Method for analyzing nickel in zinc-nickel electroplating solution

Abstract

The invention provides a method for analyzing nickel in a zinc-nickel electroplating solution. The method comprises the following steps: carrying out standard coulometric titration on a sodium thiosulfate standard solution, and calculating an actual molar concentration c standard; and adding an oxidant solution and a sodium hydroxide solution into the zinc-nickel electroplating solution to obtain a high nickel hydroxide precipitate, then adding a KI solution and sulfuric acid to obtain a sample solution, adding the sample solution, a second part of the sodium thiosulfate standard solution and a second part of the KI electrolyte into an electrolytic cup, carrying out coulometric titration on the sample, and calculating the molar concentration c nickel of nickel in the zinc-nickel electroplating solution. According to the method, the cyanide-free coulometric titration method capable of quantitatively determining the nickel content in the zinc-nickel electroplating solution is obtained by combining the oxidation-reduction reaction with the indirect coulometric titration method according to the valence state change of nickel hydroxide, a standard solution does not need to be prepared, nickel is indirectly determined under the background of high-concentration zinc and other metal ions by using electrogenerated iodine, and the method is simple and convenient to operate. High-selectivity, high-precision and on-line nickel content measurement is realized, and the detection efficiency can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of solution analysis, and more particularly, to a method for analyzing nickel in a zinc-nickel electroplating solution. Background Art

[0002] The standard methods for nickel determination include atomic absorption spectrometry and dimethylglyoxime spectrophotometry (rapid determination method for nickel in industrial wastewater). These analytical methods require the preparation of a standard working curve, which is time-consuming and laborious and cannot achieve on-line testing. When using traditional analytical methods to determine the nickel content, sample pretreatment is generally carried out, and then the determination is carried out in a solution state. The determination principle is the four major chemical reaction equilibria in the solution, including the EDTA titration method using complexation reaction, the dimethylglyoxime spectrophotometry for the complex of dimethylglyoxime and high-valent nickel ions, the method disclosed in Chinese application CN201710259374.9 in which triethylenetetramine forms a stable purple-red complex with nickel ions, and the dimethylglyoxime gravimetric method using precipitation-dissolution equilibrium.

[0003] However, the zinc-nickel plating solution has a complex composition, with a large number of interfering ions such as high-concentration zinc and strong complexing agents such as polyene polyamines, making it difficult to analyze the plating solution. For the determination of nickel content in a zinc-nickel plating solution with a complex composition, in the complexometric titration method, due to the very similar coordination properties between zinc and nickel, the complexing agent cannot distinguish between the two; the dimethylglyoxime gravimetric method for nickel determination is time-consuming and laborious and cannot achieve on-line determination. Therefore, traditional analytical methods are difficult to accurately and rapidly determine the nickel content and cannot perform highly selective determination. Summary of the Invention

[0004] The main object of the present invention is to provide a method for analyzing nickel in a zinc-nickel electroplating solution to solve the problem in the prior art that the nickel content in the zinc-nickel plating solution cannot be rapidly and accurately determined with high selectivity.

[0005] To achieve the above object, according to one aspect of the present invention, there is provided a method for analyzing nickel in a zinc-nickel electroplating solution, including the following steps: Step S1, dividing the sodium thiosulfate standard solution into two parts, namely the first part of the sodium thiosulfate standard solution and the second part of the sodium thiosulfate standard solution; dividing the KI electrolyte with a pH of 2.5 to 3.5 into two parts, namely the first part of the KI electrolyte and the second part of the KI electrolyte; adding the first part of the sodium thiosulfate standard solution and the first part of the KI electrolyte into an electrolytic cell for standard coulometric titration, and calculating the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 ;

[0006] wherein, i 1 is the electrolysis current of the standard coulometric titration, with the unit of mA; t 1is the titration end point time of the standard coulometric titration, with the unit of s; z is the conversion coefficient, dimensionless, with a value of 1; F is the Faraday constant, 96485 C / mol; V 标准1 is the volume of the first part of the sodium thiosulfate standard solution, with the unit of mL;

[0007] Step S2: Add an oxidant solution and a sodium hydroxide solution to the zinc-nickel electroplating solution, and after washing with water, obtain nickel hydroxide precipitate; add a KI solution and sulfuric acid to the nickel hydroxide precipitate to obtain a sample solution; Step S3: Add the sample solution, the second part of the sodium thiosulfate standard solution, and the second part of the KI electrolyte to an electrolysis cell, perform sample coulometric titration, and calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formula (2) 镍 ;

[0008] wherein, V 标准2 is the volume of the second part of the sodium thiosulfate standard solution, with the unit of mL; V 样品 is the volume of the sample solution, with the unit of mL; t 测定 is the titration end point time of the sample coulometric titration, with the unit of s; t 2 is the theoretical titration end point time of the second part of the sodium thiosulfate standard solution, calculated according to formula (3), with the unit of s; wherein, i 2 is the electrolysis current of the sample coulometric titration, with the unit of mA.

[0009] Furthermore, separately and independently, in the standard coulometric titration and the sample coulometric titration, the electrolysis current is 5 - 15 mA, and the minimum electrolysis time is 0.5 - 2 s.

[0010] Furthermore, separately and independently, in the standard coulometric titration and the sample coulometric titration, the equilibrium potential is 0.5 - 2 mV, and the equilibrium time is 1 - 20 s.

[0011] Furthermore, separately and independently, in the standard coulometric titration and the sample coulometric titration, the positive electrode of the electrolysis electrode used is a double platinum plate, the negative electrode of the electrolysis electrode is a single platinum wire, and the glass tube is filled with 2.5 - 3.5 mol / L sulfuric acid; and / or the indicating electrode used is a single platinum plate, the reference electrode is a black tungsten wire, and the glass tube is filled with a saturated potassium sulfate solution.

[0012] Furthermore, the theoretical molar concentration of the sodium thiosulfate standard solution is 0.05 - 0.2 mol / L.

[0013] Furthermore, the molar concentration of the KI electrolyte is 0.05 - 0.5 mol / L.

