Method for accurately and rapidly determining COD (Chemical Oxygen Demand) in high-chlorine wastewater

Abstract

The invention relates to the technical field of sewage detection, and discloses a method for accurately and rapidly determining COD (Chemical Oxygen Demand) in high-chlorine wastewater, which comprises the following steps: step 1, preparing a solution; step 2, sampling; step 3, adding a masking agent; step 4, oscillating and complexing; step 5, oxidation; step 6, catalyzing; step 7, performing reflux digestion; step 8, titration determination; step 9, calculating a result; according to the method, the detection range is wider, and the chlorine ion concentration interference range is increased from 1000 mg / L to 40100 mg / L; the detection limit is improved to 15.9 mg / L from 20, and the precision is superior to the national standard; nitrogen stripping is removed, multiple batches can be measured at a time, and the efficiency is doubled; the water sample to be detected does not need to be diluted, nitrogen stripping equipment is not needed, and the operation cost is reduced; and the reagent is added by special equipment, so that the result is reliable and controllable.

Description

Technical Field

[0001] This invention relates to the field of wastewater testing technology, and in particular to a method for accurately and rapidly determining COD in high-chlorine wastewater. Background Technology

[0002] In existing technologies, the determination of chemical oxygen demand (COD) in surface water, domestic sewage, and industrial wastewater generally employs two standards. When the concentration of chloride compounds is less than 1000 mg / L, the People's Republic of China National Environmental Protection Standard HJ828-2017 is used for determination. This method is the dichromate method, which is not suitable for saline water with a chloride ion concentration greater than 1000 mg / L. When the chloride ion concentration is greater than 1000 mg / L, the People's Republic of China National Ecological and Environmental Standard HJ / T 70-2001 is used for determination. A nitrogen stripping-assisted chlorine correction method is employed to eliminate chloride ion interference, which can mask water samples with chloride ion concentrations >1000 mg / L. The method detection limit is 30 mg / L. However, this method is only applicable to chloride ion concentrations of 1000 mg / L to 20000 mg / L. When the chloride ion concentration is greater than 20000 mg / L, there is currently no suitable method for measurement. The only option is to dilute the water sample before measurement. However, dilution may cause the COD to fall below the detection limit, resulting in inaccurate measurement results.

[0003] The HJ 70-2025 standard, which will be implemented on July 1, 2026, also adopts nitrogen stripping and chlorine absorption correction. Nitrogen is continuously introduced during the water sample reflux digestion process, and the chlorine generated in the reaction is stripped into the sodium hydroxide absorption solution to complete quantitative absorption. The detection is completed in a reflux Erlenmeyer flask and can mask chloride ion interference in the concentration range of 1000 mg / L to 20000 mg / L. The detection limit of this method has been optimized to 20 mg / L.

[0004] Both standards for chlorine correction methods applicable to high-chlorine wastewater employ nitrogen stripping and chlorine absorption correction to eliminate chloride ion interference. This requires continuous nitrogen flow during the reflux digestion process to strip the generated chlorine into a sodium hydroxide absorbent for quantitative absorption. The entire detection process is completed in a reflux Erlenmeyer flask. This method not only requires titration to determine the COD value of the water sample but also necessitates independent titration of the nitrogen stripping absorbent to calculate the correction value, adding an extra titration step. The nitrogen stripping process requires strict control, consumes nitrogen, and involves cumbersome and complex procedures, resulting in long detection times, high manpower costs, low detection efficiency, and difficulty in consistently guaranteeing the accuracy and precision of the results. Furthermore, it cannot achieve simultaneous and efficient monitoring of large batches of high-chlorine water samples.

