentration of potassium dichromate is selected to be 0.075 mol / L-0.1 mol / L, preferably 0.075 mol / L; when the predicted chemical oxygen demand of the sample to be tested is between 180 mg / L and 270 mg / L, the concentration of potassium dichromate is selected to be 0.075 mol / L-0.1 mol / L, preferably 0.075 mol / L; When the chemical oxygen demand is between 270mg / L-360mg / L, the potassium dichromate concentration is selected to be 0.1mol / L-0.125mol / L, preferably 0.1mol / L; when the predicted chemical oxygen demand of the sample to be tested is between 360mg / L-450mg / L, the potassium dichromate concentration is selected to be 0.125mol / L-0.15mol / L, preferably 0.125mol / L; when the predicted chemical oxygen demand of the sample to be tested is between 450mg / L-540mg / L When the chemical oxygen demand of the sample to be tested is predicted to be 540mg / L-630mg / L, the potassium dichromate concentration is selected to be 0.175mol / L-0.2mol / L, preferably 0.175mol / L; when the chemical oxygen demand of the sample to be tested is predicted to be 630mg / L-720mg / L, the potassium dichromate concentration is selected to be 0.2mol / L-0. 225mol / L, preferably 0.2mol / L; when the predicted chemical oxygen demand of the sample to be tested is 720mg / L-810mg / L, the potassium dichromate concentration is selected to be 0.225mol / L-0.25mol / L, preferably 0.225mol / L; when the predicted chemical oxygen demand of the sample to be tested is 810mg / L-900mg / L, the potassium dichromate concentration is selected to be 0.25mol / L-0.275mol / L, preferably 0.25mol / L.

[0033] As a preferred embodiment, the catalyst in step (1) is a silver sulfate-sulfuric acid solution formed by mixing silver sulfate and sulfuric acid, and the concentration of silver sulfate in the silver sulfate-sulfuric acid solution is 3g / L-20g / L.

[0034] In the detection method of the present application, under acidic potassium dichromate conditions, aromatic hydrocarbons and pyridine in the sample may be difficult to be oxidized by potassium dichromate, resulting in low oxidation efficiency. By using silver sulfate as a catalyst, potassium dichromate can effectively oxidize straight-chain aliphatic compounds and improve the oxidation efficiency.

[0035] As a preferred embodiment, the concentration of silver sulfate in the silver sulfate-sulfuric acid solution is within the range of one or any two of 3 g / L, 4 g / L, 5 g / L, 6 g / L, 7 g / L, 8 g / L, 9 g / L, 10 g / L, 11 g / L, 12 g / L, 13 g / L, 14 g / L, 15 g / L, 16 g / L, 17 g / L, 18 g / L, 19 g / L, and 20 g / L.

[0036] As a preferred embodiment, the total mass of silver sulfate in the silver sulfate-sulfuric acid solution is R2, in g, R2 ≥ Y2 × V2ˊ / 2000, V2ˊ is the volume of the sample to be tested in step (1), in mL;

[0037] The Y2=0.0007X2+29.254, X2 is the chloride ion concentration of the sample to be tested, in mg / L.

[0038] As a preferred solution, the difference between the R2 value and Y2×V2ˊ / 2000 is in the range of 0-50.

[0039] As a preferred embodiment, the difference between the R2 value and Y2×V2ˊ / 2000 is within the range of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any two thereof.

[0040] As a preferred embodiment, the volume fraction of the sulfuric acid solution in step (1) is 5wt%-50wt%.

[0041] As a preferred embodiment, the volume fraction of the sulfuric acid solution in step (1) is within the range of one or any two of 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, and 50wt%.

[0042] As a preferred embodiment, the sample to be tested is at least one of surface water, domestic sewage, industrial wastewater and seawater.

[0043] As a preferred embodiment, the sample to be tested is wastewater from oil fields, chlor-alkali plants, seawater, deep-sea wastewater discharge, oil depots, and refineries.

[0044] As a preferred embodiment, the chloride ion concentration in the sample to be tested is below 40000 mg / L.

[0045] As a preferred embodiment, the chloride ion concentration in the sample to be tested is 1000 mg / L-40000 mg / L.

[0046] As a preferred solution, the detection limit of the detection method is 3-4 mg / L, and the lower limit of determination is 14-16 mg / L.

[0047] As a preferred embodiment, the indicator contains 1,10-phenanthroline to provide an indication function, the reducing agent contains ammonium ferrous sulfate, and the remaining oxidant is titrated with ammonium ferrous sulfate.

[0048] As a preferred embodiment, the indicator is ferrocyanide indicator, and the reducing agent is ammonium ferrous sulfate standard solution.

[0049] As a preferred embodiment, the concentration of ferrous sulfate heptahydrate in the ferrous sulfate indicator is 5g / L-10g / L, and the concentration of 1,10-phenanthroline is 10g / L-20g / L.

[0050] As a preferred embodiment, the concentration of ferrous sulfate heptahydrate in the ferrous sulfate indicator is within the range of one or any two of 5g / L, 6g / L, 7g / L, 8g / L, 9g / L, and 10g / L.

[0051] As a preferred embodiment, the concentration of 1,10-phenanthroline in the ferroline indicator is within the range of one or any two of 10g / L, 11g / L, 12g / L, 13g / L, 14g / L, 15g / L, 16g / L, 17g / L, 18g / L, 19g / L, and 20g / L.

[0052] As a preferred embodiment, the concentration of ammonium ferrous sulfate in the ammonium ferrous sulfate standard solution is 0.01 mol / L-0.1 mol / L.

[0053] As a preferred embodiment, the concentration of sulfuric acid in the ammonium ferrous sulfate standard solution is 0.5wt%-2wt%.

[0054] As a preferred embodiment, the concentration of ammonium ferrous sulfate in the ammonium ferrous sulfate standard solution is in the range of one or any two of 0.01 mol / L, 0.03 mol / L, 0.05 mol / L, 0.08 mol / L, and 0.1 mol / L.

[0055] In the detection method of the present application, for the selected concentration of ammonium ferrous sulfate in the ammonium ferrous sulfate standard solution, those skilled in the art can routinely adjust it accordingly with the different concentrations of the oxidant. When the selected oxidant concentration is relatively high, the concentration of ammonium ferrous sulfate in the potassium ammonium ferrous sulfate standard solution is also preferably relatively high. For example, when the concentration of the oxidant is 0.025 mol / L-0.05 mol / L, the concentration of ammonium ferrous sulfate in the potassium ammonium ferrous sulfate standard solution can be selected within the range of 0.01 mol / L-0.05 mol / L, and the preferred concentration is 0.01 mol / L. When the concentration of the oxidant is 0.05 mol / L-0.5 mol / L, the concentration of ammonium ferrous sulfate in the potassium ammonium ferrous sulfate standard solution is selected within the range of 0.05 mol / L-0.1 mol / L, and the preferred concentration is 0.05 mol / L.

