Method for determining chloride ions in ammonia chloride process electrolytic zinc solution
By producing silver chloride precipitation in the electrolytic zinc solution of chlorammonia method and titrating excess silver nitrate, combined with indicators and electrochemical monitoring, the problem of large error in chloride ion determination is solved, and a fast and accurate determination of chloride ion concentration is achieved.
Patent Information
- Application Number
- CN202510715544.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-30
- Publication Date
- 2025-08-26
AI Technical Summary
The existing chlorammonia electrolytic zinc solution lacks suitable chloride ion determination methods, and the traditional spectrophotometry is not suitable for high chlorine raw materials, resulting in large measurement errors and high cost.
The silver nitrate standard solution was used to react with chloride ions to form silver chloride precipitate, and excess silver nitrate was titrated with ammonium thiocyanate standard solution. The titration end point was monitored by combining indicators and electrochemical indicators, and the chloride ion concentration was determined through a nonlinear titration model.
It realizes rapid and accurate determination of chloride ion concentration, reduces errors during spectrophotometry dilution process, and improves detection speed and accuracy.
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Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of determination methods, in particular to a method for determining chloride ions in a zinc electrolysis solution using a chloramine process. Background Art
[0002] Traditional electrolytic zinc production typically utilizes an acid process. This process utilizes lead as the anode, which is highly corrosive to chloride ions. During the corrosion process, a large amount of lead ions enter the solution and precipitate at the cathode, causing excessive lead content in the zinc flakes. Therefore, the raw materials must contain very low levels of chloride. Recently, the increasingly popular chlorammonia-chlorine process eliminates the need for chlorine removal, allowing the use of high-chloride raw materials directly, significantly reducing production costs. However, due to the low chloride content in the raw materials used in the acid process, spectrophotometry is typically used for chloride analysis. However, the high chloride content of raw materials used in the chlorammonia-chlorine process makes spectrophotometric determination unsuitable. Therefore, a method for determining chloride ions in electrolytic zinc solutions using the chlorammonia-chlorine process is needed. Summary of the Invention
[0003] In order to solve the above technical problems, the present invention provides a method for determining chloride ions in a zinc electrolytic solution by a chloramine process, and the specific technical solution is as follows:
[0004] A method for determining chloride ions in a zinc electrolytic solution using a chloramine process comprises the following steps:
[0005] Step a: providing W mL of a chlorine-containing solution and preparing it into a slightly acidic sample solution;
[0006] Step b, adding a certain amount of silver nitrate standard solution to the sample solution to allow chloride ions to react with the silver nitrate to form silver chloride precipitate;
[0007] Step c, heating the reaction mixture to a slight boiling state for 1 to 2 minutes, and then cooling to room temperature;
[0008] Step d, adding an indicator to the cooled mixture to monitor the reaction changes during the titration process;
[0009] Step e, titrating the excess silver nitrate with an ammonium thiocyanate standard solution, determining the titration endpoint based on the change in the indicator, and recording the consumed volume of the ammonium thiocyanate standard solution;
[0010] Step f, performing a blank experiment: adding the same amount of silver nitrate standard solution as in step b to the same volume of slightly acidic solution without chloride ions, heating and cooling, adding an indicator, and titrating with ammonium thiocyanate standard solution to the endpoint, recording the consumed volume of the ammonium thiocyanate standard solution;
[0011] Step g, calculate the concentration of chloride ions in the chlorine-containing solution according to the titration result, and the calculation formula is:
[0012]
[0013] Among them, C Cl - is the chloride ion concentration (g / L), V0 is the consumed volume of ammonium thiocyanate standard solution in the blank experiment (mL), V is the consumed volume of ammonium thiocyanate standard solution in the sample solution titration (mL), c is the concentration of ammonium thiocyanate standard solution (mol / L), and W is the volume of chlorine-containing solution (mL).
[0014] Preferably:
[0015] The pH of the slightly acidic sample solution is 4 to 6;
[0016] The concentration of the silver nitrate standard solution is 0.1 mol / L;
[0017] The concentration of the ammonium thiocyanate standard solution is 0.1 mol / L;
[0018] The amount of silver nitrate standard solution added in step b is 8 mL.
