Method for determining the content of chloride ions in waste lye

Abstract

The present application belongs to the technical field of detection, and particularly relates to a method for determining the content of chloride ions in waste alkali liquor. The steps of the method are as follows: adding methyl orange indicator to the waste alkali liquor sample, and using acid liquid to adjust the pH of the solution to 3-4 to obtain a sample to be measured; adding hydrogen peroxide solution to the sample to be measured, stirring and mixing, and then sequentially performing standing and heating treatment to obtain a decolorized sample; after the decolorized sample is diluted to a constant volume, adding acid liquid and titrating the decolorized sample by using silver nitrate potentiometric titration, and calculating the content of chloride ions according to the titration result. The present application effectively solves the problems of electrode pollution, titration end point drift, color interference, poor detection repeatability and the like in the prior art, has the advantages of strong anti-interference ability, simple operation, high precision, high standardization degree, wide application range and the like, and can be widely applied to environmental protection monitoring, process control and discharge standard detection of industrial waste alkali liquor.

Description

Technical Field

[0001] This invention belongs to the field of detection technology, specifically relating to a method for determining the chloride ion content in waste alkaline solution. Background Technology

[0002] In the production processes of petrochemicals, chemical fibers, and alkaline washing, large quantities of highly alkaline waste alkaline solutions with high organic content and high color (appearing as black and turbid) are discharged. Among these, chloride ion content is a core indicator affecting equipment corrosion control, the feasibility of process reuse, and environmental emission compliance. The accuracy of its detection results is directly related to the decision-making on adjusting the treatment process and determining emission compliance.

[0003] However, current methods for determining chloride ions in complex-matrix waste alkaline solutions generally suffer from the following problems: First, directly applying common standard methods such as GB / T 15453-2018 without necessary pretreatment adjustments for the strong alkalinity of the waste alkaline solution can disrupt the chemical equilibrium of the titration system due to the high pH value of the sample, leading to difficulties in endpoint determination and distorted results. Second, the deep black color and high turbidity of the sample render visual titration completely ineffective; high concentrations of organic matter adsorb onto the electrode surface, causing electrode damage, resulting in response lag, potential drift, and distorted titration curves, making accurate endpoint determination impossible with instrumental titration. Third, due to the failure to eliminate the aforementioned interferences, the data obtained by traditional direct titration methods exhibit extremely poor parallelism, with significant dispersion in multiple measurements of the same sample, rendering them unusable for production guidance. Finally, persistent contamination forms on the electrode surface after measurement, causing a rapid decline in electrode performance or even rendering the electrode unusable, requiring frequent cleaning or replacement, significantly increasing operating costs, and failing to meet the demands of high-frequency, rapid on-site detection.

[0004] To circumvent the aforementioned problems, existing technologies employ advanced methods such as "oxygen bomb incineration ashing pretreatment combined with ion chromatography." While this method can yield accurate results, it relies on expensive equipment, complex operating procedures, lengthy pretreatment and measurement times, and high maintenance costs, making it difficult to scale up in conventional industrial laboratories and unable to meet the demands of high-frequency, rapid testing in production settings. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the chloride ion content in waste alkaline solution, thereby overcoming the shortcomings of the prior art. It has the advantages of strong anti-interference ability, simple operation, high precision, high standardization and wide applicability, and can be widely used in environmental monitoring, process control and emission compliance testing of industrial waste alkaline solution.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: This invention provides a method for determining the chloride ion content in waste alkaline solution, comprising the following steps: (1) Add methyl orange indicator to the waste alkaline sample and adjust the pH of the solution to 3-4 with acid to obtain the test solution; (2) Add hydrogen peroxide solution to the test solution, stir and mix well, and then let it stand and heat it in sequence to obtain the decolorized sample; (3) After the decolorized sample is brought to a certain volume, acid solution is added and the decolorized sample is titrated by silver nitrate potentiometric titration. The chloride ion content is calculated based on the titration results.

[0007] In some other embodiments, in step (1), the chloride ion content in the waste alkaline sample is 5-5000 mg / L and the color is >50.