[0014] Further, in the standard coulometric titration, the volume ratio of the first part of the sodium thiosulfate standard solution to the first part of the KI electrolyte solution is (110 - 500 μL) : (70 - 90 mL).

[0015] Further, in the sample coulometric titration, the volume ratio of the zinc-nickel electroplating solution, the second part of the sodium thiosulfate standard solution to the second part of the KI electrolyte solution is 1.5 mL : (0.4 - 0.6 mL) : (70 - 90 mL).

[0016] Further, the oxidant solution is a sodium persulfate solution or a potassium persulfate solution. The molar concentration of the sodium persulfate solution is 1.5 - 2.5 mol / L, and the molar concentration of the potassium persulfate solution is 0.05 - 0.2 mol / L. The volume ratio of the zinc-nickel electroplating solution to the oxidant solution is 1.5 : (0.5 - 1.5); and / or the molar concentration of the sodium hydroxide solution is 1.5 - 2.5 mol / L, and the sodium hydroxide solution is added until the pH of the feed liquid ≥ 10.

[0017] Further, the molar concentration of the KI solution is 0.1 - 0.3 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5 : (1.5 - 10); and / or the molar concentration of the sulfuric acid is 1.5 - 2.5 mol / L, and the sulfuric acid is added until the pH of the feed liquid ≤ 4.

[0018] Applying the technical solution of the present invention, by utilizing the valence change of nickel hydroxide, combining the oxidation-reduction reaction with the indirect coulometric titration method, a cyanide-free coulometric titration method for quantitatively determining the nickel content in the zinc-nickel electroplating solution is obtained. Indirectly determine nickel under the background of high-concentration metal ions such as zinc by electrogenerated iodine, realizing high-selectivity and high-precision determination of nickel content. Moreover, coulometric analysis does not require the preparation of a standard solution and is easy to realize on-line determination; there is no need to prepare a titrant. Applying Faraday's law of electrolysis, a constant current is used to in-situ electrogenerate the titrant I 2 , ensuring the accuracy of the analysis results. Using the coulometric titration method of the present invention to determine nickel can complete the determination quickly, requires a small amount of sample, and can achieve the accuracy of macro analysis by using a trace amount of sample for determination, which can significantly improve the detection efficiency. Specific Embodiments

[0019] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0020] It should be noted that the "first part" and "second part" in the present invention are only used to distinguish different parts of the same material for the convenience of description, and their ratios are not limited.

[0021] Unless otherwise specified, the "solution" in the present invention is an aqueous solution.

[0022] As described in the background art of the present invention, in the prior art, due to the presence of high-concentration zinc in the zinc-nickel plating solution, the problem that the rapid, accurate and highly selective determination of the nickel content cannot be achieved exists. To solve the above problems, in a typical embodiment of the present invention, a method for analyzing nickel in a zinc-nickel electroplating solution is provided, including the following steps: Step S1, divide the sodium thiosulfate standard solution into two parts, namely the first part of the sodium thiosulfate standard solution and the second part of the sodium thiosulfate standard solution; divide the KI electrolyte with a pH of 2.5 to 3.5 into two parts, namely the first part of the KI electrolyte and the second part of the KI electrolyte; add the first part of the sodium thiosulfate standard solution and the first part of the KI electrolyte into the electrolytic cell, perform standard coulometric titration, and calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 ;

[0023] wherein, i 1 is the electrolytic current of the standard coulometric titration, with the unit of mA; t 1 is the titration end point time of the standard coulometric titration, with the unit of s; z is the conversion coefficient, dimensionless, with a value of 1; F is the Faraday constant, 96485 C / mol; V 标准1 is the volume of the first part of the sodium thiosulfate standard solution, with the unit of mL;

[0024] Step S2, add an oxidant solution and a sodium hydroxide solution to the zinc-nickel electroplating solution, and obtain nickel hydroxide precipitate after washing with water; add a KI solution and sulfuric acid to the nickel hydroxide precipitate to obtain a sample solution;

[0025] Step S3, add the sample solution, the second part of the sodium thiosulfate standard solution and the second part of the KI electrolyte into the electrolytic cell, perform sample coulometric titration, and calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formula (2) 镍 ;

[0026] wherein, V 标准2 is the volume of the second part of the sodium thiosulfate standard solution, with the unit of mL; V 样品 is the volume of the sample solution, with the unit of mL; t 测定 is the titration end point time of the sample coulometric titration, with the unit of s; t 2 is the theoretical titration end point time of the second part of the sodium thiosulfate standard solution, calculated according to formula (3), with the unit of s; wherein, i 2 is the electrolytic current of the sample coulometric titration, with the unit of mA.

[0027]

Analysis Principle

[0028] During the research process, the inventor unexpectedly found that nickel ions would be converted into nickel hydroxide in an alkaline system. The reaction formula is Ni 2+ +OH - = Ni(OH) 2 ↓. Then, adding an oxidizing agent solution can further convert nickel hydroxide into nickel peroxide hydroxide precipitate Ni(OH) 3 . After that, adding a KI solution and adjusting the pH to acidic, under this condition, nickel peroxide hydroxide Ni(OH) 3 can oxidize KI, and the generated I 2 can be quantitatively reduced by sodium thiosulfate. The concentrations of the accurately added sodium thiosulfate solution before and after the reaction can be accurately measured by coulometric titration, and then the content of nickel can be indirectly calculated. Therefore, through redox reactions, by utilizing the valence change of nickel hydroxide, the present invention adopts an indirect method to determine the content of nickel ions by coulometric titration with potassium iodide as the electrolyte. Even in the background of a large amount of other metal ions such as zinc ions, the content of nickel can be independently and accurately determined.

[0029]

Coulometric Titration

[0030] The present invention uses a coulometric titrator for coulometric titration analysis. This instrument integrates a terminal control unit, a coulometric titration unit, i.e., a constant current and potential (mV / pH) measurement unit, which can ensure high precision and high stability of electrolysis control and potential measurement. The basic principle of the analysis method is as follows: Coulometric titration, also known as constant current coulometric analysis, and potentiostatic coulometric analysis are collectively called coulometric analysis. The basic basis of coulometric analysis is Faraday's electrolysis law. It is an electroanalytical method based on measuring the amount of electricity consumed by the analyte in an electrochemical reaction on the electrode during electrolysis. The theoretical basis of this method requires a current efficiency of 100%.