[0056] As a preferred embodiment, the concentration of the ammonium ferrous sulfate standard solution is accurately calibrated with an oxidant standard solution before use.

[0057] As a preferred solution, while testing the sample to be tested, an equal amount of water is used to replace the sample to be tested, and a blank test is performed according to the same testing steps, and the consumption of the reducing agent in the blank test is recorded.

[0058] As a preferred solution, in step (2), the chemical oxygen demand calculation formula of the sample to be tested is: Wherein, ρ is the chemical oxygen demand of the sample to be tested, mg / L; C1 is the concentration of the reducing agent, mol / L; V0 is the volume of the reducing agent consumed in the blank test, mL; V1: the volume of the reducing agent consumed by the sample to be tested, mL; V2: the volume of the water sample to be tested in step (1), mL.

[0059] As a preferred embodiment, in step (1), the reaction time after adding the masking agent and the sulfuric acid solution is 5 min to 20 min.

[0060] As a preferred solution, in step (1), the masking agent and sulfuric acid solution must be fully mixed until the yellow powder in the sample disappears.

[0061] As a preferred embodiment, in the step (1), after adding the oxidant and the catalyst, the solution is heated to a slight boil, and the heating reaction time is 1 h to 4 h to obtain a digestion solution.

[0062] As a preferred embodiment, in step (1), the heating reaction is carried out in a reflux reaction device, and after the heating reaction is completed, the inner wall of the condenser of the reflux reaction device is cooled and flushed with water to obtain a digestion solution.

[0063] As a preferred embodiment, in step (2), a reducing agent is used to titrate the remaining oxidizing agent in the reaction, and the end point is when the color of the solution changes from yellow to blue-green and then to reddish brown.

[0064] As a preferred embodiment, in step (1), when the color of the sample solution changes to green instead of yellow during the digestion process, it means that the amount of potassium dichromate added in step (1) is insufficient, and the predicted chemical oxygen demand of the sample to be tested is lower than its actual oxygen demand. It is necessary to re-predict the chemical demand of the sample to be tested in step (1), and adjust the amount of potassium dichromate added for re-testing.

[0065] As a preferred embodiment, in step (1), the amount of water added for flushing can be added according to the standard "Chlorine Correction Method for Determination of Chemical Oxygen Demand of High Chlorine Wastewater" (HJ / T 70-2001).

[0066] As a preferred embodiment, in step (1), the amount of water added to flush the inner wall of the condenser tube of the reflux reaction device is R3, in mL, R3≥Y3×V3′ / 20, V3′ is the volume of the sample to be tested in step (1), in mL, Y3=0.0018X3+79.845, X3 is the chloride ion concentration of the sample to be tested, in mg / L.

[0067] As a preferred solution, the difference between the R3 value and Y3×V3ˊ / 20 is in the range of 0-10.

[0068] As a preferred embodiment, the difference between the R3 value and Y3×V3ˊ / 20 is within the range of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any two thereof.

[0069] As a preferred embodiment, in step (1), a sample to be tested is taken, a masking agent and a sulfuric acid solution are added, and after the reaction is complete, an oxidant of known concentration is added and mixed evenly, and a digestion reaction is carried out in a reflux reaction device. Cooling water is passed through the condenser of the reflux reaction device, a catalyst is added to the upper end of the condenser, and the reaction is heated. After the reaction is completed, water is added to rinse the inner wall of the condenser of the reflux reaction device to obtain a digestion solution.

[0070] As a preferred embodiment, in step (1), explosion-proof glass beads with a diameter of 4 mm to 8 mm are added before the heating reaction in the reflux reaction device.

[0071] As a preferred solution, the water used in the reagent preparation or added after reflux during the detection process is at least one of distilled water, deionized water, ultrapure water, and reverse osmosis water.

[0072] As a preferred embodiment, in step (1), the volume of the sample to be tested is 5 mL-100 mL.

[0073] As a preferred embodiment, the reflux reaction device comprises a conical flask and a condenser, and the lower end of the condenser is tightly connected to the mouth of the conical flask.

[0074] The detection method of the present application is applicable to the detection of low-chlorine wastewater and high-chlorine wastewater. For high-chlorine wastewater, the masking agent can completely shield the chloride ions in the sample and does not produce chlorine gas. Therefore, there is no need to perform chlorine correction treatment, nor is there a need to introduce protective gas as a carrier gas during the digestion reaction, which reduces the cumbersomeness of the detection steps and improves the detection efficiency.

[0075] As a preferred solution, a heating device is provided at the bottom of the conical flask.

[0076] Compared with the prior art, this application has the following beneficial effects:

[0077] 1. The detection method of the present application can realize the detection of chemical oxygen demand for wastewater with high chlorine and low chemical oxygen demand, that is, it is suitable for the detection of high chlorine wastewater with a chloride ion concentration of 1000mg / L-40000mg / L or above, and is also suitable for the detection of wastewater below 1000mg / L. For the detection of high chlorine wastewater, a high detection accuracy can be achieved without chlorine correction and dilution. The sample detection limit of high chlorine wastewater can be as low as 3.5mg / L, which is far lower than the detection limit of 30mg / L in the current high chlorine wastewater detection method in the field. The detection method has simple steps, easy operation, and high accuracy and precision of the test results.

[0078] 2. In the detection method of the present application, the content of sulfuric acid solution added during the digestion process is limited, which helps to strengthen the shielding effect of mercuric sulfate on chloride ions. At the same time, the mass ratio of the amount of mercuric sulfate added to the chloride ion content needs to be greater than or equal to 9. When this detection method is used to detect high-chlorine wastewater, there is no need for chlorine correction and dilution, and the final result can obtain high accuracy and precision.

[0079] 3. In the detection method of the present application, the detection accuracy when mercuric sulfate is added in solid form is significantly higher than that when it is added in liquid form. It is speculated that this is because the collision frequency between solid reactant particles is higher and the reaction rate is faster, while the molecular freedom of liquid reactants is larger, resulting in a lower collision frequency and a relatively slow reaction rate. The mercuric sulfate added in liquid state cannot completely shield the interference of chloride ions, resulting in lower detection accuracy.