[0005] Neither of the two standards covers the chloride ion concentration range of 20,000 mg / L to 40,000 mg / L. For water samples with low COD levels within this range, the data obtained from high-concentration and low-concentration monitoring are drastically different. If the water sample is diluted, the COD value is easily reduced to below 50 mg / L, requiring a switch to a low-concentration method, where the data obtained from the two methods differ significantly. When the COD of the water sample is already low, dilution is not advisable regardless of whether a high-concentration or low-concentration detection system is used, as the COD after dilution is very likely to fall below the method detection limit, resulting in unreliable data. Therefore, dilution is not suitable for monitoring water samples with high chloride ion concentrations and low COD values, and existing standard methods are insufficient to meet the accurate measurement requirements of such high-chlorine, low-COD water samples.

[0006] For wastewater treatment plants in coastal areas, the chloride ion concentration in the industrial wastewater they receive and the water samples from various treatment units (such as equalization tanks, coagulation tanks, biological treatment tanks, and hydrolysis tanks) generally exceeds 1000 mg / L. Relying entirely on the chlorine correction method for COD monitoring would impose a significant workload and cost burden on these enterprises. Therefore, developing a method for rapidly and accurately determining the COD of high-chlorine wastewater with chloride ion concentrations generally exceeding 1000 mg / L has become a pressing technical challenge for those skilled in the art. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide an accurate and rapid method for determining COD in high-chlorine wastewater, which has a significantly widened chloride ion masking range, a wide COD detection range, stronger applicability, and accuracy and precision that are comprehensively superior to national standards. This method simplifies the detection process, eliminates the need for nitrogen stripping and double titration, eliminates the need for water sample dilution, completely eliminates dilution errors, is suitable for large-scale simultaneous detection, has higher detection efficiency, strong method stability, outstanding anti-interference ability, controllable operating conditions, and higher reliability of results.

[0008] To solve the above-mentioned technical problems, the technical solution of the present invention is: a method for accurately and rapidly determining COD in high-chlorine wastewater, characterized by comprising the following steps: Step 1, prepare solutions: Prepare 0.2500 mol / L and 0.0250 mol / L potassium dichromate standard solutions respectively; prepare 0.050 mol / L and 0.005 mol / L ferrous ammonium sulfate standard solutions respectively; prepare 30% mercuric sulfate solution, silver sulfate-sulfuric acid solution and ferroin indicator respectively; Step 2, Sampling: Take 10ml of the water sample to be tested and add it into a COD-specific digestion tube; Step 3, add masking agent: Add the corresponding volume of the 30% mercuric sulfate solution prepared in Step 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 18-30:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well; Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well; Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group. Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0009] As a preferred technical solution, in step five, an appropriate potassium dichromate standard solution is added according to the estimated COD concentration in the water sample to be tested. When the COD concentration in the water sample to be tested is greater than 50 mg / L, a 0.2500 mol / L potassium dichromate standard solution is selected; when the COD concentration in the water sample to be tested is less than 50 mg / L, a 0.0250 mol / L potassium dichromate standard solution is selected.

[0010] As a preferred technical solution, in step eight, when the mass concentration of COD in the water sample to be tested is greater than 50 mg / L, it is titrated with a 0.050 mol / L ferrous ammonium sulfate standard solution; when the mass concentration of COD in the water sample to be tested is less than 50 mg / L, it is titrated with a 0.005 mol / L ferrous ammonium sulfate standard solution.

[0011] As a preferred technical solution, the chloride ion concentration in the water sample to be tested is 1000–40100 mg / L.

[0012] As a preferred technical solution, the COD detection range of the water sample to be tested is 15.9–260 mg / L.

[0013] Due to the adoption of the above technical solution, the beneficial effects of the present invention are: (1) The range of chloride ion masking is significantly broadened. It can stably mask chloride ion interference in an ultra-wide concentration range of 1000–40100 mg / L, breaking through the limitation of existing national standard methods that are only applicable to the detection of chloride ions ≤20000 mg / L, and is suitable for more complex high-chlorine water samples.

[0014] (2) The COD detection range is wider and the applicability is stronger. It can directly measure water samples with COD ranging from 15.9 to 260 mg / L, covering both low-concentration and medium-to-high-concentration ranges, meeting the needs of various high-chlorine water quality testing.