[0031] In ordinary chemical volumetric analysis, the titrant is added from a burette, while in coulometric titration, a constant current is used for electrolysis to in-situ generate a titrant for quantitative reaction with the analyte. Since the amount of the electrochemically generated titrant is proportional to the amount of electricity consumed in electrolysis, this method is actually a volumetric analysis method with electrons as the titrant. Its advantages include high sensitivity, good accuracy, no need to prepare a standard solution for titration, unstable titrants can be electrolytically generated, and current and time can be accurately measured, etc. The end-point indication of coulometric titration can adopt the chemical indicator method, the potentiometric method, or the double platinum electrode current indication method (also known as the dead-stop method).

[0032]

Standard Coulometric Titration

[0033] Specifically, before performing the coulometric titration of the sample solution, the standard solution of sodium thiosulfate is first calibrated to obtain the reaction concentration of the standard solution of sodium thiosulfate accurately added in the coulometric titration of the sample. The standard solution of sodium thiosulfate and the KI electrolyte solution are added to the electrolytic cell, and pre-stirred for about 10 seconds with the immersion electrode at the minimum volume, and then the standard coulometric titration is carried out.

[0034] The KI electrolyte solution can be used in the coulometric titration method to determine the thiosulfate ion in the solution. By electrolyzing KI with a constant current, I 2 is oxidized to obtain I - -2e→I 2 .

[0035] Using the generated I 2 as the titrant, it undergoes a quantitative redox reaction with the excessive S 2 O 3 2- . In a neutral or weakly acidic solution, the titration reaction is as follows: 2Na 2 S 2 O 3 +I 2 =Na 2 S 4 O 6 +2NaI.

[0036] Using a coulometric titrator to record the electrolysis current and the titration end point time, the content of S 2 O 3 2- in the solution can be calculated as follows:

[0037] The electricity consumption Q during the electrolysis process = electrolysis current i × electrolysis time t, and the electrolysis time is the titration end point time;

[0038] At this time, the electricity consumption Q 标准 of the standard coulometric titration 1 =i 1 ×t

[0039] The amount of substance n 2 O 3 2- of sodium thiosulfate in the standard solution 标准 =Q 标准 / z / F=c 标准 ×V 标准1 :

[0040] The calculation formula for the actual molar concentration c 标准 of the sodium thiosulfate standard solution is as follows:

[0041]

Sample Solution Preparation

[0042] Specifically, first add an oxidant solution and a sodium hydroxide solution to the zinc-nickel electroplating solution. Under alkaline conditions, nickel ions will produce a light green Ni(OH) 2 precipitate: Ni 2+ + OH - = Ni(OH) 2 ↓. Under the action of the added oxidant solution, the color of the Ni(OH) 2 precipitate gradually changes: from light green to blue-gray and then to black, indicating the complete formation of black nickel hydroxide Ni(OH) 3 precipitate. Taking potassium persulfate as the oxidant solution, the precipitation conversion process is as follows: Filter and wash thoroughly with water to remove the oxidant solution in the solution, obtaining nickel hydroxide precipitate. Then add KI solution and sulfuric acid to the black nickel hydroxide precipitate to make the pH of the feed liquid acidic. At this time, Ni(OH) 3 oxidizes KI to generate I 2 , obtaining a sample solution containing nickel ions and I 2 (with a molar amount of n 碘 ).

[0043]

Sample Coulometric Titration

[0044] The concentration of the sodium thiosulfate standard solution has been accurately determined by standard coulometric titration.

[0045] Add the sample solution, sodium thiosulfate standard solution (with a molar amount of n 标 , corresponding to V 标准2 ) and KI electrolyte into the electrolysis cell. Using the immersion electrode as the minimum volume, pre-stir for about 10 seconds and conduct sample coulometric titration with it as the test solution. Determine the remaining amount n 测 of sodium thiosulfate after the reaction through coulometric titration. According to n 标 - n 测 = 2n 碘 = n 镍 , the molar concentration c 镍 of nickel in the zinc-nickel electroplating solution can be calculated.

[0046] During the sample coulometric titration, there are two sources of I 2 . One is the I 2 obtained by oxidizing the KI electrolyte using constant current electrolysis. The electrode reaction: 2I - - 2e → I 2 , which acts as the titrant. The other is the I 3 generated by the oxidation of the KI solution by Ni(OH) 2 in the sample solution, which acts to indirectly determine the nickel content.

[0047] The I from the above two sources2 Quantitatively undergoes an oxidation-reduction reaction with the excess S in the sodium thiosulfate standard solution in the test solution. In a neutral or weakly acidic solution, the titration reaction is as follows: 2Na 2 O 3 2- S 2 O 2 +I 3 =Na 2 S 2 O 4 +2NaI. By using a coulometric titrator to record the electrolytic current and the titration end point time, the content of S 6 in the solution can be calculated based on the amount of electricity consumed during the electrolysis process: 2 O 3 2-

[0048] Since the I 3 generated by the oxidation of KI by Ni(OH) 2 in the sample solution will consume part of the S 2 O 3 2- , when measuring the sample solution, part of the I 2 O 3 2- oxidized by S 2 comes from the oxidation of KI by Ni(OH) 3 , and the other part comes from the electrolysis of KI. Compared with the standard coulometric titration of the sodium thiosulfate standard solution, the titration end point time of the sample coulometric titration will be shorter.

[0049] At this time, Q 样品溶液中由Ni(OH)3氧化KI生成的I2消耗的 =Q 标准 -Q 样品溶液中由电解KI生成的I2消耗的 ;

[0050] According to Q 标准 =i 1 ×t 2 ; Q 样品溶液中由电解KI生成的I2消耗的 =i 2 ×t 测定 ,

[0051] It can be obtained that:

[0052]

[0053]

[0054] Finally, the calculation formula for the molar concentration c 镍 of nickel in the zinc-nickel electroplating solution is as follows:

[0055]

[0056] Among them, t​2 is the theoretical titration end point time of the second part of the sodium thiosulfate standard solution, which is calculated according to the following formula:

[0057]

[0058] It should be noted that when the volumes of the first part and the second part of the sodium thiosulfate standard solution are the same (V 标准1 = V 标准2 ), under the condition of constant electrolysis current, the corresponding theoretical titration end point times of the two are also the same (t 1 = t 2 ), and at this time C 标准 *V 标准1 > C 镍 *V 样品 .