[0080] 4. In the detection method of this application, the concentration of potassium dichromate solution will affect the detection upper limit and detection limit of the detection method. If the concentration of potassium dichromate standard solution changes, the detection method will Cr The upper detection limit and detection limit of the concentration also change. By adjusting the concentration of potassium dichromate solution, the detection method can be adapted to the detection indicators of wastewater samples with different chemical oxygen demands. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 : is a schematic diagram of the structure of a reflux reaction device A used in a method for detecting chemical oxygen demand in water quality in a test example of this application;

[0082] Figure 2 : It is a structural schematic diagram of a reflux reaction device B used in a method for detecting chemical oxygen demand in water quality in a test example of this application;

[0083] Figure 3 : A digital photo of a reflux absorption device B used in a method for detecting chemical oxygen demand in water quality in a test example of this application;

[0084] Figure 4:A digital photo of a reflux absorption device A used in a method for detecting chemical oxygen demand in water quality in a test example of this application;

[0085] Figure 5 : The working process of titration in step (3) in a method for detecting chemical oxygen demand in water quality in the test example of this application;

[0086] Figure 6 : is a linear regression curve of the relationship between the silver sulfate-sulfuric acid standard solution used in a method for detecting chemical oxygen demand in water quality in a test example of this application and the chloride ion concentration in 20 mL of the sample to be tested;

[0087] Figure 7 : is a linear regression curve of the relationship between the minimum addition volume of sulfuric acid solution and the chloride ion concentration in 20 mL of the sample to be tested in a method for detecting chemical oxygen demand in water quality in the test example of this application;

[0088] The instruction manual is attached Figure 2 The accompanying drawings are marked as follows: 1. Erlenmeyer flask; 2. Condenser; 3. Lead-out tube; 4-5. Silicone rubber connecting tube; 6. Absorption bottle. DETAILED DESCRIPTION

[0089] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0090] The reagents and equipment used in the test examples of this application are as follows. Unless otherwise specified, the reagents used in the examples and comparative examples of this application are analytically pure reagents that meet national standards, and the test water is freshly prepared ultrapure water, distilled water or water of equivalent purity.

[0091] 1. Sample A to be tested: Chloride ion concentration 40000mg / L, COD Cr Concentration 20 mg / L, weigh 66.20g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000mg / L, take 2mL of standard substance solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0092] 2. Sample B to be tested: Chloride ion concentration 20000mg / L, COD CrConcentration 20 mg / L, weigh 33.10 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 20000 mg / L, take 2 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0093] 3. Sample C to be tested: Chloride ion concentration 10000mg / L, COD Cr Concentration 20 mg / L, weigh 16.55g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 10000 mg / L, take 2mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-prepared brine.

[0094] 4. Test sample D: Chloride ion concentration 5000mg / L, COD Cr Concentration 20 mg / L, weigh 8.28 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 5000 mg / L, take 2 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0095] 5. Sample E to be tested: Chloride ion concentration 3000mg / L, COD Cr Concentration 20 mg / L, weigh 4.96g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 3000mg / L, take 2mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0096] 6. Test sample F: Chloride ion concentration 1000mg / L, COD CrConcentration 20 mg / L, weigh 1.66g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 1000mg / L, take 2mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0097] 7. Test sample G: Chloride ion concentration 40000mg / L, COD Cr Concentration 80 mg / L, weigh 66.20g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000mg / L, take 0.8mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0098] 8. Sample H to be tested: Chloride ion concentration 40000mg / L, COD Cr Concentration 90 mg / L, weigh 66.20g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000mg / L, take 0.9mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0099] 9. Test sample I: Chloride ion concentration 40000mg / L, COD Cr The concentration is 100 mg / L. Weigh 66.20 g of analytical grade sodium chloride in laboratory ultrapure water. After it is completely dissolved, dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40,000 mg / L. Take 1 mL of the standard solution with the production number BW20003-1000-WS-100 (B24010076) of Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10,000 mg / L in a 100 mL volumetric flask and dilute it to the scale line with the above-prepared brine.

[0100] 10. Test sample J: Chloride ion concentration 40000mg / L, COD CrConcentration 160mg / L, weigh 66.20g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000mg / L, take 1.6mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0101] 11. Test sample K: Chloride ion concentration 40000mg / L, COD Cr Concentration 180 mg / L, weigh 66.20 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000 mg / L, take 1.8 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10000 mg / L in a 100 mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0102] 12. Sample L to be tested: Chloride ion concentration 40000mg / L, COD Cr The concentration is 200 mg / L. Weigh 66.20 g of analytical grade sodium chloride in laboratory ultrapure water. After completely dissolving, dilute to a 1 L volumetric flask to obtain brine with a chloride ion concentration of 40,000 mg / L. Take 2 mL of the standard solution with the production number BW20003-1000-WS-100 (B24010076) of Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 10,000 mg / L in a 100 mL volumetric flask and dilute to the scale with the above-prepared brine.

[0103] 13. Sample M to be tested: Chloride ion concentration 40000mg / L, COD Cr Concentration 15 mg / L, weigh 66.20g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000mg / L, take 1.5mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000mg / L in a 100mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0104] 14. Sample N to be tested: Chloride ion concentration 20000mg / L, COD CrConcentration 15 mg / L, weigh 33.10 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000 mg / L, take 1.5 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100 mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0105] 15. Sample O to be tested: Chloride ion concentration 5000mg / L, COD Cr Concentration 15 mg / L, weigh 8.28 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000 mg / L, take 1.5 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100 mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0106] 16. Sample P to be tested: Chloride ion concentration 1000mg / L, COD Cr Concentration 15 mg / L, weigh 1.66 g of analytical grade sodium chloride in laboratory ultrapure water, dissolve it completely and dilute it in a 1L volumetric flask to obtain brine with a chloride ion concentration of 40000 mg / L, take 1.5 mL of standard solution with product number BW20003-1000-WS-100 (B24010076) produced by Tanmo Quality Inspection Technology Co., Ltd. and a chemical oxygen demand concentration of 1000 mg / L in a 100 mL volumetric flask, and dilute it to the scale line with the above-configured brine.

[0107] 17. Sulfuric acid (H2SO4): ρ = 1.84 g / mL.

[0108] 18. Sulfuric acid solution A: 1+9(V H2SO4 / V H2O ), mix 1 volume of sulfuric acid and 9 volumes of ultrapure water.

[0109] 19. Sulfuric acid solution B: 1+5(V H2SO4 / V H2O ), mix 1 volume of sulfuric acid and 5 volumes of ultrapure water.

[0110] 20. Potassium dichromate (K2Cr2O7) standard solution: Accurately weigh potassium dichromate and dissolve it in ultrapure water. Use ultrapure water to make up to 1000mL. Prepare standard solutions of potassium dichromate with concentrations of 0.025mol / L and 0.05mol / L respectively.

[0111] 21. Silver sulfate-sulfuric acid standard solution: Accurately weigh silver sulfate and dissolve it in sulfuric acid. Use sulfuric acid to make the volume to 1000mL to prepare a standard solution with a silver sulfate concentration of 10g / L.

[0112] 22. Mercury sulfate powder: Produced by Guangdong Guangshi Reagent Technology Co., Ltd., with analytical grade specification and content ≥99.5%.