[0019] Preferably, the preparation of the sample solution in step a comprises: pipetting 0.5-5 mL of the chlorine-containing solution into a 200 mL Erlenmeyer flask, diluting the solution to 80 mL with water, and adding 5 mL of 8 mol / L nitric acid solution.
[0020] Preferably, the concentration of the ammonium thiocyanate standard solution is determined by calibration, and the calibration step comprises:
[0021] (1) Pipette 5 mL of 1 mg / mL chlorine standard solution into a 200 mL Erlenmeyer flask and dilute to 80 mL with water;
[0022] (2) Add 5 mL of 8 mol / L nitric acid solution and 8 mL of 0.1 mol / L silver nitrate standard solution, heat to a gentle boil for 1 min, and then cool to room temperature;
[0023] (3) Add an indicator and titrate with ammonium thiocyanate standard solution, and determine the endpoint based on the change in the indicator;
[0024] (4) Calculate the concentration of ammonium thiocyanate standard solution according to the formula:
[0025]
[0026] in, is the concentration of ammonium thiocyanate standard solution (mol / L), M is the mass of chloride ions in the chlorine standard solution (g), V0 is the volume of ammonium thiocyanate standard solution consumed in the blank experiment (mL), and V1 is the volume of ammonium thiocyanate standard solution consumed in the calibration experiment (mL).
[0027] Preferably:
[0028] The indicator in step d is a ferric ammonium sulfate solution, which is prepared by dissolving 40 g of ferric ammonium sulfate in 100 mL of water and adding 2 mL of nitric acid;
[0029] The titration endpoint in step e is that the mixture appears red and does not fade for 30 seconds.
[0030] Preferably:
[0031] The indicator in step d is an electrochemical indicator, and the electrochemical indicator is selected from one or a combination of a silver electrode, a glass electrode or an ion-selective electrode;
[0032] In step e, the reaction progress of the excess silver nitrate during the titration is monitored by potentiometric titration or amperometric titration to determine the titration endpoint.
[0033] Preferably, the electrochemical indicator is a silver electrode, and the titration process is monitored by potentiometric titration, and the specific steps include:
[0034] (1) Prepare the electrochemical system: In step d, insert the silver electrode as the working electrode and the saturated calomel electrode as the reference electrode into the cooled sample solution and connect them to a potentiometer or electrochemical workstation;
[0035] (2) Initial potential measurement: Before the titration begins, record the initial potential of the sample solution, which reflects the excess Ag + concentration;
[0036] (3) Titration operation: In step e, add the ammonium thiocyanate standard solution dropwise, stir evenly after each addition, and record the change of the silver electrode potential with the added volume;
[0037] (4) End point determination: Continue titrating until a significant jump in potential occurs, which corresponds to excess Ag. + When the reaction with NH4SCN is complete, take the midpoint of the potential jump as the titration endpoint and record the consumed volume V of the ammonium thiocyanate standard solution.
[0038] Preferably, in step e, the titration data is analyzed by a nonlinear titration model to determine the titration endpoint, wherein the nonlinear titration model is based on a curve fitting of the nonlinear changes of volume and indicator signal during the titration process, and the curve is determined by the least squares method, Gaussian function or logistic function to determine the equivalence point, and the equivalence point corresponds to the endpoint at which the reaction of excess silver nitrate and ammonium thiocyanate is complete.
[0039] Preferably, the nonlinear titration model uses a logistic function to fit the titration curve, and the specific steps include:
[0040] (1) Data acquisition: During the titration process in step e, the volume of the ammonium thiocyanate standard solution added each time and the corresponding data of the indicator signal are recorded to form a titration data set;
[0041] (2) Curve fitting: Input the titration data set into the Logistic function Fitting curve, where y is the signal value, x is the titration volume, A is the maximum value of the curve, k is the slope parameter, and x0 is the inflection point volume;
[0042] (3) Endpoint calculation: Calculate the derivative of the fitting curve and find the volume x0 corresponding to the maximum value of the derivative, which is the equivalence point. The equivalence point corresponds to the volume at which the excess silver nitrate and ammonium thiocyanate react completely.
[0043] (4) Result output: x0 is used as the volume V of the ammonium thiocyanate standard solution consumed at the titration endpoint for subsequent calculation of chloride ion concentration.