[0059] In summary, considering the presence of a large number of interfering ions in the zinc-nickel electroplating solution, the present invention establishes a coulometric titration method for quantitatively determining nickel, and indirectly determines nickel under the background of high-concentration metal ions such as zinc by electrogenerated iodine. The present invention can effectively and highly selectively determine nickel by utilizing the valence state change of nickel hydroxide, and it is an efficient analysis method for nickel ions Ni 2+ in the zinc-nickel electroplating solution, and it is also a cyanide-free coulometric titration method that can quantitatively determine nickel. Moreover, coulometric analysis does not require the preparation of standard solutions and is easy to realize on-line determination; there is no need to prepare titrants, and the Faraday's law of electrolysis is applied to electrogenerate titrants in situ with a constant current, which ensures the accuracy of the analysis results. Using the coulometric titration method of the present invention to determine nickel can complete the determination quickly, requires a small amount of sample, and can achieve the accuracy of macroanalysis by using a small amount of sample for determination, which can significantly improve the detection efficiency and provide more reliable data support for the quality control of zinc-nickel electroplating solutions.

[0060] It can be seen that the present invention utilizes the valence state change characteristics of nickel hydroxide, directly converts nickel into nickel hydroxide, and uses its reaction with potassium iodide to generate iodine, and then indirectly determines the content of nickel through the reduction reaction with sodium thiosulfate solution. By indirectly coulometric titration to determine the concentration of nickel ions, the interference of other metal ions such as zinc can be effectively excluded, high-selectivity determination can be realized, the accuracy and efficiency of nickel content determination are improved, the content of nickel can be obtained quickly and accurately, which provides strong data support for the control and optimization of electroplating processes. At the same time, this method does not require the preparation of standard solutions, is easy to realize on-line determination, saves time and cost, solves the problem of accurate determination of nickel content in complex electroplating solutions, especially the problem that it is difficult to accurately, quickly and selectively determine the nickel content by traditional analysis methods in the presence of a large amount of other metal ions such as zinc ions.

[0061] It should be noted that during the preparation of the sample solution, the precipitates formed in each reaction are separated by filtration or suction filtration, rather than by centrifugation. This is because the inventor unexpectedly found during the research process that a small amount of precipitate floats during centrifugation, and the loss of the filtration method is less. One suction filtration is sufficient to complete the recovery of the precipitate, and no obvious sample filtration loss will occur during the precipitate conversion.

[0062] In a preferred embodiment, independently, in the standard coulometric titration and the sample coulometric titration, the electrolysis current is 5 - 15 mA, and the minimum electrolysis time is 0.5 - 2 s. By controlling the electrolysis parameters within the above ranges, the electrolysis rate can be effectively controlled, the current efficiency can be maintained, the measurement accuracy can be improved, and at the same time, the precision of the electrolysis reaction can be taken into account. The smaller the electrolysis current and the minimum electrolysis time, the higher the precision of the electrolysis reaction, but the current efficiency will be affected; if the electrolysis current and the minimum electrolysis time are too large, the current efficiency will also be affected, and the reaction may not be complete, affecting the measurement accuracy. Within the above ranges, the reaction can proceed more completely, while shortening the analysis time and improving the work efficiency, so as to further ensure the high efficiency and accuracy of the electrolysis process and more effectively selectively determine the nickel ion concentration in the zinc-nickel alloy plating solution.

[0063] For the purpose of accurately measuring the potential of the solution system, then accurately determining the titration end point, and at the same time taking into account the reaction completion degree and work efficiency, in a preferred embodiment, independently, in the standard coulometric titration and the sample coulometric titration, the equilibrium potential is 0.5 - 2 mV, and the equilibrium time is 1 - 20 s. By precisely controlling the potential equilibrium conditions of the solution system, the influence of potential fluctuations on the measurement results can be further reduced, and the stability, precision, and accuracy of the measurement results can be improved.

[0064] Preferably, the electrolysis current, minimum electrolysis time, equilibrium potential, and equilibrium time of the standard coulometric titration are the same as those of the sample coulometric titration, and the accuracy of the measurement results is higher.

[0065] Considering the complexity of the composition of the zinc-nickel alloy plating solution sample, platinum electrodes with good stability and strong corrosion resistance are preferably used as the working electrode and the auxiliary electrode. In a preferred embodiment, independently, in the standard coulometric titration and the sample coulometric titration, the positive electrode of the electrolysis electrode used is a double platinum sheet, the negative electrode of the electrolysis electrode is a single platinum wire, and the glass tube is filled with 2.5 - 3.5 mol / L sulfuric acid; and / or the indicating electrode used is a single platinum sheet, the reference electrode is a black tungsten wire, and the glass tube is filled with saturated potassium sulfate solution. Such an electrode configuration is more conducive to achieving a 100% current efficiency during electrolysis and accurate and stable potential measurement, and can effectively improve the electrolysis efficiency, reduce electrode contamination, and achieve long-term stable operation.

[0066] In order to further promote the titration reaction between the electrogenerated titrant and the analyte after the electrolysis reaction to proceed rapidly and completely according to the stoichiometric ratio, in a preferred embodiment, the theoretical molar concentration of the sodium thiosulfate standard solution is 0.05 - 0.2 mol / L. By adjusting the concentration and added volume of the standard solution, it is possible to adapt to the determination of different nickel contents, further expand the applicable range of the method, and make it applicable to the analysis of electroplating solutions with various concentrations.

[0067] In a preferred embodiment, the molar concentration of the KI electrolyte is 0.05 - 0.5 mol / L. The selection of the concentration of the KI electrolyte can further stabilize the electrolysis process, reduce side reactions, and is more conducive to maintaining a current efficiency of 100% during the electrolysis reaction to electrogenerate the titrant, thereby improving the accuracy of the measurement results.