[0113] 23. Mercuric sulfate solution: Weigh mercuric sulfate powder and dissolve it in sulfuric acid solution A. Use sulfuric acid solution A to make the volume to 1000 mL to prepare a mercuric sulfate solution with a mercuric sulfate concentration of 100 g / L.

[0114] 24. Ammonium ferrous sulfate standard solution: Accurately weigh ammonium ferrous sulfate and dissolve it in water. Add 1 mL of sulfuric acid and dilute to 1000 mL to prepare a standard solution with a concentration of 0.01 mol / L. Before use, the concentration of the ammonium ferrous sulfate solution needs to be accurately calibrated with potassium dichromate standard solution.

[0115] 25. Sodium thiosulfate standard solution: Dissolve 1.24 g of sodium thiosulfate in ultrapure water, add 0.1 g of anhydrous sodium carbonate, dilute to 1000 mL with ultrapure water, shake well and leave for one week before use. If there is precipitation in the solution, filter it to obtain a sodium thiosulfate standard solution with a concentration of 0.005 mol / L.

[0116] 26. Test the ferrous sulfate indicator solution: dissolve 0.7 g of ferrous sulfate heptahydrate in ultrapure water, add 1.5 g of 1,10-phenanthroline to dissolve, and dilute to 100 mL with ultrapure water.

[0117] 27. Sodium hydroxide solution: Weigh sodium hydroxide and dissolve it in a small amount of ultrapure water. Use ultrapure water to make the volume to 1000 mL to prepare a sodium hydroxide solution with a mass fraction of 2 wt%.

[0118] 28. Starch solution: Weigh soluble starch and a small amount of water to make a paste, add boiling water to make up to 1000 mL, continue boiling until the solution is clear, and after cooling, obtain a starch solution with a concentration of 10000 mg / L.

[0119] 29. Reflux reaction device A: Figure 1 As shown, it includes a conical flask and a condenser, cooling water flows on the inner wall of the condenser, the lower end of the condenser is tightly connected to the mouth of the conical flask, the upper end of the condenser is open, and a heating device is placed at the bottom of the conical flask.

[0120] 30. Reflux reaction device B: Figure 2-3As shown, it includes a conical flask, an air guide tube, a condenser, a lead-out tube and an absorption bottle. The air guide tube extends from the side wall of the conical flask into the interior, cooling water flows on the inner wall of the condenser, the lower end of the condenser is tightly connected to the mouth of the conical flask, the upper end of the condenser is tightly connected to one end of the lead-out tube, the other end of the lead-out tube is completely extended into the solution in the absorption bottle, and a heating device is placed at the bottom of the conical flask.

[0121] 31. In the detection method of dichromate method, according to the literature, the linear regression curve of the relationship between the reagent silver sulfate-sulfuric acid standard solution and the chloride ion concentration in 20mL of the sample to be tested is as follows: Figure 6 As shown, the linear regression line equation Y2=0.0007X2+29.254, R 2 =0.9979, Y2 is the required minimum dosage of the silver sulfate-sulfuric acid standard solution, in mL, and X2 is the sample chloride ion concentration, in mg / L. In the detection method of the following test example, the required silver sulfate-sulfuric acid standard solution is greater than or equal to the Y2 value, and the required silver sulfate-sulfuric acid standard solution corresponding to different chloride ion concentrations in the sample to be tested is shown in Table 1 below.

[0122] 32. In the detection method of dichromate method, according to the literature, the relationship between the amount of reflux water flushing after the digestion reaction and the chloride ion concentration in 20mL of the sample to be tested satisfies the linear regression line equation Y3=0.0018X3+79.845, R 2 =0.9995, Y3 is the minimum amount of water required for reflux flushing, in mL, and X3 is the sample chloride ion concentration, in mg / L. In the following test method, the required reflux water addition amount is greater than or equal to the Y3 value. The required post-reflux water addition amount corresponding to different chloride ion concentrations in the sample to be tested is shown in Table 1 below.

[0123] Table 1 - Relationship between silver sulfate, amount of water added after reflux and chloride ion concentration in 20mL sample

[0124]

[0125] Test Examples 1-6

[0126] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0127] (1) Take 20 mL of sample A to be tested in a conical flask. The chloride ion concentration of sample A to be tested is 40000 mg / L, COD CrThe concentration is 20 mg / L, and then 8 g of mercuric sulfate powder solid and sulfuric acid solution A are added in sequence, wherein the mass ratio of mercuric sulfate to the chloride ion content in the sample A to be tested is 10:1, and the volume ratio of sulfuric acid solution A to the sample to be tested is (2-4.5):1. The specific addition amount of sulfuric acid solution A is shown in Table 1 below. After standing for 10 minutes, 10 mL of potassium dichromate standard solution with a concentration of 0.05 mol / L and explosion-proof boiling glass beads with a diameter of 6 mm are added and mixed evenly;

[0128] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device B, connect the condensed water, add 60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, assemble the outlet tube and absorption bottle of the reflux absorption device, add 20 mL of sodium hydroxide solution to the absorption bottle, and dilute it to 200 mL with water, extend the outlet tube below the liquid level of the absorption bottle, insert the air guide tube and pass nitrogen, start heating the conical flask, and keep the solution at a slight boiling reflux for 2 h to perform the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0129] (3) After cooling, add 152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested A for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The apparent COD is calculated by Cr value;

[0130] (4) After the digestion reaction is completed, add 1g of potassium iodide to the absorption bottle, shake and dissolve it completely, then add sulfuric acid solution B to adjust the pH to 3, shake well and let it stand in the dark for 10 minutes, titrate the solution with sodium thiosulfate standard solution until it turns light yellow, add starch solution, and continue titrating with sodium thiosulfate standard solution until the blue color just disappears, which is the end point. Record the consumption volume of sodium thiosulfate standard solution, calculate the chlorine correction value, and calculate the sample COD Cr As a result, the experiment was repeated 6 times in parallel, and the results are shown in Table 2.

[0131] Among them, apparent COD Cr The value is calculated as follows:

[0132]

[0133] The calculation formula for the chlorine correction value (mg / L) is as follows:

[0134]

[0135] Where, ρ: apparent COD of the sample to be tested Cr Result, mg / L; C1: concentration of ammonium ferrous sulfate standard solution, mol / L; C2: concentration of sodium thiosulfate standard solution, mol / L; V0: volume of ammonium ferrous sulfate standard solution consumed in blank test, mL; V1: volume of ammonium ferrous sulfate standard solution consumed in determination of water sample to be tested, mL; V2: volume of water sample to be tested taken during heating and reflux; V3: volume consumed in titration of sodium thiosulfate standard solution; 8000: conversion value of 1 / 4 molar mass of O2 in mg / L.