[0008] In some other embodiments, in step (1), the acid solution is nitric acid, and the concentration of the acid solution is 0.030-0.10 mol / L, preferably 0.050 mol / L.

[0009] In some other embodiments, in step (2), the volume concentration of the hydrogen peroxide solution is 20-40%.

[0010] In some other implementations, the settling time in step (2) is 5-10 min.

[0011] In some other embodiments, in step (2), the temperature of the heat treatment is 100℃-110℃ and the time is 5-10 min.

[0012] In some other embodiments, in step (3), the acid solution is nitric acid, and the concentration of the acid solution is 0.030-0.10 mol / L, preferably 0.050 mol / L.

[0013] In some other embodiments, the formula for calculating the chloride ion content in step (3) is as follows: ρ=(V1 V0)×C×35.45×1000×100 / (V sample×50); Wherein, ρ - chloride ion mass concentration, mg / L; V1 - volume of silver nitrate consumed by the sample, mL; V0 - volume of silver nitrate consumed by the blank, mL; C - concentration of silver nitrate standard solution, mol / L; Vsample - volume of original waste alkali solution transferred, mL.

[0014] In some other embodiments, the acid, hydrogen peroxide, and silver nitrate solution are all analytical grade reagents.

[0015] In some other embodiments, when the chloride ion content is <100 mg / L, the absolute difference between parallel determinations is ≤0.5 mg / L; When the chloride ion content is 100-200 mg / L, the absolute difference between parallel determinations is ≤1.0 mg / L; When the chloride ion content is >200 mg / L, the absolute difference between parallel determinations should be ≤0.01 times the average value of the determinations.

[0016] The beneficial effects of this invention are: This invention employs a three-stage pretreatment process comprising neutralization, oxidative decolorization, and heating to remove oxidants. This process completely eliminates interference from strong alkalinity, high color intensity, high organic matter content, and residual oxidants in samples, resolving core industry pain points such as sample blackening, electrode adsorption contamination, and titration curve distortion in traditional methods. The assay method of this invention exhibits good stability and repeatability, with minimal relative deviation between results obtained by different operators, and good linearity, fully meeting the requirements of compliance testing for data accuracy, precision, and comparability. Samples pretreated with this invention have effectively removed organic matter and colored substances, significantly reducing electrode contamination. After measurement, the electrodes can be directly used for subsequent sample testing without special treatment, avoiding frequent cleaning, maintenance, or replacement of electrodes, and significantly saving testing costs.

[0017] This invention requires only conventional laboratory equipment such as potentiometric titrators, eliminating the need for expensive, large-scale instruments like oxygen bomb incinerators and ion chromatographs. The operation is easy to master and can be rapidly adopted in ordinary laboratories. Strictly adhering to the GB / T15453-2018 standard, it establishes standardized operating procedures and clear parameter controls, ensuring the comparability and traceability of results across different testing units and batches. It exhibits excellent concentration adaptability, allowing for adjustments to the sampling volume to handle high, medium, and low concentrations of waste alkali samples, meeting the needs of various scenarios and all-weather batch testing in industrial production. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0019] Figure 1 This is a schematic diagram of the measuring device in Embodiment 1 of the present invention; Among them, 1. silver electrode, 2. reference electrode, 3. titration cup, 4. silver nitrate standard titration solution, 5. potentiometric titrator, and 6. magnetic stir bar; Figure 2 This is a flowchart of the determination method in Embodiment 1 of the present invention. Detailed Implementation

[0020] It should be noted that the following detailed description is exemplary and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The instruments and reagents used are as follows: potentiometric titrator, silver electrode, reference electrode, hot plate, 10 mL graduated pipette, 100 mL volumetric flask; nitric acid (analytical grade), 30% hydrogen peroxide (analytical grade), silver nitrate standard solution (0.0100 mol / L), methyl orange indicator, and grade III water.