[0068] In order to ensure that the sodium thiosulfate standard solution is fully reacted and accurately calibrated, in a preferred embodiment, in the standard coulomb titration, the volume ratio of the first part of the sodium thiosulfate standard solution to the first part of the KI electrolyte is (110 - 500 μL):(70 - 90 mL), thereby further improving the accuracy of the nickel content determination in the zinc-nickel alloy electroplating solution. Such a volume ratio can also promote the full contact between the standard solution and the electrolyte, which is beneficial to further improving the reaction efficiency.

[0069] In a preferred embodiment, in the sample coulomb titration, the volume ratio of the zinc-nickel electroplating solution, the second part of the sodium thiosulfate standard solution to the second part of the KI electrolyte is 1.5 mL:(0.4 - 0.6 mL):(70 - 90 mL). Under the above conditions, it is more conducive to indirectly using coulomb titration to complete the determination of nickel in the sample, that is, the oxidized sample solution consumes part of the sodium thiosulfate, and the remaining sodium thiosulfate is accurately measured by coulomb titration, promoting the accurate generation of the electrogenerated titrant. By optimizing the volume ratio of the sample, the standard solution, and the electrolyte, the precision and accuracy of the measurement can be further improved.

[0070] For the purpose of more fully precipitating zinc and nickel in the zinc-nickel electroplating solution and more fully oxidizing the precipitate to the target valence state (nickel hydroxide peroxide in the trivalent state), in a preferred embodiment, the oxidizing agent solution is a sodium persulfate solution or a potassium persulfate solution. The molar concentration of the sodium persulfate solution is 1.5 - 2.5 mol / L, the molar concentration of the potassium persulfate solution is 0.05 - 0.2 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the oxidizing agent solution is 1.5:(0.5 - 1.5); and / or the molar concentration of the sodium hydroxide solution is 1.5 - 2.5 mol / L, and the sodium hydroxide solution is added until the pH of the feed liquid ≥ 10.

[0071] In a preferred embodiment, the molar concentration of the KI solution is 0.1 - 0.3 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5:(1.5 - 10); and / or the molar concentration of sulfuric acid is 1.5 - 2.5 mol / L, and sulfuric acid is added until the pH of the feed liquid ≤ 4. Under the above conditions, nickel hydroxide can be more fully reduced to divalent nickel ions, and finally a sample solution containing divalent nickel ions and I 2 is obtained, which is more convenient for subsequent coulometric titration and improves the accuracy of the measurement results.

[0072]

Technical Path of Coulometric Titration Analysis

[0073] In the actual coulometric titration process, the supporting electrolyte (such as KI of the present invention) undergoes an electrode reaction at the working electrode to generate a titrant that reacts with the sample solution to obtain a titration product. After the end point indication appears, the electric quantity is recorded, and the component concentration is calculated according to the formula.

[0074] Exemplarily, the specific operation of the coulometric titration analysis of the present invention is as follows:

[0075] 1) Detection device setup and parameter setting: Set up the detection device according to the information in the following table, and finely set the parameters of electrolysis and end point indication.

[0076] Table 1

[0077]

[0078] Using constant current electrolysis of KI, the oxidized I 2 is used as a titrant, which can quantitatively oxidize thiosulfate ions to determine the S 2 O 3 2- concentration. The electrode reactions are as follows:

[0079] Working electrode reaction:

[0080] Titration reaction:

[0081] The electrolysis product I 2 in the solution reacts quantitatively with S 2 O 3 2- Using a coulometer, the content of S 2 O 3 2- in the solution can be calculated according to the electric quantity consumed during the electrolysis process.

[0082] 2) Titration mode parameter setting: According to the specific requirements of the experiment, reasonably set the titration mode parameters, including key elements such as electrolysis current, minimum electrolysis time, program control of electrolysis time and equilibrium time designed according to the equilibrium potential, endpoint jump amount, and pre-electrolysis time.

[0083] 3) Sample treatment and determination: Accurately and quantitatively pipette the sample solution of the zinc-nickel electroplating solution, and perform appropriate sample pretreatment as needed to facilitate further content determination. Subsequently, perform coulometric titration analysis using the established and improved device.

[0084] 4) Titration process monitoring: During the titration process, the instrument will display the current potential value and electrolysis time in real-time, and synchronously plot the titration curve, including the potential vs. electrolysis time curve and the first derivative vs. electrolysis time curve, to achieve visual monitoring of the titration process.

[0085] 5) Endpoint determination: According to the extreme value of the first derivative vs. electrolysis time curve, the instrument can automatically and accurately find the titration endpoint, thus ensuring the reliability of the analysis results.

[0086] 6) Result calculation: Calculate the concentration of the component to be measured in the sample based on the electricity consumed during electrolysis, and use the following classical formula for calculation:

[0087]

[0088] where c represents the concentration of the component to be measured in the sample (mol / L); I represents the electrolysis current (mA); t ep represents the electrolysis time (s) corresponding to the endpoint; V 0 represents the sample volume (mL); z represents the conversion coefficient; F represents the Faraday constant (96485 C / mol). This formula ensures the accuracy of the determination of metal ion concentration.

[0089] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0090]

Coulometric titrator

[0091] Equipped with a positive electrolysis electrode (double platinum plates), a negative electrolysis electrode (single platinum wire, 3 mol / L sulfuric acid filled in a glass tube); an indicating electrode (single platinum plate), and a reference electrode (black tungsten wire, saturated potassium sulfate solution filled in a glass tube).

[0092]

Standardization of sodium thiosulfate standard solution

[0093] Add 1 drop (about 0.05 mL) of H 2 SO 4A solution (2 mol / L) and 80 mL of KI solution (0.2 mol / L) are added to an electrolysis cup. The minimum volume is to submerge the electrodes of the double platinum plates, and the pH is adjusted so that the pH of the KI electrolyte is around 3.