[0136] Table 2 - Amount of sulfuric acid solution A added and COD in step (1) of the detection method of test examples 1-6 Cr Measurement results

[0137]

[0138]

[0139] In the above digestion reaction process, a known amount of potassium dichromate is added to the sample to be tested as an oxidant, and silver sulfate is used as a catalyst. Potassium dichromate oxidizes the reducing substances in the sample under acidic conditions. Silver nitrate can also catalyze the effective oxidation of straight-chain aliphatic compounds. Mercuric sulfate is used to shield the consumption of potassium dichromate by chloride ions in the sample. After digestion, ferrous iron is used as an indicator, and ammonium ferrous sulfate is used to titrate the unreduced potassium dichromate in the water sample. The apparent COD is calculated based on the consumed potassium dichromate content. Cr However, the chloride ions in the sample will consume potassium dichromate, especially when the chloride ion concentration in the water sample is high, the chloride ions are oxidized into chlorine gas, and the chlorine correction method is used to guide the oxidized chlorine gas into the absorption bottle to be absorbed by the sodium hydroxide solution. According to the consumption during the sodium thiosulfate titration, the chlorine correction value is converted to the sample COD Cr The result is apparent COD Cr Subtract the chlorine correction value.

[0140] Regarding the amount of sulfuric acid solution A added to high-chlorine wastewater, the inventors conducted the above research and test. As shown in Table 2 above, the sample to be tested is high-chlorine wastewater with a chloride ion concentration of 40,000 mg / L, and the sample to be tested has not been diluted. The detection method found that when the volume ratio of sulfuric acid solution A added to the sample to be tested is (2-2.5): 1, the apparent COD Cr The actual COD of the sample to be tested Cr The results are quite different, and the chlorine correction value is large. As the amount of sulfuric acid solution added decreases, the apparent COD Cr When the volume ratio of sulfuric acid solution A to the sample to be tested is (3-4.5):1, the apparent CODCr The actual COD of the sample to be tested Cr The results are similar. The apparent COD of Test Examples 3-6 Cr The relative error of the numerical value is between 0.7% and 5.5%, with high accuracy, and the chlorine correction value of Test Example 4-6 is 0, indicating that the detection method of the present application adds sufficient sulfuric acid solution during the digestion reaction, which can make mercuric sulfate effectively shield the consumption of chloride ions on potassium dichromate. Test Example 4-6 does not need to use the chlorine correction method or dilute the sample to be tested, and can achieve the COD detection of high-chlorine wastewater. Cr The detection is simple to operate and the detection data has high accuracy and precision.

[0141] In this regard, the inventor conducted multiple tests on samples with different chloride ion concentrations and found that as the chloride ion concentration in the sample changes, in order to allow mercuric sulfate to completely shield the consumption of chloride ions on potassium dichromate and to omit the chlorine correction step, the minimum amount of sulfuric acid solution A required for the digestion reaction process changes with the chloride ion concentration. The inventor obtained the minimum sulfuric acid solution A addition volume required for samples with different chloride ion concentrations of 1000mg / L-40000mg / L when the chlorine correction method was not performed through multiple tests. On the premise of meeting the high accuracy when the chlorine correction method was not performed, 20mL samples with different chloride ion concentrations were subjected to linear regression statistical analysis with the minimum sulfuric acid solution A addition volume required, as shown in Figure 2. Figure 7 As shown in the figure, the linear regression equation is Y1=0.0017X1+0.4635, R 2 =0.9999, Y1 is the volume of sulfuric acid solution A added in step (1), in mL, and X1 is the chloride ion concentration in 20 mL of the sample to be tested, in mg / L. Therefore, the volume of sulfuric acid solution A added in step (1) needs to be greater than or equal to the value of Y1 to meet a higher accuracy.

[0142] The following test examples 7-12 are used to verify the samples with different chloride ion concentrations. The added volume of sulfuric acid solution A during the digestion reaction is greater than or equal to the Y1 value of the above linear regression equation. The added volume of sulfuric acid solution A is the value rounded up after the Y1 value is calculated. The measurement results are shown in Table 3. The COD of the sample finally detected is Cr The results showed that the standard relative deviation was less than 3.6%, and the relative error value was between -3.7% and 2.1%. The precision and accuracy were high, indicating that mercuric sulfate could effectively shield the consumption of potassium dichromate by chloride ions, and no chlorine correction treatment was required, which was easy to operate.

[0143] Test Example 7-12

[0144] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0145] (1) Take 20 mL of the sample to be tested in a conical flask. The COD Cr The concentration is 20 mg / L, the chloride ion concentration is 1000 mg / L-40000 mg / L, and then 0.2g-8g of mercuric sulfate powder solid and 2mL-70mL of sulfuric acid solution A are added in sequence, wherein the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested is 10:1. After standing for 10 minutes, 10mL of potassium dichromate standard solution with a concentration of 0.05mol / L and explosion-proof boiling glass beads with a diameter of 6mm are added and mixed evenly;

[0146] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device A, connect the condensed water, add 30mL-60mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2h from the time it boils to perform the digestion reaction. The nitrogen flow rate during the digestion is 10ml / min;

[0147] The specific addition amounts of the chloride ion concentration of the sample to be tested, sulfuric acid solution A, mercuric sulfate and silver sulfate-sulfuric acid standard solution are shown in Table 3 below;

[0148] (3) After cooling, add ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The COD of the sample is calculated by formula (1): Cr As a result, the experiment was repeated three times in parallel.

[0149] Table 3 - The amount of reagents used in the detection method of test examples 7-12 of this application and the COD measured by the samples Cr result

[0150]

[0151]

[0152] Test Examples 13-16

[0153] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0154] (1) Take 20 mL of sample A to be tested in a conical flask. The chloride ion concentration of sample A to be tested is 40000 mg / L, COD CrThe concentration is 20 mg / L, and then 6.4 g-8.8 g of mercuric sulfate powder solid and 70 mL of sulfuric acid solution A are added in sequence, wherein the mass ratio of mercuric sulfate to the chloride ion content in the sample A to be tested is (8-11): 1, and the specific dosage of mercuric sulfate is shown in Table 4 below. After standing for 10 minutes, 10 mL of potassium dichromate standard solution with a concentration of 0.05 mol / L and explosion-proof boiling glass beads with a diameter of 6 mm are added and mixed evenly;

[0155] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device B, connect the condensed water, add 60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, assemble the outlet tube and absorption bottle of the reflux absorption device, add 20 mL of sodium hydroxide solution to the absorption bottle, and dilute it to 200 mL with water, extend the outlet tube below the liquid level of the absorption bottle, insert the air guide tube and pass nitrogen, start heating the conical flask, and keep the solution at a slight boiling reflux for 2 h to perform the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0156] (3) After cooling, add 152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested A for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The apparent COD is calculated by Cr result;

[0157] (4) After the digestion reaction is completed, add 1g of potassium iodide to the absorption bottle, shake and dissolve it completely, then add sulfuric acid solution B to adjust the pH to 3, shake well and let it stand in the dark for 10 minutes, titrate the solution with sodium thiosulfate standard solution until it turns light yellow, add starch solution, and continue titrating with sodium thiosulfate standard solution until the blue color just disappears, which is the end point. Record the consumption volume of sodium thiosulfate standard solution, calculate the chlorine correction value, and calculate the sample COD Cr As a result, the experiment was repeated 6 times in parallel.