[0021] Existing chloride ion detection methods generally suffer from problems such as method inapplicability, severe interference, unreliable results, high cost, and low efficiency when detecting highly alkaline, high-organic, and high-chroma waste alkaline solutions. Furthermore, the lack of standardized, efficient, robust, and cost-effective pretreatment and measurement procedures leads to poor data comparability between different laboratories, making it difficult to support the timely and accurate monitoring of key indicators in industrial settings.

[0022] This invention provides a method for determining the chloride ion content in waste alkaline solution, such as... Figure 2 As shown, the specific steps include: (1) Sample pretreatment: Shake the waste alkaline sample thoroughly, accurately transfer the waste alkaline sample into a beaker using a graduated pipette, add methyl orange indicator, slowly neutralize with nitric acid solution, stir until the solution changes from yellow to orange-red, adjust the pH to 3~4, continue stirring to ensure complete and uniform neutralization.

[0023] (2) Oxidation and decolorization: Add hydrogen peroxide dropwise to the neutralized sample until the solution becomes turbid. Stir well and let stand so that the organic matter is completely oxidized and decomposed and the chromophores are destroyed. The sample changes from black to colorless or pale yellow clear liquid.

[0024] (3) Remove hydrogen peroxide: Place the sample on a hot plate and heat it to a gentle boil and maintain for 5 min to completely decompose the excess hydrogen peroxide; cool to room temperature, transfer to a volumetric flask, and dilute to the mark with grade III water according to GB / T 6682-2008, and shake well; if necessary, filter through qualitative filter paper and take the filtrate as the test solution.

[0025] (4) Potentiometric titration: Transfer the test solution to the titration cup, add nitric acid solution to acidify, and add a stir bar; adjust the instrument and calibrate the silver electrode and reference electrode according to GB / T15453-2018; titrate with the calibrated silver nitrate standard solution, and record the potential jump point as the endpoint; perform three parallel determinations and perform a blank test at the same time.

[0026] (5) Calculation and Judgment of Results: The mass concentration of chloride ions is calculated according to the following formula: ρ=(V1 V0)×C×35.45×1000×100 / (V sample×50); Wherein, ρ - chloride ion mass concentration, mg / L; V1 - volume of silver nitrate consumed by the sample, mL; V0 - volume of silver nitrate consumed by the blank, mL; C - concentration of silver nitrate standard solution, mol / L; Vsample - volume of original waste alkali solution transferred, mL.

[0027] The difference between parallel determinations is allowed to be no more than 10% of the average value, and the average value is taken as the final result.

[0028] The measurement method of the present invention will be further explained below with reference to specific embodiments and comparative examples: Example 1 This embodiment provides a method for determining chloride ions in waste alkaline solutions with a chloride ion content of 300-600 mg / L and a color value of 300 or higher. The method specifically includes the following steps: (1) Sample pretreatment: Take 10.00 mL of waste alkaline solution, add 2 drops of methyl orange, neutralize with 0.050 mol / L nitric acid aqueous solution until orange-red, pH=3.5, stir for 3 min.

[0029] (2) Oxidation and decolorization: Add 30% hydrogen peroxide dropwise (hydrogen peroxide must be added dropwise to avoid excessive boiling at one time) until the solution becomes turbid, stir, and let stand for 5 minutes. The sample changes from black to light yellow clear liquid.

[0030] (3) Remove hydrogen peroxide: Heat the hot plate to a gentle boil and maintain for 5 minutes (a gentle boil for 5 minutes can completely decompose hydrogen peroxide without causing chloride ion loss), cool, and transfer to a final volume of 100 mL.

[0031] (4) Potentiometric titration: Transfer 50.00 mL of the test solution and titrate according to GB / T 15453-2018 standard. Blank V0 = 0.02 mL, the volume of the three samples consumed was 8.52 mL, 8.48 mL and 8.50 mL respectively, with an average value of 8.50 mL.