[0094] Accurately pipette 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.1 mol / L) and add it to the above-mentioned KI electrolyte (volume ratio 110 μL:80 mL). Pre-stir for 10 seconds and conduct a standard coulometric titration (electrolysis current 10 mA, minimum electrolysis time 1 s, equilibrium potential 1 mV, equilibrium time 10 s). Calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1). 标准 Measure multiple times, and the calculated average value and precision are shown in the following table.

[0095] As can be seen from the following table, for the sodium thiosulfate standard solution, the coulometric titration method of the present invention with KI as the supporting electrolyte has high precision and meets the measurement requirements.

[0096] Table 2

[0097]

[0098]

[0099] Determination of nickel standard solution in Example 1

[0100] Step S1, the determination steps of the actual molar concentration c of the sodium thiosulfate standard solution are shown in the calibration part of the sodium thiosulfate standard solution. 标准

[0101] Step S2, accurately pipette 0.100 mL (V 样品 = 0.100 mL) of nickel standard solution (theoretical molar concentration 0.3060 mol / L) into an electrolysis cup. Add sodium hydroxide solution (molar concentration 2 mol / L) to the nickel standard solution until the pH of the feed liquid ≥ 10. Add an oxidizing agent solution (potassium persulfate solution, molar concentration 0.1 mol / L, volume ratio of nickel standard solution to oxidizing agent solution 0.1:1). Filter and wash thoroughly with water to remove the oxidizing agent solution in the solution to obtain nickel hydroxide precipitate; add KI solution (molar concentration 0.2 mol / L, volume ratio of nickel standard solution to KI solution 0.1:10) and sulfuric acid (molar concentration 2 mol / L) to the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3 to obtain a sample solution.

[0102] Step S3: Add 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte solution to the sample solution. Using the immersion electrode as the minimum volume, control the pH to be around 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current is 10 mA, minimum electrolysis time is 1 s, equilibrium potential is 1 mV, equilibrium time is 10 s). The quantity of electricity value Q measured by the coulometric titrator 测定 = 1882 mC, t 测定 = 188.2 s. Calculate the molar concentration c of nickel in the nickel standard solution according to formulas (2) and (3) 镍 :[[]]

[0103] According to formula (3), we can get

[0104] The theoretical quantity of electricity value Q of 0.500 mL (V 标准2 = 0.500 mL) of sodium thiosulfate standard solution is Q = c 标准 *z*F*V 标准2 = i 2 ×t 2 = 4830 mC, t 2 = 483.0 s;

[0105] Substitute into formula (2) to get

[0106] The relative error of the measurement result is 0.2%. Relative error = |measured value - theoretical value| ÷ theoretical value × 100%.

[0107] Example 2: Determination of zinc-nickel standard solution

[0108] Step S1: The steps for determining the actual molar concentration c 标准 of the sodium thiosulfate standard solution can be found in the calibration part of the sodium thiosulfate standard solution.

[0109] Step S2: Accurately pipette 0.200 mL (V 样品 = 0.200 mL) of zinc-nickel standard solution (the theoretical concentration of zinc is 0.1368 mol / L, and the theoretical concentration of nickel is 0.1530 mol / L) into the electrolysis cell. Add sodium hydroxide solution (molar concentration is 2 mol / L) to the zinc-nickel standard solution until the pH of the feed liquid ≥ 10. Add oxidant solution (potassium persulfate solution, molar concentration is 0.1 mol / L, and the volume ratio of zinc-nickel standard solution to oxidant solution is 0.2:1). Filter and wash thoroughly with water to remove the oxidant solution in the solution to obtain nickel hydroxide precipitate; add KI solution (molar concentration is 0.2 mol / L, and the volume ratio of zinc-nickel standard solution to KI solution is 0.2:10) and sulfuric acid (molar concentration is 2 mol / L) to the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3 to obtain the sample solution.

[0110] Step S3: Add 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte solution to the sample solution. Using the immersion electrode as the minimum volume, control the pH to be around 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current is 10 mA, minimum electrolysis time is 1 s, equilibrium potential is 1 mV, equilibrium time is 10 s). The t measured by the coulometric titrator 测定 = 186.0 s. Calculate the molar concentration c of nickel in the zinc-nickel standard solution according to formulas (2) and (3) 镍 :

[0111] According to formula (3), we can get:

[0112] The theoretical charge value Q of 0.500 mL (V 标准2 = 0.500 mL) of sodium thiosulfate standard solution is Q = c 标准 *z*F*V 标准2 = i 2 ×t 2 = 4830 mC, t 2 = 483.0 s;

[0113] Substitute into formula (2) to get:

[0114] The relative error of the measurement result is 0.6%.

[0115] Determination of the zinc-nickel electroplating solution sample in Example 3

[0116] Step S1: The procedure for determining the actual molar concentration c of the sodium thiosulfate standard solution can be found in the calibration section of the sodium thiosulfate standard solution. 标准

[0117] Step S2: Accurately pipette 1.500 mL (V 样品 = 1.500 mL) of zinc-nickel electroplating solution (the theoretical concentration of nickel is 0.02167 mol / L) into the electrolysis cell. Add sodium hydroxide solution (molar concentration is 2 mol / L) to the zinc-nickel electroplating solution until the pH of the feed liquid ≥ 10. Add oxidant solution (sodium persulfate solution, molar concentration is 2 mol / L, the volume ratio of zinc-nickel electroplating solution to oxidant solution is 1.5:1). Filter and wash thoroughly with water to remove the oxidant solution in the solution to obtain nickel hydroxide precipitate; add KI solution (molar concentration is 0.2 mol / L, the volume ratio of zinc-nickel electroplating solution to KI solution is 1.5:10) and sulfuric acid (molar concentration is 2 mol / L) to the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3 to obtain the sample solution.

[0118] ​Step S3: Add 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte solution to the sample solution. Using the immersion electrode as the minimum volume, control the pH to be around 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current is 10 mA, minimum electrolysis time is 1 s, equilibrium potential is 1 mV, equilibrium time is 10 s). The t measured by the coulometric titrator 测定 = 168.6 s. Calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formulas (2) and (3) 镍 :

[0119] According to formula (3), we can get

[0120] 0.500 mL (V 标准2 = 0.500 mL) of the theoretical charge value Q of the sodium thiosulfate standard solution = c 标准 *z*F*V 标准2 = i 2 ×t 2 = 4830 mC, t 2 = 483.0 s;

[0121] Substitute into formula (2) to get

[0122] The relative error of the measurement result is 0.2%.

