Method for the analysis of nickel in zinc-nickel plating solutions

Abstract

The application provides a method for analyzing nickel in a zinc-nickel electroplating solution. The method comprises: performing standard coulometric titration on a sodium thiosulfate standard solution to calculate an actual molar concentration c 标准 ; adding an oxidizing agent solution and a sodium hydroxide solution to 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 a second part of the sodium thiosulfate standard solution and a second part of a KI electrolyte to an electrolytic cup to perform sample coulometric titration and calculate the molar concentration c 镍 of nickel in the zinc-nickel electroplating solution. The application uses the change in the valence state of nickel hydroxide, combines a redox reaction with an indirect coulometric titration method, and obtains a cyanide-free coulometric titration method for quantitatively determining the content of nickel in a zinc-nickel electroplating solution. The method does not need to prepare a standard solution, indirectly determines nickel by using electrically generated iodine in a high-concentration zinc metal ion background, realizes high selectivity, high precision, and online determination of the content of nickel, and can significantly improve the detection efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solution analysis, in particular to a method for analyzing nickel in zinc-nickel electroplating solution. BACKGROUND

[0002] The standard method for determining nickel includes atomic absorption spectrometry and dimethylglyoxime spectrophotometry (rapid determination method of nickel in industrial wastewater). These analysis methods need to draw a standard working curve, which is time-consuming and laborious, and cannot realize online testing. When using traditional analysis methods to determine the content of nickel, sample pretreatment is generally performed, and then determination is performed in a solution state. The determination principle is the four chemical reaction equilibriums in the solution, including EDTA titration method using complexation reaction, dimethylglyoxime spectrophotometry of complex of high-valence nickel ion, method for generating stable purple-red complex of triethylenetetramine and nickel ion disclosed in Chinese application CN201710259374.9, and dimethylglyoxime gravimetric method using precipitation-dissolution equilibrium.

[0003] However, the zinc-nickel plating solution is complex in composition, contains a large amount of interfering ions such as high-concentration zinc, and also contains a strong complexing agent of polyene polyamine, so it is difficult to analyze the plating solution. For the determination of the content of nickel in the complex zinc-nickel plating solution, the complexometric titration method cannot distinguish between zinc and nickel because of the very close coordination properties between zinc and nickel; the dimethylglyoxime gravimetric method for determining nickel is time-consuming and laborious, and cannot realize online determination. Therefore, the traditional analysis method cannot accurately and quickly determine the content of nickel, and cannot perform high-selectivity determination. SUMMARY

[0004] The main purpose of the present application is to provide a method for analyzing nickel in zinc-nickel electroplating solution, so as to solve the problem that the zinc-nickel plating solution cannot realize rapid, accurate and high-selectivity determination of the content of nickel in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a method for analyzing nickel in zinc-nickel electroplating solution is provided, comprising the following steps: step S1, dividing a sodium thiosulfate standard solution into two parts, which are a first part of sodium thiosulfate standard solution and a second part of sodium thiosulfate standard solution; dividing a KI electrolyte with a pH of 2.5-3.5 into two parts, which are a first part of KI electrolyte and a second part of KI electrolyte; adding the first part of sodium thiosulfate standard solution and the first part of KI electrolyte into an electrolytic cup, and performing standard coulometric titration; calculating the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 ;

[0006] Wherein, i1 is the electrolytic current of standard coulometric titration, unit is mA; t1 is the titration end time of standard coulometric titration, unit is s; z is the conversion coefficient, dimensionless, the value is 1; F is the Faraday constant, 96485 C / mol; V is the voltage of the indicator electrode, unit is V. 标准1 V1 is the volume of the first part of sodium thiosulfate standard solution, unit is mL;

[0007] Step S2, adding an oxidizing agent solution and a sodium hydroxide solution into the zinc-nickel electroplating solution, and washing with water to obtain a high-nickel hydroxide precipitate; adding a KI solution and sulfuric acid into the high-nickel hydroxide precipitate to obtain a sample solution; Step S3, adding the sample solution, the second part of sodium thiosulfate standard solution and the second part of KI electrolyte into an electrolytic cup to perform sample coulometric titration, and calculating 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 sodium thiosulfate standard solution, unit is mL; V 样品 is the volume of the sample solution, unit is mL; t 测定 is the titration end time of sample coulometric titration, unit is s; t2 is the theoretical titration end time of the second part of sodium thiosulfate standard solution, calculated according to formula (3), unit is s; Wherein, i2 is the electrolytic current of sample coulometric titration, unit is mA.

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

[0010] Further, 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] Further, 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 saturated potassium sulfate solution.

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

[0013] Further, 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 is (110-500 μL):(70-90 mL).

[0015] Further, in the sample coulometric titration, the volume ratio of the zinc-nickel plating 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).

[0016] Further, 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, the volume ratio of the zinc-nickel plating 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 to the feed liquid until the pH is ≥10.

[0017] Further, the molar concentration of the KI solution is 0.1-0.3 mol / L, the volume ratio of the zinc-nickel plating 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 to the feed liquid until the pH is ≤4.