[0158] Table 4 - Mercuric sulfate input and COD measured in the detection method of test examples 13-16 of this application Cr result

[0159]

[0160]

[0161] As shown in Table 4, tests were conducted on different mass ratios of mercuric sulfate added to the chloride ion content in the sample to be tested during the test process, and it was found that the amount of mercuric sulfate added would affect the accuracy of the test results. The sample to be tested was high-chlorine wastewater with a chloride ion concentration of 40,000 mg / L, and the sample to be tested was not diluted. When the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested was (8-9):1, the apparent COD Cr The actual COD of the sample to be tested Cr The results are quite different, and the chlorine correction value is large, indicating that mercuric sulfate cannot effectively shield the consumption of potassium dichromate by chloride ions during the digestion process. The relative error of the result reaches 137%, and the accuracy is significantly lower. When the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested is (10-11):1, the apparent COD Cr The actual COD of the sample to be tested Cr The results are similar, indicating that the amount of mercuric sulfate added in Test Examples 15-16 can ensure that the consumption of potassium dichromate by chloride ions is effectively shielded during the digestion process. The relative error of the test result is below 7.1%, and the detection accuracy is high.

[0162] Test Example 17

[0163] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0164] (1) Take 20 mL of sample A to be tested in a conical flask. The chloride ion concentration of sample A to be tested is 40000 mg / L, COD Cr Concentration 20mg / L, 8g of mercuric sulfate is fully dissolved in 70mL of sulfuric acid solution A, then added to a conical flask, where the mass ratio of mercuric sulfate to chloride ion content in the sample A to be tested is 10:1. After standing for 10 minutes, 10mL of 0.05mol / L potassium dichromate standard solution and 6mm diameter explosion-proof glass beads are added and mixed evenly;

[0165] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device B, connect the condensed water, add 60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, assemble the outlet tube and absorption bottle of the reflux absorption device, add 20 mL of sodium hydroxide solution to the absorption bottle, and dilute it to 200 mL with water, extend the outlet tube below the liquid level of the absorption bottle, insert the air guide tube and pass nitrogen, start heating the conical flask, and keep the solution at a slight boiling reflux for 2 h to perform the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0166] (3) After cooling, add 152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested A for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The apparent COD is calculated by Cr result;

[0167] (4) After the digestion reaction is completed, add 1 g of potassium iodide to the absorption bottle, shake it to dissolve completely, add sulfuric acid solution B to adjust the pH to 3, shake it well and let it stand in the dark for 10 min. Titrate the solution with sodium thiosulfate standard solution until it turns light yellow, add starch solution, and continue to titrate with sodium thiosulfate standard solution until the blue color just disappears, which is the end point. Record the consumption volume of sodium thiosulfate standard solution, calculate the chlorine correction value, and calculate the sample COD Cr As a result, the experiment was repeated 6 times in parallel, and the results are shown in Table 5.

[0168] Table 5 - COD of samples measured in the detection methods of test examples 4 and 17 of this application Cr result

[0169]

[0170] As shown in Table 5, the inventors found through multiple experiments that the masking effects of adding mercuric sulfate in solid form or in liquid form dissolved with sulfuric acid are quite different. Cr The actual COD of the sample to be tested Cr The results are similar, and a higher accuracy can be obtained without the need for chlorine correction. It is speculated that the chemical reaction occurs in the collision between the reactant molecules. In the solid reactant, the distance between molecules is closer, and the collision frequency between particles is higher, which makes the reaction rate faster. The reaction rate of mercuric sulfate and chloride ions to form a stable mercuric chloride complex is faster, thus achieving effective interference. In Test Example 17, when mercuric sulfate was dissolved in sulfuric acid solution and added as liquid, the apparent COD detected was Cr The actual COD of the sample to be tested Cr The results are quite different, and the COD of the samples calculated after chlorine correction method is Cr The results have a high relative error and low accuracy. It is speculated that the molecular freedom of the liquid reactant is large, resulting in a low collision frequency, and the reaction rate of mercuric sulfate and chloride ions to form a stable mercuric chloride complex is relatively slow. Mercuric sulfate cannot fully shield the interference of chloride ions, resulting in low detection accuracy.

[0171] Test Examples 18-23

[0172] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0173] (1) Take 20 mL of the sample to be tested in a conical flask. The chloride ion concentration of the sample to be tested is 40000 mg / L, COD Cr The concentration is 80mg / L-200mg / L, and then 8g of mercuric sulfate powder solid and 70mL of sulfuric acid solution A are added in sequence, wherein the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested is 10:1. After standing for 10 minutes, 10mL of potassium dichromate standard solution with a concentration of 0.025mol / L-0.05mol / L and explosion-proof boiling glass beads with a diameter of 6mm are added and mixed evenly. The COD Cr The specific content and the specific concentration of potassium dichromate standard solution are shown in Table 6 below;

[0174] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device B, connect the condensed water, add 60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2 h to carry out the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0175] (3) After cooling, add 152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The COD of the sample is calculated. Cr As a result, the experiment was repeated twice in parallel, and the results are shown in subscript 6.

[0176] Table 6 - COD of samples measured in the detection method of test examples 18-23 of this application Cr result

[0177]

[0178] As shown in Table 6, the inventors found through multiple studies and experiments that the concentration of potassium dichromate standard solution would affect the detection limit of the detection method. In Test Examples 18-20, the concentration of potassium dichromate standard solution was 0.025 mol / L. It was found that the detection method could only detect COD Cr The sample with a concentration below 90mg / L can be accurately tested. CrWhen the concentration is 100 mg / L, the digested solution shows green and the titration endpoint cannot be observed. In Test Examples 21-23, the concentration of the potassium dichromate standard solution is 0.05 mol / L. It is found that this detection method can only detect COD Cr The sample with a concentration below 180mg / L can be accurately tested. Cr When the concentration is 200 mg / L, the digested solution shows green and the titration endpoint cannot be observed. Therefore, it can be seen that the potassium dichromate standard solution has a higher detection limit when the concentration is 0.05 mg / L, which can reach 180 mg / L, and can be used for the detection of wastewater samples with higher chemical oxygen demand.