[0032] The potentiometric titration apparatus used is as follows: Figure 1 As shown, it includes a silver nitrate standard titration solution (4) and a potentiometric titrator (5), wherein the silver nitrate standard titration solution (4) is connected to the inlet of the potentiometric titrator (5). The potentiometric titrator (5) is an existing device, including a silver electrode (1), a reference electrode (2) and a titration cup (3), and a magnetic stir bar (6) is provided in the titration cup (3).

[0033] The titration process of the potentiometric titration apparatus is as follows: Place the titration cup (3) containing the waste alkaline solution sample to be tested on the titration stage of the potentiometric titrator (5), and add an appropriate amount of pretreated sample to the titration cup. Properly install the silver electrode (1) and the reference electrode (2) and insert them below the liquid surface of the titration cup. Turn on the magnetic stirrer and stir the solution at a uniform speed with the stir bar (6). Start the potentiometric titrator (5), and the instrument automatically controls the addition process of the silver nitrate standard titration solution (4) through the injection port. During the titration, the instrument monitors the potential change between the silver electrode (1) and the reference electrode (2) in real time, and automatically records the titration curve of the potential value changing with the titration volume. When a sudden jump in potential is detected, the instrument automatically determines the titration endpoint according to the preset endpoint identification algorithm and accurately records the volume of silver nitrate standard titration solution consumed at the endpoint. Calculate the chloride ion content in the sample based on the titration degree and the volume consumed. After the titration is completed, clean the electrode and the titration cup in time for the next use.

[0034] It is important to note that electrodes should be activated before use and rinsed with chloride-free water to prevent silver chloride deposition and passivation. During titration, the instrument should be kept stable to avoid vibration interfering with potential acquisition. Silver nitrate standard solution should be stored in a light-proof, sealed container and calibrated periodically. Laboratory equipment should be clean and free of chlorine residue, and waste solutions should be treated as hazardous waste.

[0035] (5) Calculate: ρ = (8.50) 0.02)×0.0100×35.45×1000×100 / (10×50)=601.232mg / L.

[0036] The results are valid if the difference between parallel measurements is ≤10% of the average.

[0037] Example 2 This embodiment provides a method for determining chloride ions in high-chromatic waste alkaline solutions with a chloride ion content of 500-1500 mg / L and a color value of 300 or higher. The method specifically includes the following steps: Unlike Example 1, in step (1), the volume of waste alkali solution transferred is changed to 5.00 mL, while the remaining steps are the same as in Example 1.

[0038] The measured chloride ion concentration was 1186.32 mg / L, with good parallelism and a clear endpoint jump.

[0039] Example 3 This embodiment provides a method for determining chloride ions in ultra-high chromatic waste alkaline solutions with a chloride ion content of 1000-3000 mg / L and a chromaticity value of 300 or higher. The method specifically includes the following steps: Unlike Example 1, in step (2), the oxidation and decolorization standing time is extended to 8 minutes to ensure complete decolorization. The remaining steps are the same as in Example 1, and the decolorization is thorough with no adsorption on the electrode.

[0040] The measured chloride ion concentration was 1602.12 mg / L, and the detection results were stable and reliable.

[0041] Comparative Example 1 Unlike Example 1, steps (1) to (3) are omitted, and the chloride ion content in the waste alkali solution is determined directly by titration. The determination steps are the same as in Example 1.

[0042] The study found that if the sample was not neutralized, decolorized, and treated to remove hydrogen peroxide before direct potentiometric titration, the solution would turn black, the electrode adsorption would be severe, the titration curve would be distorted, the endpoint would be difficult to identify, the RSD of parallel determinations would be >9%, the spiked recovery rate would be <85%, and the results would be unusable.

[0043] Comparative Example 2 Unlike Example 1, the nitric acid in steps (1) and (4) is replaced with sulfuric acid in equal amounts, while the remaining steps are the same as in Example 1.

[0044] The study found that when sulfuric acid was used instead of nitric acid for acidity adjustment and titration environment construction, the overall pH and ionic strength changes of the solution had a smaller impact on potentiometric titration. The titration curve still formed a clear and sharp potential jump peak, and the instrument could accurately identify the titration endpoint. The relative standard deviation (RSD) of the parallel determination results was <2%, and the deviation from the determination results using nitric acid in Example 1 was within the allowable error range.