[0044] Preferably, for the determination of solid samples, the sample solution is prepared by weighing 10 g of solid sample into a 200 mL volumetric flask, diluting to 200 mL with water, shaking for 30 minutes, and then dry filtering, and analyzing the filtrate according to steps b to g.
[0045] The present invention adds a quantitative silver nitrate standard solution to a slightly acidic sample solution to convert chloride ions into silver chloride precipitates, and adds an indicator to titrate excess silver nitrate with an ammonium thiocyanate standard solution. This method can accurately and quickly measure the chloride ion concentration in an electrolytic zinc solution, thereby improving the detection speed and reducing the error caused by the dilution process of the spectrophotometric method. DETAILED DESCRIPTION
[0046] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is described in detail below. The description in this section is only exemplary and explanatory and should not have any limiting effect on the scope of protection of the present invention.
[0047] This embodiment provides a method for determining chloride ions in a zinc solution electrolyzed by ammonia chloride method, comprising the following steps:
[0048] Step a: Provide W mL of chlorine-containing solution and prepare a slightly acidic sample solution.
[0049] Step b: adding a certain amount of silver nitrate standard solution to the sample solution to allow chloride ions to react with the silver nitrate to form silver chloride precipitate.
[0050] Step c: heating the reaction mixture to a slight boiling state for 1 to 2 minutes, and then cooling to room temperature.
[0051] Step d: adding an indicator to the cooled mixture to monitor the reaction changes during the titration process.
[0052] Step e: titrate the excess silver nitrate with an ammonium thiocyanate standard solution, determine the titration endpoint according to the change of the indicator, and record the consumed volume of the ammonium thiocyanate standard solution.
[0053] Step f, performing a blank experiment: adding the same amount of silver nitrate standard solution as in step b to the same volume of slightly acidic solution without chloride ions, heating and cooling, adding an indicator, and titrating with ammonium thiocyanate standard solution to the endpoint, recording the consumed volume of the ammonium thiocyanate standard solution.
[0054] Step g, calculate the concentration of chloride ions in the chlorine-containing solution according to the titration result, and the calculation formula is:
[0055]
[0056] Among them, C Cl - is the chloride ion concentration (g / L), V0 is the consumed volume of ammonium thiocyanate standard solution in the blank experiment (mL), V is the consumed volume of ammonium thiocyanate standard solution in the sample solution titration (mL), c is the concentration of ammonium thiocyanate standard solution (mol / L), and W is the volume of chlorine-containing solution (mL).
[0057] Specifically, chloride ions react with silver nitrate to form silver chloride precipitate. The excess silver nitrate is titrated with a standard ammonium thiocyanate solution, and the consumed volume is V. V0-V represents the volume of ammonium thiocyanate corresponding to the silver nitrate that reacts with the chloride ions in the sample. The mass of the chloride ions can be converted from the known ammonium thiocyanate concentration c and the molar mass of the chloride ions. Finally, the mass is divided by the volume of the chloride-containing solution W and multiplied by the unit conversion factor 1000 to obtain the chloride ion concentration (g / L).
[0058] This embodiment adds a fixed amount of silver nitrate standard solution to a slightly acidic sample solution to convert chloride ions into silver chloride precipitates, and then adds an indicator and titrates the excess silver nitrate with an ammonium thiocyanate standard solution. This allows for accurate and rapid determination of chloride ion concentration in an electrolytic zinc solution, thereby increasing detection speed and reducing errors caused by the dilution process of the spectrophotometric method.
[0059] Further:
[0060] The pH of the slightly acidic sample solution is 4 to 6.
[0061] The concentration of silver nitrate standard solution is 0.1 mol / L.
[0062] The concentration of ammonium thiocyanate standard solution is 0.1 mol / L.
[0063] The amount of silver nitrate standard solution added in step b is 8 mL.
[0064] Furthermore, the preparation of the sample solution in step a includes: pipetting 0.5 to 5 mL of the chlorine-containing solution into a 200 mL Erlenmeyer flask, diluting the solution to 80 mL with water, and adding 5 mL of 8 mol / L nitric acid solution.
[0065] Furthermore, the concentration of the ammonium thiocyanate standard solution is determined by calibration, and the calibration step comprises:
[0066] (1) Pipette 5 mL of 1 mg / mL chlorine standard solution into a 200 mL Erlenmeyer flask and dilute to 80 mL with water.