[0179] Test Examples 24-27

[0180] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0181] (1) Take 20mL of the sample to be tested in a conical flask. The chloride ion concentration of the sample to be tested is 1000-40000mg / L, COD Cr The concentration is 15 mg / L, followed by the addition of mercuric sulfate powder solid and 2-70 mL of sulfuric acid solution A, wherein the mass ratio of mercuric sulfate to chloride ion content in the sample to be tested is 10:1. The specific COD Cr The concentrations are shown in Table 7 below. After standing for 10 minutes, 10 mL of a 0.05 mol / L potassium dichromate standard solution and 6 mm diameter explosion-proof glass beads were added and mixed evenly.

[0182] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device A, connect the condensed water, add 30-60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2 hours from the time it boils to perform a digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0183] (3) After cooling, add 85-152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution, and then titrate with ammonium ferrous sulfate standard solution. When the color of the solution changes from yellow to blue-green to reddish brown, the end point is recorded. The consumed volume of ammonium ferrous sulfate standard solution is recorded. While testing the sample to be tested, an equal amount of ultrapure water is used to replace the sample to be tested for a blank test. The volume of ammonium ferrous sulfate standard solution consumed during the blank titration is recorded. The apparent COD is calculated by formula (1): Cr As a result, the experiment was repeated 12 times in parallel, and the results are shown in Table 7.

[0184] Table 7 - COD of samples measured in the detection method of test examples 24-27 of this application Cr result

[0185]

[0186]

[0187]

[0188] As shown in Table 7, after many studies and experiments, the inventors have concluded that the minimum detection limit of the detection method of the present application is about 3.5 mg / L, the determination lower limit is about 15 mg / L, and the detection lower limit is about four times the detection limit. It can be applied to chloride ion concentrations below 40000 mg / L and COD Cr The detection of wastewater samples with a concentration of 180 mg / L effectively improved the detection limit of high-chlorine wastewater and the accuracy and precision of the detection data. The detection steps are simple and easy to operate.

[0189] In order to verify the accuracy and precision of the detection method of this application for the detection of high-chlorine wastewater generated in the actual production process, the wastewater treated by the etching copper process of an electronics factory is used as the test sample. The chloride ion concentration in the wastewater is 3000 mg / L. The following test example 28 and the existing industry standard "Chlorine Correction Method for Determination of Chemical Oxygen Demand of High-Chlorine Wastewater" (HJ / T 70-2001) are used for detection, and the accuracy and precision of the two detection methods are compared. The test results are shown in Table 8 below. For the detection of wastewater with a chloride ion concentration of 1000 mg / L-20000 mg / L, the detection results of the detection method of this application and the HJ / T70-2001 standard are not much different, and the standard relative deviation is low, and the accuracy and precision are high.

[0190] Test Example 28

[0191] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0192] (1) Take 20 mL of the sample to be tested in a conical flask, then add 0.6 g of mercuric sulfate powder and 6 mL of sulfuric acid solution A in sequence, where the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested is 10:1. After standing for 10 minutes, add 10 mL of 0.05 mol / L potassium dichromate standard solution and 6 mm diameter explosion-proof glass beads and mix well;

[0193] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device A, connect the condensed water, add 32 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2 h to carry out the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0194] (3) After cooling, add 85 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution and then titrate with ammonium ferrous sulfate standard solution. The end point is when the color of the solution changes from yellow to blue-green to reddish brown. Record the consumed volume of ammonium ferrous sulfate standard solution. At the same time, use ultrapure water to replace the sample water sample to perform a blank test. Record the volume of ammonium ferrous sulfate standard solution consumed during the blank titration. Calculate the sample COD by formula (1): Cr As a result, the test was repeated three times in parallel, and the test results are shown in Table 8 below.

[0195] Table 8 - COD of samples measured in the test example 28 of the present application and the chlorine correction method Cr result

[0196]

[0197] In order to verify the accuracy and precision of the detection method of this application for the detection of low-chlorine wastewater generated in the actual production process, the wastewater treated by a hospital is used as the sample to be tested. The chloride ion concentration in the wastewater is 140 mg / L. The following test example 29 and the existing industry standard "Determination of Chemical Oxygen Demand of Water Quality by Dichromate Method" (HJ 828-2017) are used for detection, and the accuracy and precision of the two detection methods are compared. The test results are shown in Table 9 below. For the detection of wastewater with a chloride ion concentration below 1000 mg / L, the detection results of the detection method of this application and the HJ / T 70-2001 standard are not much different, and the standard relative deviation is low, and the accuracy and precision are high.

[0198] Test Example 29

[0199] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0200] (1) Take 20 mL of the sample to be tested in a conical flask, then add 0.028 g of mercuric sulfate powder and 0.8 mL of sulfuric acid solution A in sequence, where the mass ratio of mercuric sulfate to the chloride ion content in the sample to be tested is 10:1. After standing for 10 minutes, add 10 mL of 0.05 mol / L potassium dichromate standard solution and 6 mm diameter explosion-proof glass beads and mix well.

[0201] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device A, connect the condensed water, add 32 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2 h to carry out the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0202] (3) After cooling, add 85 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution and then titrate with ammonium ferrous sulfate standard solution. The end point is when the color of the solution changes from yellow to blue-green to reddish brown. Record the consumed volume of ammonium ferrous sulfate standard solution. At the same time, use ultrapure water to replace the sample water sample to perform a blank test. Record the volume of ammonium ferrous sulfate standard solution consumed during the blank titration. Calculate the sample COD by formula (1): Cr As a result, the test was repeated three times in parallel, and the test results are shown in Table 9 below.

[0203] Table 9 - COD of samples measured in the test example 29 of the present application and the detection method of the dichromate method Cr result

[0204]

[0205]

[0206] In order to verify the accuracy and precision of the detection method of this application for the detection of high-chlorine wastewater generated in the actual production process, the wastewater of Nanjian was used as the test sample. The COD Cr The value is 5.63 mg / L. The chloride ion concentration in the wastewater is 3000 mg / L. The following test example 30 method is used for detection to evaluate the accuracy and precision of the detection method for high-chlorine wastewater. The test results are shown in Table 10 below. For the detection of wastewater with a chloride ion concentration of 20000 mg / L-40000 mg / L, the detection method of this application is consistent with the COD measured by the online TOC instrument. Cr The values ​​are not much different, and the standard relative deviations are low, indicating high accuracy and precision.

[0207] Test Example 30

[0208] A method for detecting chemical oxygen demand in water quality comprises the following steps:

[0209] (1) Take 20 mL of the sample to be tested in a conical flask, then add 8 g of mercuric sulfate powder and 70 mL of sulfuric acid solution A in sequence. After standing for 10 min, add 10 mL of 0.05 mol / L potassium dichromate standard solution and 6 mm diameter explosion-proof glass beads and mix well.