[0015] (3) The accuracy and precision are superior to the national standard requirements. Based on the national standard "Determination of Chemical Oxygen Demand in Water - Dichromate Method" (HJ 828-2017) and the COD detection standard requirements for high-chlorine wastewater, this invention, under the optimized masking ratio, achieves a relative error of ≤4% and an RSD of ≤2.40%, with stable and reliable data. The accuracy and precision are superior to the national standard quality control requirements, effectively solving the core problems of large deviations and inaccurate data in high-chlorine, low-COD water samples.

[0016] (4) Simplify the detection process and eliminate nitrogen stripping and double titration. By eliminating the need for traditional nitrogen stripping devices and separate titration steps with absorbent liquid, COD determination can be completed in a single titration, significantly reducing operational steps and minimizing sources of human error.

[0017] (5) No need to dilute water samples, completely eliminating dilution errors. For water samples with low COD and high chlorine, direct detection is possible without dilution, avoiding problems such as COD falling below the detection limit or large differences in high and low concentration ranges due to dilution, thus ensuring accurate and reliable results.

[0018] (6) Suitable for simultaneous testing of large batches of water samples, with higher testing efficiency. By using a dedicated large-diameter COD digestion tube and a batch digestion device, multiple water samples can be simultaneously shaken, digested, and measured, solving the problems of small processing capacity, long processing time, and high manpower input of traditional methods.

[0019] (7) The method has strong stability and outstanding anti-interference ability. Excellent precision can be maintained with different masking ratios. The ratio can be flexibly selected according to the actual concentration of chloride ions. The method has good reproducibility and is applicable to a variety of scenarios.

[0020] (8) The operating conditions are controllable, and the results are more reliable. The entire process does not require strict control of complex conditions such as nitrogen flow rate and absorption efficiency, making the detection process easier to control and significantly improving the accuracy and stability of the detection results. Detailed Implementation

[0021] To make the objectives and technical solutions of this invention clearer and easier to understand, the invention will be further described in detail below. Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions; unless otherwise specified, the reagents and materials are commercially available.

[0022] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0023] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0024] The chemical preparations involved in the following examples are all commonly used in the field.

[0025] Example 1: The COD of the following water samples was measured respectively; Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 45.6 ± 2.9 mg / L. Water sample 2 had a chloride ion concentration of 5056 mg / L and a COD concentration of 33.5 ± 2.3 mg / L. Water sample three had a chloride ion concentration of 16080 mg / L and a COD concentration of 28.7 ± 1.9 mg / L. Water sample four had a chloride ion concentration of 21020 mg / L and a COD concentration of 24.0 ± 1.8 mg / L. Water sample 5 had a chloride ion concentration of 30,400 mg / L and a COD concentration of 33.0 ± 1.5 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 15.9 ± 1.2 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0026] The product batch number used for water sample 1 is B25010015; the product batch number used for water sample 2 is B25020416; the product batch number used for water sample 3 is B25020224; the product batch number used for water sample 4 is B24110133; the product batch number used for water sample 5 is B21110178; and the product batch number used for water sample 6 is BY40001. Step 1, Solution Preparation: Since the average COD concentration in the water samples to be tested is less than 50 mg / L, the following solutions are prepared: 0.0250 mol / L potassium dichromate standard solution; 0.005 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 18:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well; Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well; Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group. Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0027]

[0028] Table 1. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is less than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 18:1. As shown in Table 1, when the mass ratio of mercuric sulfate to chloride ions is 18:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-40100 mg / L and COD less than 50 mg / L.