[0067] (2) Add 5 mL of 8 mol / L nitric acid solution and 8 mL of 0.1 mol / L silver nitrate standard solution, heat to a gentle boil for 1 min, and then cool to room temperature.
[0068] (3) Add an indicator and titrate with ammonium thiocyanate standard solution. Determine the end point based on the change in the indicator.
[0069] (4) Calculate the concentration of ammonium thiocyanate standard solution according to the formula:
[0070]
[0071] in, is the concentration of ammonium thiocyanate standard solution (mol / L), M is the mass of chloride ions in the chlorine standard solution (g), V0 is the volume of ammonium thiocyanate standard solution consumed in the blank experiment (mL), and V1 is the volume of ammonium thiocyanate standard solution consumed in the calibration experiment (mL).
[0072] The purpose of calibration is to determine the actual concentration of ammonium thiocyanate solution through experiments so that the chloride ion content can be accurately calculated in subsequent analysis. In the calibration experiment, a known amount of silver nitrate standard solution is first added. It will react with the chloride ions in the chlorine standard solution to form silver chloride precipitate. Since the silver nitrate is added in excess, the unreacted silver nitrate will react with the ammonium thiocyanate. V0-V1 represents the volume of ammonium thiocyanate corresponding to the silver nitrate that reacts with the chloride ions. The molar amount of chloride ions can be converted from the known mass of chloride ions in the chlorine standard solution and the molar mass of chloride ions, thereby indirectly calculating the concentration of ammonium thiocyanate.
[0073] Further:
[0074] In step d, the indicator is ferric ammonium sulfate solution, which is prepared by dissolving 40 g of ferric ammonium sulfate in 100 mL of water and adding 2 mL of nitric acid.
[0075] The titration endpoint in step e is when the mixture turns red and remains red for 30 seconds without fading.
[0076] Further:
[0077] The indicator in step d is an electrochemical indicator, which is selected from one or a combination of a silver electrode, a glass electrode or an ion-selective electrode.
[0078] In step e, the reaction progress of the excess silver nitrate during the titration is monitored by potentiometric titration or amperometric titration to determine the titration endpoint.
[0079] Furthermore, the electrochemical indicator is a silver electrode, and the titration process is monitored by potentiometric titration, and the specific steps include:
[0080] (1) Prepare the electrochemical system: In step d, insert the silver electrode as the working electrode and the saturated calomel electrode as the reference electrode into the cooled sample solution and connect them to a potentiometer or electrochemical workstation.
[0081] (2) Initial potential measurement: Before the titration begins, record the initial potential of the sample solution. The potential reflects the excess Ag + concentration.
[0082] (3) Titration operation: In step e, add the ammonium thiocyanate standard solution dropwise, stir evenly after each addition, and record the change of the silver electrode potential with the added volume.
[0083] (4) End point determination: Continue titrating until a significant jump in potential occurs, which corresponds to excess Ag. + When the reaction with NH4SCN is complete, take the midpoint of the potential jump as the titration endpoint and record the consumed volume V of the ammonium thiocyanate standard solution.
[0084] Among them, the concentration change was monitored by potentiometric titration using a silver electrode, and the potential jump point directly reflected the excess NH 4 When the SCN reaction is complete, compared with the visual judgment of traditional color indicators, human error is reduced and the endpoint judgment is more accurate; combined with the electrochemical workstation and reference electrode, the titration process can realize real-time recording and automatic analysis of potential data, reducing manual operation time and improving experimental efficiency and repeatability.
[0085] Furthermore, in step e, the titration data is analyzed by a nonlinear titration model to determine the titration endpoint. The nonlinear titration model is based on a curve fitting of the nonlinear changes in volume and indicator signal during the titration process. The curve is determined by the least squares method, Gaussian function or logistic function to determine the equivalence point. The equivalence point corresponds to the endpoint at which the reaction of excess silver nitrate and ammonium thiocyanate is complete.
[0086] Furthermore, the nonlinear titration model uses a logistic function to fit the titration curve, and the specific steps include:
[0087] (1) Data acquisition: During the titration process in step e, the volume of the ammonium thiocyanate standard solution added each time and the corresponding data of the indicator signal are recorded to form a titration data set.