[0123] Example 4

[0124] Step S1: Add 2 drops (about 0.10 mL) of H 2 SO 4 solution (2 mol / L), 70 mL of KI solution (0.2 mol / L) into the electrolysis cell. Using the immersion electrolysis of the double platinum electrodes as the minimum volume, adjust the pH so that the pH of the KI electrolyte is around 3. Accurately pipette 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.1 mol / L), add it to the above KI electrolyte solution (volume ratio is 110 μL:80 mL), pre-stir for 10 seconds, and perform standard coulometric titration (electrolysis current is 10 mA, minimum electrolysis time is 1 s, equilibrium potential is 1 mV, equilibrium time is 10 s). Calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 .

[0125] Step S2: Accurately pipette 1.500 mL (V 样品Transfer 1.500 mL of a zinc-nickel electroplating solution (the theoretical concentration of nickel is 0.02167 mol / L) into an electrolytic cell. Add sodium hydroxide solution (molar concentration is 2 mol / L) to the zinc-nickel electroplating solution until the pH of the feed liquid ≥ 10. Then add an oxidant solution (sodium persulfate solution, molar concentration is 2 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the oxidant solution is 1.5:1). After filtration and thorough washing with water, remove the oxidant solution from the solution to obtain nickel hydroxide precipitate. Add KI solution (molar concentration is 0.2 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5:2) and sulfuric acid (molar concentration is 2 mol / L) to the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3 to obtain a sample solution.

[0126] Step S3: Add 0.500 mL of sodium thiosulfate standard solution and 70 mL of KI electrolyte solution to the sample solution. Using the immersion electrode as the minimum volume, control the pH to be around 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current is 10 mA, minimum electrolysis time is 1 s, equilibrium potential is 1 mV, and equilibrium time is 10 s). Calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formulas (2) and (3). 镍 , Measure multiple times, and the results are shown in the following table.

[0127] Table 3

[0128]

[0129] The average molar concentration c of nickel in the zinc-nickel electroplating solution 镍 is 0.02172 mol / L, the relative error of the measurement result is 0.2%, and the experimental error (relative range) of two measurements is 0.1%.

[0130] Example 5

[0131] The difference from Example 3 is that:

[0132] The molar concentration of the KI electrolyte solution is 0.05 mol / L, and the pH is 3.5.

[0133] Step S1: Accurately transfer 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.05 mol / L) and add it to the above KI electrolyte solution (volume ratio is 110 μL:90 mL). Pre-stir for 10 seconds and perform standard coulometric titration (electrolysis current is 5 mA, minimum electrolysis time is 2 s, equilibrium potential is 0.5 mV, and equilibrium time is 20 s). Calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1). 标准 .

[0134] Step S2: Accurately transfer 1.500 mL (V 样品Transfer 1.500 mL of zinc-nickel electroplating solution (the theoretical concentration of nickel is 0.02167 mol / L) into an electrolytic cell. Add sodium hydroxide solution (molar concentration is 1.5 mol / L) to the zinc-nickel electroplating solution until the pH of the feed liquid ≥ 10. Add an oxidizing agent solution (sodium persulfate solution, molar concentration is 1.5 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the oxidizing agent solution is 1.5:1.5). Filter and wash thoroughly with water to remove the oxidizing agent solution in the solution, obtaining nickel hydroxide precipitate; add KI solution (molar concentration is 0.1 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5:10) and sulfuric acid (molar concentration is 1.5 mol / L) to the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3, obtaining a sample solution.

[0135] Step S3, add 0.400 mL of sodium thiosulfate standard solution and 70 mL of KI electrolyte solution to the sample solution. Using the immersion electrode as the minimum volume, control the pH at about 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current is 5 mA, minimum electrolysis time is 2 s, equilibrium potential is 0.5 mV, equilibrium time is 20 s). Calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formulas (2) and (3) 镍 。

[0136] Example 6

[0137] The difference from Example 3 is that:

[0138] The molar concentration of the KI electrolyte solution is 0.5 mol / L and the pH is 2.5.

[0139] Step S1, accurately pipette 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.2 mol / L) and add it to the above KI electrolyte solution (volume ratio is 110 μL:70 mL). Pre-stir for 10 seconds and perform standard coulometric titration (electrolysis current is 15 mA, minimum electrolysis time is 0.5 s, equilibrium potential is 2 mV, equilibrium time is 2 s). Calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 。

[0140] Step S2, accurately pipette 1.500 mL (V 样品Add 1.500 mL of zinc-nickel electroplating solution (the theoretical concentration of nickel is 0.02167 mol / L) into the electrolytic cell. Add sodium hydroxide solution (molar concentration is 2.5 mol / L) into the zinc-nickel electroplating solution until the pH of the feed liquid ≥ 10. Add oxidant solution (sodium persulfate solution, molar concentration is 2.5 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the oxidant solution is 1.5:0.5). After filtration and thorough washing with water, remove the oxidant solution in the solution to obtain nickel hydroxide precipitate. Add KI solution (molar concentration is 0.3 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5:1.5) and sulfuric acid (molar concentration is 2.5 mol / L) into the nickel hydroxide precipitate until the pH of the feed liquid ≤ 3 to obtain the sample solution.

[0141] Step S3: Add 0.600 mL of sodium thiosulfate standard solution and 90 mL of KI electrolyte solution into the sample solution. Using the immersion electrode as the minimum volume, control the pH around 3, pre-stir for 10 seconds, and conduct sample coulometric titration (electrolysis current is 15 mA, minimum electrolysis time is 0.5 s, equilibrium potential is 2 mV, and equilibrium time is 2 s). Calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formulas (2) and (3). 镍 。

[0142] Comparative Example 1

[0143] Use the dimethylglyoxime gravimetric method (standard number: GB / T 223.25-1994 "Methods for Chemical Analysis of Iron, Steel and Alloys - Determination of Nickel Content by Dimethylglyoxime Gravimetric Method") to determine the nickel content in the zinc-nickel electroplating solution (the theoretical concentration of nickel is 0.02167 mol / L). The measured nickel concentration is 0.01897 mol / L, and the relative error of the measurement result is 12.5%.