[0029] Example 2: COD was measured on the following water samples respectively. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 45.6 ± 2.9 mg / L. Water sample 2 had a chloride ion concentration of 5056 mg / L and a COD concentration of 33.5 ± 2.3 mg / L. Water sample three had a chloride ion concentration of 16080 mg / L and a COD concentration of 28.7 ± 1.9 mg / L. Water sample four had a chloride ion concentration of 21020 mg / L and a COD concentration of 24.0 ± 1.8 mg / L. Water sample 5 had a chloride ion concentration of 30,400 mg / L and a COD concentration of 33.0 ± 1.5 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 15.9 ± 1.2 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0030] The product batch number used for water sample 1 is B25010015; the product batch number used for water sample 2 is B25020416; the product batch number used for water sample 3 is B25020224; the product batch number used for water sample 4 is B24110133; the product batch number used for water sample 5 is B21110178; and the product batch number used for water sample 6 is BY40001. Step 1, Solution Preparation: Since the average COD concentration in the water samples to be tested is less than 50 mg / L, the following solutions are prepared: 0.0250 mol / L potassium dichromate standard solution; 0.005 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mercuric sulfate to chloride ion mass ratio of 22.5:1; Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well; Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well; Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group. Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0031]

[0032] Table 2. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is less than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 22.5:1. As shown in Table 2, when the mass ratio of mercuric sulfate to chloride ions is 22.5:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-40100 mg / L and COD less than 50 mg / L.

[0033] Unlike Example 1, in Example 2, the mass ratio of mercuric sulfate to chloride ions is 22.5:1, so the amount of mercuric sulfate added is greater than in Example 1. Although both can meet the test requirements, Example 1 is superior to Example 2 in order to reduce costs.

[0034] Example 3: COD was measured on the following water samples respectively. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 45.6 ± 2.9 mg / L. Water sample 2 had a chloride ion concentration of 5056 mg / L and a COD concentration of 33.5 ± 2.3 mg / L. Water sample three had a chloride ion concentration of 16080 mg / L and a COD concentration of 28.7 ± 1.9 mg / L. Water sample four had a chloride ion concentration of 21020 mg / L and a COD concentration of 24.0 ± 1.8 mg / L. Water sample 5 had a chloride ion concentration of 30,400 mg / L and a COD concentration of 33.0 ± 1.5 mg / L. The COD standard samples of the above five water samples were obtained by using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix and adding the corresponding sodium chloride to obtain high-chlorine water samples with known chloride ion concentrations.

[0035] The product batch number used for water sample 1 was B25010015; for water sample 2 it was B25020416; for water sample 3 it was B25020224; for water sample 4 it was B24110133; and for water sample 5 it was B21110178. Step 1, Solution Preparation: Since the average COD concentration in the water samples to be tested is less than 50 mg / L, the following solutions are prepared: 0.0250 mol / L potassium dichromate standard solution; 0.005 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 30:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0036] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0037] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0038] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0039]

[0040] Table 3. Detection results of COD in water samples when chloride ion concentration is 1000-30400 mg / L, COD is less than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 30:1. As shown in Table 3, when the mass ratio of mercuric sulfate to chloride ions is 30:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-30400 mg / L and COD less than 50 mg / L.

[0041] Unlike Example 1, in Example 3 the mass ratio of mercuric sulfate to chloride ions is 30:1, so the amount of mercuric sulfate added is greater than in Example 1. Although both can meet the test requirements, Example 1 is better than Example 3 in order to reduce costs.

[0042] Example 4: COD was measured on the following water samples respectively. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 45.6 ± 2.9 mg / L. Water sample 2 had a chloride ion concentration of 5056 mg / L and a COD concentration of 33.5 ± 2.3 mg / L. Water sample three had a chloride ion concentration of 16080 mg / L and a COD concentration of 28.7 ± 1.9 mg / L. Water sample four had a chloride ion concentration of 21020 mg / L and a COD concentration of 24.0 ± 1.8 mg / L. Water sample 5 had a chloride ion concentration of 30,400 mg / L and a COD concentration of 33.0 ± 1.5 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 15.9 ± 1.2 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0043] The product batch number used for water sample 1 is B25010015; the product batch number used for water sample 2 is B25020416; the product batch number used for water sample 3 is B25020224; the product batch number used for water sample 4 is B24110133; the product batch number used for water sample 5 is B21110178; and the product batch number used for water sample 6 is BY40001. Step 1, Solution Preparation: Since the average COD concentration in the water samples to be tested is less than 50 mg / L, the following solutions are prepared: 0.0250 mol / L potassium dichromate standard solution; 0.005 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 9:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0044] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0045] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0046] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0047]