[0088] (2) Curve fitting: Input the titration data set into the Logistic function Fit the curve, where y is the signal value, x is the titration volume, A is the maximum value of the curve, k is the slope parameter, and x0 is the inflection point volume.
[0089] (3) End point calculation: Calculate the derivative of the fitting curve and find the volume x0 corresponding to the maximum value of the derivative, which is the equivalence point. The equivalence point corresponds to the volume where the excess silver nitrate and ammonium thiocyanate react completely.
[0090] (4) Result output: x0 is used as the volume V of the ammonium thiocyanate standard solution consumed at the titration endpoint for subsequent calculation of chloride ion concentration.
[0091] Among them, fitting the titration curve with the Logistic function and calculating the maximum value of the derivative can accurately determine the equivalence point. Even when the titration curve is flat or nonlinear, the volume at which the excess silver nitrate and ammonium thiocyanate react completely can be accurately identified, which can improve the accuracy of the calculation of the chloride ion content.
[0092] Furthermore, for the determination of solid samples, the sample solution was prepared by weighing 10 g of the solid sample into a 200 mL volumetric flask, diluting to 200 mL with water, shaking for 30 min, and then dry filtering. The filtrate was analyzed according to steps b to g.
[0093] Specific examples are provided below. The provided examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.
[0094] Example 1
[0095] 1. Experimental Procedure
[0096] Take the chloramine method electrolytic zinc solution and accurately transfer 2.0 mL into a 200 mL conical flask.
[0097] Dilute to 80 mL with distilled water, add 5 mL of 8 mol / L nitric acid solution (concentrated nitric acid: water = 1:1), shake well, and prepare a slightly acidic sample solution.
[0098] Use a micro-pipette to add 8 mL of 0.1 mol / L silver nitrate standard solution and mix well.
[0099] Place the flask on a hot plate and heat to a slight boil (98°C, with a small amount of bubbles appearing on the surface of the solution), maintain for 1 minute, and cool to room temperature.
[0100] Add 2 mL of ammonium ferric sulfate indicator to the cooled sample solution and titrate with 0.1 mol / L ammonium thiocyanate standard solution. Shake vigorously and slowly add dropwise near the endpoint. The solution should turn red and remain so for 30 seconds. Record the consumed volumes (V = 6.45 mL, 6.50 mL, and 6.40 mL) for three groups. The average value (V) is 6.45 mL.
[0101] Blank experiment: Take 80 mL of distilled water in another 200 mL Erlenmeyer flask, add 5 mL of 8 mol / L nitric acid solution and 8 mL of 0.1 mol / L silver nitrate standard solution, and heat and cool under the same conditions.
[0102] Add 2 mL of ferric ammonium sulfate indicator (40 g of ferric ammonium sulfate dissolved in 100 mL of water + 2 mL of nitric acid), titrate with 0.1 mol / L ammonium thiocyanate standard solution until it turns orange-red, and record the consumed volume V0 = 8.10 mL.
[0103] Calculate the chloride ion content:
[0104] Given: V0 = 8.10 mL, V = 6.45 mL, c = 0.1 mol / L, W = 2.0 mL;
[0105] formula:
[0106] calculate:
[0107] 2. Experimental Data
[0108] Measurement results:
[0109] C Cl -=2.92 g / L (repeated three times: 2.92, 2.90, 2.94 g / L, average value 2.92 g / L).
[0110] Ion chromatography results: C Cl -=3.00g / L
[0111] 3. Data comparison
[0112] method Measured value (g / L) deviation(%) Ammonium ferric sulfate indicator 2.92 -2.67％ Ion Chromatograph 3.00 -
[0113] 4. Experimental Conclusion
[0114] The measured chloride ion content was 2.92 g / L, with a deviation of -2.67% from the result of ion chromatography, indicating high accuracy.
[0115] Example 2
[0116] 1. Experimental Procedure
[0117] Take the same chloramine electrolytic zinc solution and accurately transfer 2.0 mL into a 200 mL conical flask.
[0118] Dilute to 80 mL with distilled water, add 5 mL of 8 mol / L nitric acid solution, and shake well.
[0119] Use a micro-pipette to add 8 mL of 0.1 mol / L silver nitrate standard solution and mix well.
[0120] Heat to a gentle boil (98°C), maintain for 1 minute, and cool to room temperature.