[0045] Further research revealed that when the sulfate concentration in the sample is high, it readily reacts with silver ions to form slightly soluble silver sulfate, which may lead to slight adsorption contamination on the electrode surface. Furthermore, high sulfate concentrations increase the background of the solution matrix, potentially impacting electrode lifespan with long-term use. Therefore, while this method meets the detection requirements, considering the stability and long-term applicability of the method, nitric acid is still the preferred acidity adjuster.

[0046] Comparative Example 3 Unlike Example 1, steps (2) and (3) are omitted, while the remaining steps are the same as in Example 1.

[0047] The study found that organic impurities in the sample digestion solution could not be fully oxidized and decomposed, and the remaining large organic molecules in the solution would form colloidal or complexed systems. The impact of this phenomenon on the potentiometric titration process and results is as follows: Electrode contamination and response hysteresis: Organic colloids are easily adsorbed on the surface of the indicator electrode, forming a contamination film, which slows down the electrode response speed, causes potential drift, increases the fluctuation of the potential curve during titration, and makes it impossible to form a clear and sharp abrupt peak, making it difficult for the instrument to accurately identify the endpoint.

[0048] Complexation interference of the analyte: Some organic impurities can form stable complexes with the analyte ions, reducing the concentration of free ions, resulting in incomplete titration reaction, reduced potential jump amplitude, premature endpoint determination, and ultimately leading to a systematically lower measurement result.

[0049] Accuracy verification: In the digestion system without hydrogen peroxide, the relative standard deviation (RSD) of parallel sample determinations was >1.0%, which exceeded the allowable range of accuracy set by this method and could not meet the requirements of potentiometric titration analysis.

[0050] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining the chloride ion content in waste alkaline solution, characterized in that, Includes the following steps: (1) Add methyl orange indicator to the waste alkaline sample and adjust the pH of the solution to 3-4 with acid to obtain the test solution; (2) Add hydrogen peroxide solution to the test solution, stir and mix well, and then let it stand and heat it in sequence to obtain the decolorized sample; (3) After the decolorized sample is brought to a certain volume, acid solution is added and the decolorized sample is titrated by silver nitrate potentiometric titration. The chloride ion content is calculated based on the titration results.

2. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (1), the chloride ion content in the waste alkaline solution sample is 5-5000 mg / L, and the color is >50.

3. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (1), the acid solution is nitric acid, and the concentration of the acid solution is 0.030-0.10 mol / L, preferably 0.050 mol / L.

4. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (2), the volume concentration of the hydrogen peroxide solution is 20-40%.

5. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (2), the settling time is 5-10 min.

6. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (2), the temperature of the heat treatment is 100℃-110℃ and the time is 5-10 min.

7. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (3), the acid solution is nitric acid, and the concentration of the acid solution is 0.030-0.10 mol / L, preferably 0.050 mol / L.

8. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, In step (3), the formula for calculating the chloride ion content is as follows: ρ=(V1 V0)×C×35.45×1000×100 / (V sample×50); Wherein, ρ - chloride ion mass concentration, mg / L; V1 - volume of silver nitrate consumed by the sample, mL; V0 - volume of silver nitrate consumed by the blank, mL; C - concentration of silver nitrate standard solution, mol / L; Vsample - volume of original waste alkali solution transferred, mL.

9. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, The acid solution, hydrogen peroxide, and silver nitrate solution were all analytical grade reagents.

10. The method for determining the chloride ion content in waste alkaline solution according to claim 1, characterized in that, When the chloride ion content is <100 mg / L, the absolute difference between parallel determinations should be ≤0.5 mg / L; When the chloride ion content is 100-200 mg / L, the absolute difference between parallel determinations is ≤1.0 mg / L; When the chloride ion content is >200 mg / L, the absolute difference between parallel determinations should be ≤0.01 times the average value of the determinations.

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