[0048] Table 4. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is less than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 9:1. As shown in Table 2, when the mass ratio of mercuric sulfate to chloride ions is 9:1, the relative error of the measured COD result is higher than the national standard requirement of -4.1% to 5.8%. Therefore, the mercuric sulfate to chloride ion mass ratio in this embodiment is not suitable for measuring the COD of high-chlorine wastewater with a chloride ion concentration of 1000-40100 mg / L and a COD of less than 50 mg / L. Thus, when the mass ratio of mercuric sulfate to chloride ions is 9:1, the amount of mercuric sulfate added is too small to mask the chloride ions.

[0049] Example 5: COD was measured on the following water samples respectively. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 107 ± 10 mg / L; Water sample 2 had a chloride ion concentration of 5512 mg / L and a COD concentration of 251 ± 19 mg / L. Water sample 3 had a chloride ion concentration of 16020 mg / L and a COD concentration of 144±8 mg / L. Water sample four had a chloride ion concentration of 21030 mg / L and a COD concentration of 260 ± 17 mg / L. Water sample 5 had a chloride ion concentration of 30,500 mg / L and a COD concentration of 183 ± 8 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 183 ± 8 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0050] The product batch number used for water sample 1 is B25090590; the product batch number used for water sample 2 is B25090712; the product batch number used for water sample 3 is B25080192; the product batch number used for water sample 4 is B2601025; the product batch number used for water sample 5 is B22100140; and the product batch number used for water sample 6 is B26020053. Step 1: Since the average COD concentration in the water samples to be tested is greater than 50 mg / L, prepare the following solutions: 0.250 mol / L potassium dichromate standard solution; 0.05 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 9:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0051] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0052] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0053] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0054]

[0055] Table 5. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is greater than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 18:1. As shown in Table 5, when the mass ratio of mercuric sulfate to chloride ions is 18:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-40100 mg / L and COD greater than 50 mg / L.

[0056] Example 6: COD was measured on the following water samples. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 107 ± 10 mg / L; Water sample 2 had a chloride ion concentration of 5512 mg / L and a COD concentration of 251 ± 19 mg / L. Water sample 3 had a chloride ion concentration of 16020 mg / L and a COD concentration of 144±8 mg / L. Water sample four had a chloride ion concentration of 21030 mg / L and a COD concentration of 260 ± 17 mg / L. Water sample 5 had a chloride ion concentration of 30,500 mg / L and a COD concentration of 183 ± 8 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 183 ± 8 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0057] The product batch number used for water sample 1 is B25090590; the product batch number used for water sample 2 is B25090712; the product batch number used for water sample 3 is B25080192; the product batch number used for water sample 4 is B2601025; the product batch number used for water sample 5 is B22100140; and the product batch number used for water sample 6 is B26020053. Step 1: Since the average COD concentration in the water samples to be tested is greater than 50 mg / L, prepare the following solutions: 0.250 mol / L potassium dichromate standard solution; 0.05 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mercuric sulfate to chloride ion mass ratio of 22.5:1; Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0058] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0059] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0060] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0061]

[0062] Table 6. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is greater than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 22.5:1. As shown in Table 6, when the mass ratio of mercuric sulfate to chloride ions is 22.5:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-40100 mg / L and COD greater than 50 mg / L.

[0063] Unlike Example 5, in Example 6 the mass ratio of mercuric sulfate to chloride ions is 22.5:1, so the amount of mercuric sulfate added is greater than in Example 5. Although both can meet the test requirements, Example 5 is better than Example 6 in order to reduce costs.