[0144] The nickel concentration measurement results of the above examples and comparative examples are shown in Table 4.

[0145] Table 4

[0146]

[0147] As can be seen from the above, compared with the comparative examples, each embodiment of the present invention utilizes the valence change of nickel hydroxide, combines the oxidation-reduction reaction with the indirect coulometric titration method, and obtains a cyanide-free coulometric titration method that can quantitatively determine the nickel content in the zinc-nickel electroplating solution. It indirectly determines nickel under the background of high-concentration metal ions such as zinc by electrogenerated iodine, realizing high-selectivity and high-precision determination of nickel content. Moreover, coulometric analysis does not require the preparation of standard solutions, is easy to realize on-line determination; does not require the preparation of titrants, applies Faraday's electrolysis law, and in-situ electrogenerates titrant I with a constant current. 2, which ensures the accuracy of the analysis results. Using the coulometric titration method of the present invention to determine nickel can complete the determination quickly, requires a small amount of sample, and can achieve the accuracy of macro analysis by using a trace amount of sample for determination, which can significantly improve the detection efficiency.

[0148] In addition, it can be seen that when each process parameter is within the preferred range of the present invention, the comprehensive effect is better.

[0149] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for analyzing nickel in a zinc-nickel electroplating solution, characterized in that: The following steps are involved: Step S1, dividing the sodium thiosulfate standard solution into two parts, namely a first part of the sodium thiosulfate standard solution and a second part of the sodium thiosulfate standard solution; The KI electrolyte having a pH of 2.5 to 3.5 is divided into two parts, namely a first part of the KI electrolyte and a second part of the KI electrolyte; The first part of sodium thiosulfate standard solution and the first part of KI electrolyte are added to the electrolysis cup, and standard coulometric titration is performed to calculate the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1): 标准 ; Wherein, i1 is the electrolysis current of the standard coulometric titration, in mA; t1 is the titration endpoint time of the standard coulometric titration, in seconds; z is the conversion coefficient, dimensionless, with a value of 1; F is the Faraday constant, 96485 C / mol; V 标准1 is the volume of the first part of sodium thiosulfate standard solution, in mL; Step S2, adding an oxidant solution and a sodium hydroxide solution to a zinc-nickel electroplating solution, washing with water to obtain a nickel hydroxide precipitate; adding a KI solution and sulfuric acid to the nickel hydroxide precipitate to obtain a sample solution; Step S3, adding the sample solution, the second part of the sodium thiosulfate standard solution and the second part of the KI electrolyte into an electrolytic cup, performing coulometric titration of the sample, and calculating the molar concentration c of nickel in the zinc-nickel electroplating solution according to formula (2): 镍 ; Among them, V 标准2 V is the volume of the second part of sodium thiosulfate standard solution, in mL; 样品 is the volume of the sample solution, in mL; t 测定 is the titration endpoint time of the coulometric titration of the sample, in seconds; t2 is the theoretical titration endpoint time of the second part of the sodium thiosulfate standard solution, calculated according to formula (3), in seconds; Wherein, i2 is the electrolysis current of the coulometric titration of the sample, in mA.

2. The analysis method according to claim 1, characterized in that Independently, in the standard coulometric titration and the sample coulometric titration, the electrolysis current is 5-15 mA, and the minimum electrolysis time is 0.5-2 s.

3. The analysis method according to claim 1 or 2, characterized in that Independently, in the standard coulometric titration and the sample coulometric titration, the equilibrium potential is 0.5 to 2 mV, and the equilibrium time is 1 to 20 s.

4. The analysis method according to any one of claims 1 to 3, characterized in that Independently, in the standard coulometric titration and the sample coulometric titration, The positive electrode of the electrolytic electrode used is a double platinum sheet, the negative electrode of the electrolytic electrode is a single platinum wire, and the glass tube is filled with 2.5-3.5 mol / L sulfuric acid; and / or The indicator electrode used is a single platinum sheet, the reference electrode is a black tungsten wire, and the glass tube is filled with a saturated potassium sulfate solution.

5. The analysis method according to any one of claims 1 to 4, characterized in that The theoretical molar concentration of the sodium thiosulfate standard solution is 0.05-0.2 mol / L.

6. The analysis method according to any one of claims 1 to 5, characterized in that The molar concentration of the KI electrolyte is 0.05-0.5 mol / L.

7. The analysis method according to any one of claims 1 to 6, characterized in that In the standard coulometric titration, the volume ratio of the first part of sodium thiosulfate standard solution to the first part of KI electrolyte is (110-500 μL): (70~90mL).

8. The analysis method according to any one of claims 1 to 7, characterized in that In the coulometric titration of the sample, the volume ratio of the zinc-nickel electroplating solution, the second part of the sodium thiosulfate standard solution and the second part of the KI electrolyte is 1.5 mL: (0.4-0.6 mL): (70-90 mL).

9. The analysis method according to any one of claims 1 to 8, characterized in that The oxidant solution is a sodium persulfate solution or a potassium persulfate solution, the molar concentration of the sodium persulfate solution is 1.5 to 2.5 mol / L, the molar concentration of the potassium persulfate solution is 0.05 to 0.2 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the oxidant solution is 1.5:(0.5 to 1.5); and / or The molar concentration of the sodium hydroxide solution is 1.5-2.5 mol / L, and the sodium hydroxide solution is added until the pH of the feed solution is ≥10.

10. The analysis method according to any one of claims 1 to 9, characterized in that The molar concentration of the KI solution is 0.1-0.3 mol / L, and the volume ratio of the zinc-nickel electroplating solution to the KI solution is 1.5:(1.5-10); and / or The molar concentration of the sulfuric acid is 1.5-2.5 mol / L, and the sulfuric acid is added until the pH of the feed solution is ≤4.

Images