[0210] (2) Connect the conical flask to the lower end of the condenser of the reflux reaction device A, connect the condensed water, add 60 mL of silver sulfate-sulfuric acid standard solution from the top of the condenser, mix well, start heating the conical flask, and keep the solution at a low boiling reflux for 2 h to perform the digestion reaction. The nitrogen flow rate during the digestion is 10 ml / min;

[0211] (3) After cooling, add 152 mL of ultrapure water to the upper end of the condenser for rinsing. After cooling to room temperature again, add 3 drops of ferrous sulfate indicator solution and then titrate with ammonium ferrous sulfate standard solution. The end point is when the color of the solution changes from yellow to blue-green to reddish brown. Record the consumed volume of ammonium ferrous sulfate standard solution. At the same time, use ultrapure water to replace the sample water sample to perform a blank test. Record the volume of ammonium ferrous sulfate standard solution consumed during the blank titration. Calculate the sample COD by formula (1): Cr As a result, the test was repeated three times in parallel, and the test results are shown in Table 10 below.

[0212] Table 10 - COD of samples measured in the detection method of Experiment 30 of this application Cr result

[0213]

[0214]

[0215] The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the scope of protection of the present application. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A method for detecting chemical oxygen demand in water, characterized in that: The following steps are involved: (1) taking a sample to be tested, adding a masking agent and a sulfuric acid solution, and after the reaction is complete, adding an oxidant of known concentration and in excess according to the predicted chemical oxygen demand of the sample to be tested, and adding a catalyst at the same time, heating the reaction to complete, and obtaining a digestion solution; (2) adding an indicator to the digestion solution, titrating the remaining oxidant in the reaction with a reducing agent of known concentration, recording the consumption of the reducing agent, and calculating the chemical oxygen demand result of the sample to be tested; Wherein, the amount of sulfuric acid added to the sulfuric acid solution is R1, in mL, R1 value ≥ Y1×V1′ / 200, V1′ is the volume of the sample to be tested in step (1), in mL; Y1=0.0017X1+0.4635, X1 is the chloride ion concentration of the sample to be tested, in mg / L; Among them, the masking agent is solid mercuric sulfate powder or silver nitrate, and when the masking agent is solid mercuric sulfate powder, the mass ratio of the mercuric sulfate to the chloride ion content in the sample to be tested is greater than 9; when the masking agent is silver nitrate, the molar ratio of the silver nitrate to the chloride ion content in the sample to be tested is 1:

1.

2. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: The mass ratio of the solid mercuric sulfate to the chloride ion content in the sample to be tested is 10-20.

3. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: The oxidant is a potassium dichromate solution, and the concentration of potassium dichromate in the potassium dichromate solution is 0.025 mol / L-0.5 mol / L; And / or, the total amount of potassium dichromate added to the sample to be tested is E, Wherein, E: the amount of potassium dichromate added, mol / L; ρ': the predicted chemical oxygen demand of the water sample to be tested in step (1), mg / L; a≥15, V2: the volume of the water sample to be tested in step (1), mL.

4. The method for detecting chemical oxygen demand in water according to claim 3, characterized in that: The volume ratio of the potassium dichromate solution to the sample to be tested is 1:(1-5).

5. The method for detecting chemical oxygen demand in water according to claim 4, characterized in that: The volume ratio of the potassium dichromate solution to the sample to be tested is 1:2; And / or, when the COD of the water sample to be tested in step (1) is predicted Cr When the concentration is 90 mg / L or less, the concentration of the potassium dichromate solution is 0.025 mol / L-0.05 mol / L; When the COD of the water sample to be tested is predicted in step (1) Cr When the concentration is 180 mg / L or less, the concentration of the potassium dichromate solution is 0.05 mg / L-0.075 mg / L.

6. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: The catalyst in step (1) is a silver sulfate-sulfuric acid solution formed by mixing silver sulfate and sulfuric acid, and the concentration of silver sulfate in the silver sulfate-sulfuric acid solution is 3 g / L-20 g / L; And / or, the indicator in step (2) contains 1,10-phenanthroline; And / or, the reducing agent in step (2) contains ammonium ferrous sulfate.

7. The method for detecting chemical oxygen demand in water according to claim 6, characterized in that: The total mass of silver sulfate in the silver sulfate-sulfuric acid solution is R2, in g, R2 ≥ Y2 × V2′ / 2000, V2′ is the volume of the sample to be tested in step (1), in mL; The Y2=0.0007X2+29.254, X2 is the chloride ion concentration of the sample to be tested, in mg / L.

8. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: The volume fraction of the sulfuric acid solution in step (1) is 5wt%-50wt%; And / or, the chloride ion concentration in the sample to be tested in step (1) is below 40000 mg / L.

9. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: The chloride ion concentration in the sample to be tested is 1000 mg / L-40000 mg / L.

10. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: While testing the sample to be tested, an equal amount of water is used to replace the sample to be tested, and a blank test is performed according to the same testing steps, and the consumption of the reducing agent in the blank test is recorded.

11. The method for detecting chemical oxygen demand in water according to claim 10, characterized in that: In step (2), the chemical oxygen demand calculation formula of the sample to be tested is: Wherein, ρ is the chemical oxygen demand of the sample to be tested, mg / L; C1 is the concentration of the reducing agent, mol / L; V0 is the volume of the reducing agent consumed in the blank test, mL; V1: the volume of the reducing agent consumed by the sample to be tested, mL; V2: the volume of the water sample to be tested in step (1), mL.

12. The method for detecting chemical oxygen demand in water according to claim 1, characterized in that: In step (1), the reaction time after adding the masking agent and the sulfuric acid solution is 5 min to 20 min; And / or, in the step (1), after adding the oxidant and the catalyst, the solution is heated to a slight boil, and the heating reaction time is 1h-4h to obtain a digestion solution; And / or, in the step (1), the heating reaction is carried out in a reflux reaction device, and after the heating reaction is completed, the inner wall of the condenser tube of the reflux reaction device is cooled and water is added to rinse to obtain a digestion solution; And / or, in step (2), a reducing agent is used to titrate the remaining oxidizing agent in the reaction, and the end point is when the color of the solution changes from yellow to blue-green to reddish brown.

13. The method for detecting chemical oxygen demand in water according to claim 12, characterized in that: In step (1), the amount of water added to flush the inner wall of the condenser tube of the reflux reaction device is R3, in mL, R3 ≥ Y3 × V3ˊ / 20, V3ˊ is the volume of the sample to be tested in step (1), in mL, Y3 = 0.0018X3 + 79.845, X3 is the chloride ion concentration of the sample to be tested, in mg / L.

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