[0064] Example 7: COD was measured on the following water samples. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 107 ± 10 mg / L; Water sample 2 had a chloride ion concentration of 5512 mg / L and a COD concentration of 251 ± 19 mg / L. Water sample 3 had a chloride ion concentration of 16020 mg / L and a COD concentration of 144±8 mg / L. Water sample four had a chloride ion concentration of 21030 mg / L and a COD concentration of 260 ± 17 mg / L. Water sample 5 had a chloride ion concentration of 30,500 mg / L and a COD concentration of 183 ± 8 mg / L. The COD standard samples of the above five water samples were obtained by using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix and adding the corresponding sodium chloride to obtain high-chlorine water samples with known chloride ion concentrations.

[0065] The product batch number used for water sample 1 is B25090590; the product batch number used for water sample 2 is B25090712; the product batch number used for water sample 3 is B25080192; the product batch number used for water sample 4 is B2601025; and the product batch number used for water sample 5 is B22100140. Step 1: Since the average COD concentration in the water samples to be tested is greater than 50 mg / L, prepare the following solutions: 0.250 mol / L potassium dichromate standard solution; 0.05 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 30:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0066] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0067] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0068] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0069]

[0070] Table 7. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is greater than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 30:1. As shown in Table 7, when the mass ratio of mercuric sulfate to chloride ions is 30:1, the relative errors of the measured COD results are all lower than the national standard requirements of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Therefore, this method is suitable for measuring the COD of high-chlorine wastewater with chloride ion concentrations of 1000-40100 mg / L and COD greater than 50 mg / L.

[0071] Unlike Example 5, in Example 7 the mass ratio of mercuric sulfate to chloride ions is 30:1, so the amount of mercuric sulfate added is greater than in Example 5. Although both can meet the test requirements, Example 5 is better than Example 7 in order to reduce costs.

[0072] Example 8: COD was measured on the following water samples respectively. Water sample 1 had a chloride ion concentration of 1000 mg / L and a COD concentration of 107 ± 10 mg / L; Water sample 2 had a chloride ion concentration of 5512 mg / L and a COD concentration of 251 ± 19 mg / L. Water sample 3 had a chloride ion concentration of 16020 mg / L and a COD concentration of 144±8 mg / L. Water sample four had a chloride ion concentration of 21030 mg / L and a COD concentration of 260 ± 17 mg / L. Water sample 5 had a chloride ion concentration of 30,500 mg / L and a COD concentration of 183 ± 8 mg / L. Water sample six had a chloride ion concentration of 40,100 mg / L and a COD concentration of 183 ± 8 mg / L. The COD standard samples of the above six water samples were prepared using standard samples produced by Tanmo Quality Inspection Technology Co., Ltd. as the matrix, and the corresponding sodium chloride was added to obtain high-chlorine water samples with known chloride ion concentrations.

[0073] The product batch number used for water sample 1 is B25090590; the product batch number used for water sample 2 is B25090712; the product batch number used for water sample 3 is B25080192; the product batch number used for water sample 4 is B2601025; the product batch number used for water sample 5 is B22100140; and the product batch number used for water sample 6 is B26020053. Step 1: Since the average COD concentration in the water samples to be tested is greater than 50 mg / L, prepare the following solutions: 0.250 mol / L potassium dichromate standard solution; 0.05 mol / L ferrous ammonium sulfate standard solution; 30% mercuric sulfate solution; silver sulfate-sulfuric acid solution; and ferroin indicator. The solution preparation method for step one is in accordance with national standard HJ 828-2017; Step 2, Sampling: Take six 10ml samples for each water sample, for a total of 36 samples, and add them to COD-specific digestion tubes. Step 3, add masking agent: Add the corresponding volume of 30% mercuric sulfate solution in Table 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 9:1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well.

[0074] Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well.

[0075] Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group.