[0121] A silver electrode and a saturated calomel electrode were inserted into the cooled sample solution, and the initial potential (450 mV) was recorded.
[0122] Use 0.1 mol / L ammonium thiocyanate standard solution to titrate dropwise, stir evenly, and record the potential change:
[0123] At 6.0 mL, the potential is 400 mV;
[0124] At 6.4 mL, the potential is 350 mV;
[0125] At 6.5 mL, the potential was 200 mV (jump start);
[0126] At 6.6 mL, the potential was 50 mV (jump end).
[0127] Data input software, fitting logistic function: (y is the potential, x is the volume).
[0128] The maximum value of the derivative was calculated and the equivalence point V = 6.48 mL was determined (repeated three times: 6.48, 6.47, 6.48 mL, with an average value of 6.48 mL).
[0129] Blank experiment: Take 80 mL of distilled water, add 5 mL of 8 mol / L nitric acid solution and 8 mL of 0.1 mol / L silver nitrate standard solution, heat and cool.
[0130] Insert a silver electrode and a saturated calomel electrode, connect the electrochemical workstation, titrate until the potential jumps, and record V0 = 8.10 mL.
[0131] Calculate the chloride ion content:
[0132] Known: V0 = 8.10 mL, V = 6.48 mL, c = 0.1 mol / L, W = 2.0 mL.
[0133] calculate:
[0134] 2. Experimental Data
[0135] Test results: C Cl -=2.97 g / L (repeated three times: 2.97, 2.96, 2.97 g / L, average value 2.97 g / L).
[0136] Ion chromatography results: C Cl -=3.00g / L.
[0137] 3. Data comparison
[0138] method Measured value (g / L) deviation(%) Silver electrode + Logistic model 2.97 -1.00％ Ion Chromatograph 3.00 -
[0139] 4. Experimental Conclusion
[0140] The chloride ion content measured in Example 2 was 2.97 g / L, which had a deviation of -1.00% from the result of the ion chromatograph, and was more accurate.
[0141] Silver electrode potentiometric titration and logistic model provide objective endpoint judgment, high repeatability, and more stable performance under complex conditions.
[0142] The principles and implementation methods of the present invention are described herein using specific examples. The description of the above embodiments is only intended to help understand the method and core ideas of the present invention. The above are only preferred implementation methods of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art, within the technical scope disclosed by the present invention, can make equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, and all of these should fall within the scope of protection of the present invention.
Claims
1. A method for determining chloride ions in a chloramine electrolytic zinc solution, characterized in that: The steps include: a) Provide W mL of chlorine-containing solution and prepare a slightly acidic sample solution; b) adding a quantitative silver nitrate standard solution to the sample solution to react chloride ions with the silver nitrate to form a silver chloride precipitate; c) heating the reaction mixture to a slight boil for 1 to 2 minutes, and then cooling it to room temperature; d) adding an indicator to the cooled mixture to monitor the reaction changes during the titration; e) titrating the excess silver nitrate with an ammonium thiocyanate standard solution, determining the titration endpoint based on the change in the indicator, and recording the consumed volume of the ammonium thiocyanate standard solution; f) performing a blank test: adding the same amount of silver nitrate standard solution as in step b) to the same volume of slightly acidic solution without chloride ions, heating and cooling, adding an indicator, and titrating with ammonium thiocyanate standard solution to the endpoint, recording the volume of ammonium thiocyanate standard solution consumed; g) Calculate the concentration of chloride ions in the chlorine-containing solution based on the titration results. The calculation formula is: Among them, C Cl - is the chloride ion concentration (g / L), V0 is the consumed volume of ammonium thiocyanate standard solution in the blank experiment (mL), V is the consumed volume of ammonium thiocyanate standard solution in the sample solution titration (mL), c is the concentration of ammonium thiocyanate standard solution (mol / L), and W is the volume of chlorine-containing solution (mL).
2. The assay method according to claim 1, wherein: The pH of the slightly acidic sample solution is 4 to 6; The concentration of the silver nitrate standard solution is 0.1 mol / L; The concentration of the ammonium thiocyanate standard solution is 0.1 mol / L; The amount of the silver nitrate standard solution added in step b) is 8 mL.
3. The measuring method according to claim 2, wherein The preparation of the sample solution in step a) comprises: transferring 0.5-5 mL of the chlorine-containing solution into a 200 mL conical flask, diluting the solution to 80 mL with water, and adding 5 mL of 8 mol / L nitric acid solution.