[0076] Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

[0077]

[0078] Table 8. Detection results of COD in water samples when chloride ion concentration is 1000-40100 mg / L, COD is greater than 50 mg / L, and the mass ratio of mercuric sulfate to chloride ions is 9:1. As shown in Table 8, when the mass ratio of mercuric sulfate to chloride ions is 9:1, the relative error of the measured COD result is higher than the national standard requirement of -4.1% to 5.8%. Therefore, this method is not suitable for measuring the COD of high-chlorine wastewater with chloride ion concentration of 1000-40100 mg / L and COD greater than 50 mg / L.

[0079] In summary, the method provided by this invention is suitable for detecting COD content in high-chlorine wastewater with chloride ion concentrations ranging from 1000 to 40100 mg / L and COD concentrations ranging from 15.9 to 260 mg / L. The most preferred embodiments are Examples 1 and 5. When the mass ratio of mercuric sulfate to chloride ions is 18:1, the relative errors of the measured COD results are all lower than the national standard requirement of -4.1% to 5.8%, and the relative standard deviations are also lower than the national standard requirement of ≤11%. Example 1 is suitable for COD content less than 50 mg / L, and Example 5 is suitable for COD content greater than 50 mg / L. It not only provides accurate measurement results but also requires a small amount of mercuric sulfate. When the mass ratio of mercuric sulfate to chloride ions is greater than 18:1 and less than 30:1, the measurement is also accurate, but the amount of mercuric sulfate used is larger, increasing the cost.

[0080] The beneficial effects of this invention are as follows: (1) The range of chloride ion masking is significantly broadened. It can stably mask chloride ion interference in an ultra-wide concentration range of 1000–40100 mg / L, breaking through the limitation of existing national standard methods that are only applicable to the detection of chloride ions ≤20000 mg / L, and is suitable for more complex high-chlorine water samples.

[0081] (2) The COD detection range is wider and the applicability is stronger. It can directly measure water samples with COD ranging from 15.9 to 260 mg / L, covering both low-concentration and medium-to-high-concentration ranges, meeting the needs of various high-chlorine water quality testing.

[0082] (3) The accuracy and precision are superior to the national standard requirements. Based on the national standard "Determination of Chemical Oxygen Demand in Water - Dichromate Method" (HJ 828-2017) and the COD detection standard requirements for high-chlorine wastewater, this invention, under the optimized masking ratio, achieves a relative error of ≤4% and an RSD of ≤2.40%, with stable and reliable data. The accuracy and precision are superior to the national standard quality control requirements, effectively solving the core problems of large deviations and inaccurate data in high-chlorine, low-COD water samples.

[0083] (4) Simplify the detection process and eliminate nitrogen stripping and double titration. By eliminating the need for traditional nitrogen stripping devices and separate titration steps with absorbent liquid, COD determination can be completed in a single titration, significantly reducing operational steps and minimizing sources of human error.

[0084] (5) No need to dilute water samples, completely eliminating dilution errors. For water samples with low COD and high chlorine, direct detection is possible without dilution, avoiding problems such as COD falling below the detection limit or large differences in high and low concentration ranges due to dilution, thus ensuring accurate and reliable results.

[0085] (6) Suitable for simultaneous testing of large batches of water samples, with higher testing efficiency. By using a dedicated large-diameter COD digestion tube and a batch digestion device, multiple water samples can be simultaneously shaken, digested, and measured, solving the problems of small processing capacity, long processing time, and high manpower input of traditional methods.

[0086] (7) The method has strong stability and outstanding anti-interference ability. Excellent precision can be maintained with different masking ratios. The ratio can be flexibly selected according to the actual concentration of chloride ions. The method has good reproducibility and is applicable to a variety of scenarios.

[0087] (8) The operating conditions are controllable, and the results are more reliable. The entire process does not require strict control of complex conditions such as nitrogen flow rate and absorption efficiency, making the detection process easier to control and significantly improving the accuracy and stability of the detection results.

[0088] (9) Wider range: Chlorine interference range increased from 1000 to 40100 mg / L; More accurate: Detection limit increased from 20 to 15.9 mg / L, with accuracy better than the national standard; Faster: Nitrogen stripping is eliminated, multiple batches can be tested at once, doubling the efficiency; More economical: No dilution or nitrogen equipment is required, reducing operating costs; More stable: Dedicated equipment and reagents ensure reliable and controllable results.

[0089] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above description is illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for accurately and rapidly determining COD in high-chlorine wastewater, characterized in that, Includes the following steps: Step 1, prepare solutions: Prepare 0.2500 mol / L and 0.0250 mol / L potassium dichromate standard solutions respectively; prepare 0.050 mol / L and 0.005 mol / L ferrous ammonium sulfate standard solutions respectively; prepare 30% mercuric sulfate solution, silver sulfate-sulfuric acid solution and ferroin indicator respectively; Step 2, Sampling: Take 10ml of the water sample to be tested and add it into a COD-specific digestion tube; Step 3, add masking agent: Add the corresponding volume of the 30% mercuric sulfate solution prepared in Step 1 into the COD-specific digestion tube using a graduated pipette, and calculate the dosage according to the mass ratio of mercuric sulfate to chloride ions of 18-30:

1. Step 4, Oscillating Complexation: Fix the COD-specific digestion tube from Step 3 in the oscillator and oscillate at a frequency of 60 times / minute for 30 minutes to fully complex and mask chloride ions; Step 5, Oxidation: Add oxidant, and use a 5.00 mL pipette to add 5.00 mL of the potassium dichromate standard solution prepared in Step 1 into the COD-specific digestion tube, and shake well; Step 6, Catalysis: Add 15.0 mL of the silver sulfate-sulfuric acid solution prepared in Step 1 into the COD-specific digestion tube and shake well; Step 7, reflux digestion: Install the condenser tube and place the COD-specific digestion tube in the COD constant temperature heating reflux device for heating and reflux to ensure uniform heating, complete digestion, and no volatilization loss; Step 8, titration determination: After cooling, add water to dilute to about 140 mL in the COD-specific digestion tube, and titrate to the endpoint with the ferrous ammonium sulfate standard solution of the corresponding concentration prepared in Step 1 using the ferrous ammonium sulfate indicator prepared in Step 1, and record the volume; at the same time, perform a blank test, and perform 6 parallel determinations for each group. Step 9, Calculate the result: According to the formula ρ= Calculate the mass concentration ρ (mg / L) of chemical oxygen demand in the water sample to be tested. In the formula: V0 — Volume (mL) of ferrous ammonium sulfate standard solution consumed in the blank test. V1 — Volume (mL) of ferrous ammonium sulfate standard solution consumed by the water sample to be tested; C — Concentration of ferrous ammonium sulfate standard solution (mol / L); V2 — Volume of water sample to be tested (mL); 8000 — Conversion factor.

2. The method for accurately and rapidly determining COD in high-chlorine wastewater as described in claim 1, characterized in that: In step five, an appropriate potassium dichromate standard solution is added according to the estimated COD concentration in the water sample to be tested. When the COD concentration in the water sample is greater than 50 mg / L, a 0.2500 mol / L potassium dichromate standard solution is selected; when the COD concentration in the water sample is less than 50 mg / L, a 0.0250 mol / L potassium dichromate standard solution is selected.

3. The method for accurately and rapidly determining COD in high-chlorine wastewater as described in claim 2, characterized in that: In step eight, when the COD concentration in the water sample is greater than 50 mg / L, titration is performed using a 0.050 mol / L ferrous ammonium sulfate standard solution; when the COD concentration in the water sample is less than 50 mg / L, titration is performed using a 0.005 mol / L ferrous ammonium sulfate standard solution.

4. The method for accurately and rapidly determining COD in high-chlorine wastewater as described in claim 1, characterized in that: The chloride ion concentration in the water sample to be tested was 1000–40100 mg / L.

5. The method for accurately and rapidly determining COD in high-chlorine wastewater as described in claim 1, characterized in that: The COD detection range of the water sample to be tested is 15.9–260 mg / L.

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