4. The measuring method according to claim 3, wherein The concentration of the ammonium thiocyanate standard solution is determined by calibration, and the calibration step comprises: i) Pipette 5 mL of 1 mg / mL chlorine standard solution into a 200 mL Erlenmeyer flask and dilute to 80 mL with water; ii) Add 5 mL of 8 mol / L nitric acid solution and 8 mL of 0.1 mol / L silver nitrate standard solution, heat to a gentle boil for 1 min, and then cool to room temperature; iii) adding an indicator and titrating with a standard ammonium thiocyanate solution, and determining the endpoint based on the change in the indicator; iv) Calculate the concentration of the ammonium thiocyanate standard solution according to the formula: in, is the concentration of ammonium thiocyanate standard solution (mol / L), M is the mass of chloride ions in the chlorine standard solution (g), V0 is the volume of ammonium thiocyanate standard solution consumed in the blank experiment (mL), and V1 is the volume of ammonium thiocyanate standard solution consumed in the calibration experiment (mL).
5. The measuring method according to claim 1, wherein: In step d), the indicator is a ferric ammonium sulfate solution, which is prepared by dissolving 40 g of ferric ammonium sulfate in 100 mL of water and adding 2 mL of nitric acid; The titration endpoint in step e) is when the mixture turns red and does not fade for 30 seconds.
6. The measuring method according to claim 1, wherein: The indicator in step d) is an electrochemical indicator, which is selected from one or a combination of a silver electrode, a glass electrode or an ion-selective electrode; In step e), the reaction progress of the excess silver nitrate during the titration is monitored by potentiometric titration or amperometric titration to determine the titration endpoint.
7. The measuring method according to claim 6, wherein The electrochemical indicator is a silver electrode, and the titration process is monitored by potentiometric titration. The specific steps include: i) preparing an electrochemical system: in step d), inserting a silver electrode as a working electrode and a saturated calomel electrode as a reference electrode into the cooled sample solution and connecting them to a potentiometer or an electrochemical workstation; ii) Initial potential measurement: Before the titration begins, record the initial potential of the sample solution, which reflects the excess Ag + concentration; iii) Titration: In step e), add the ammonium thiocyanate standard solution dropwise, stir evenly after each addition, and record the change in the silver electrode potential with the added volume; iv) End point determination: The titration is continued until a significant jump in potential occurs, which corresponds to excess Ag. + When the reaction with NH4SCN is complete, take the midpoint of the potential jump as the titration endpoint and record the consumed volume V of the ammonium thiocyanate standard solution.
8. The measuring method according to claim 1, wherein In step e), the titration data is analyzed using a nonlinear titration model to determine the titration endpoint. The nonlinear titration model is based on a curve fitting of the nonlinear changes in volume and indicator signal during the titration process. The curve is used to determine the equivalence point using a least squares method, a Gaussian function, or a logistic function. The equivalence point corresponds to the endpoint at which the reaction of excess silver nitrate and ammonium thiocyanate is complete.
9. The measuring method according to claim 8, characterized in that The nonlinear titration model uses a logistic function to fit the titration curve, and the specific steps include: i) Data acquisition: During the titration process in step e), the volume of the ammonium thiocyanate standard solution added each time and the corresponding data of the indicator signal are recorded to form a titration data set; ii) Curve fitting: Input the titration data set into the Logistic function Fitting curve, where y is the signal value, x is the titration volume, A is the maximum value of the curve, k is the slope parameter, and x0 is the inflection point volume; iii) Endpoint calculation: Calculate the derivative of the fitting curve and find the volume x0 corresponding to the maximum value of the derivative, which is the equivalence point. The equivalence point corresponds to the volume at which the excess silver nitrate and ammonium thiocyanate react completely. iv) Output result: Take x0 as the volume V of the ammonium thiocyanate standard solution consumed at the titration endpoint for subsequent calculation of chloride ion concentration.
10. The measuring method according to claim 1, wherein For the determination of solid samples, the sample solution was prepared by weighing 10 g of solid sample into a 200 mL volumetric flask, diluting to 200 mL with water, shaking for 30 min, and then dry filtering. The filtrate was analyzed according to steps b) to g).