[0018] By using the technical solution of the present application, the change in the valence state of the nickel hydroxide is utilized, the redox reaction is combined with the indirect coulometric titration method, a cyanide-free coulometric titration method for quantitatively determining the nickel content in the zinc-nickel plating solution is obtained, the electrically generated iodine is utilized to indirectly determine the nickel in the high-concentration zinc and other metal ion background, the high selectivity and high precision determination of the nickel content is realized. Moreover, the coulometric analysis does not need to prepare a standard solution, and is easy to realize online determination; no titrant needs to be prepared, the Faraday's law of electrolysis is applied, the titrant I2 is generated in situ under constant current, and the accuracy of the analysis result is ensured. By using the coulometric titration method of the present application to determine the nickel, the determination can be quickly completed, the sample amount required is small, and the accuracy of the constant analysis can be achieved by using a small amount of sample for determination, and the detection efficiency can be significantly improved. DETAILED DESCRIPTION

[0019] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application 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 application are only used to distinguish different parts of the same material for the purpose of description, and the ratio is not limited.

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

[0022] As described in the background of the present application, there is a problem in the prior art that the high concentration of zinc in the zinc-nickel plating solution leads to the inability to achieve rapid, accurate and high-selectivity determination of the nickel content. In order to solve the above problem, in a typical embodiment of the present application, a method for analyzing nickel in a zinc-nickel electroplating solution is provided, comprising the following steps: step S1, dividing a 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; dividing a KI electrolyte with a pH of 2.5-3.5 into two parts, namely a first part of the KI electrolyte and a 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 cup to perform standard coulometric titration, and calculating the actual molar concentration c of the sodium thiosulfate standard solution according to formula (1) 标准 ;

[0023] wherein i1 is the electrolytic current of the standard coulometric titration, in mA; t1 is the titration end time of the standard coulometric titration, in s; z is a 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, in mL;

[0024] Step S2, adding an oxidizing agent solution and a sodium hydroxide solution to the zinc-nickel electroplating solution, and after water washing, obtaining a high-nickel hydroxide precipitate; adding a KI solution and sulfuric acid to the high-nickel hydroxide precipitate to obtain a sample solution;

[0025] Step S3, adding the sample solution, the second part of the sodium thiosulfate standard solution and the second part of the KI electrolyte into the electrolytic cup to perform sample coulometric titration, and calculating 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, in mL; V 样品 is the volume of the sample solution, in mL; t 测定 is the titration end time of the sample coulometric titration, in s; t2 is the theoretical titration end time of the second part of the sodium thiosulfate standard solution, calculated according to formula (3), in s; wherein i2 is the electrolytic current of the sample coulometric titration, in mA.

[0027]

Analysis principle

[0028] The inventors unexpectedly found in the research process that in an alkaline system, nickel ions will be converted into nickel hydroxide, and the reaction formula is Ni 2+ + OH- = Ni(OH)2↓, then adding oxidant solution can make the nickel hydroxide further converted into high nickel hydroxide precipitate Ni(OH)3, after adding KI solution and adjusting pH to acidic, under this condition, high nickel hydroxide Ni(OH)3 can oxidize KI, the generated I2 can be quantitatively reduced by sodium thiosulfate. The concentration of the accurately added sodium thiosulfate solution before and after the reaction can be accurately determined by coulometric titration method, and then the content of nickel is indirectly calculated. Therefore, by the redox reaction, using the valence change of nickel hydroxide, using the indirect method, using potassium iodide as the electrolyte, the content of nickel ion is determined by coulometric titration method, and the content of nickel can also be accurately determined independently in the presence of a large amount of zinc ion and other metal ions.

[0029]

Coulometric Titration

[0030] The present application applies coulometric titrator to carry out coulometric titration analysis, which integrates terminal control unit, coulometric titration unit, i.e. 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 is also called constant current coulometric analysis, which is collectively referred to as coulometric analysis with constant potential coulometric analysis. The basic basis of coulometric analysis is Faraday's electrolysis law, which is an electric analysis method based on the measurement of the amount of electricity consumed by the electrochemical reaction of the measured substance on the electrode in the electrolysis process. The theoretical basis of this method requires that the current efficiency is 100%.

[0031] In general chemical volumetric analysis, the titrant is added from a burette, while in coulometric titration, the titrant is generated in situ by constant current electrolysis and reacts quantitatively with the measured substance. Since the amount of electrolytically generated titrant is directly proportional to the amount of electricity consumed, this method is actually a volumetric analysis method using electrons as titrant. Its advantages are high sensitivity, good accuracy, no need to prepare standard solution for titration, unstable titrant can be electrolytically generated, current and time can be accurately measured, etc. The end point indication of coulometric titration can be achieved by chemical indicator method, potential method or double platinum electrode current indication method (also known as permanent stop method).

[0032]

Standard Coulometric Titration

[0033] Specifically, before performing the coulometric titration of the sample solution, the sodium thiosulfate standard solution is calibrated to obtain the post-reaction concentration of the accurately added sodium thiosulfate standard solution in the sample coulometric titration. The sodium thiosulfate standard solution and KI electrolyte are added to the electrolytic cup with the lowest volume immersed in the electrode, pre-stirring for about 10 seconds, and then standard coulometric titration is performed.

[0034] KI electrolyte solution can be used for coulometric titration method to determine the thiosulfate ion in the solution, which uses constant current electrolysis of KI to oxidize I2, and the electrode reaction is: 2I- - 2e→ I2.

[0035] The generated I2 is used as a titrant, and it is mixed with excess S2O3 2- The quantitative redox reaction occurs in a neutral or weakly acidic solution, and the titration reaction is as follows: 2Na2S2O3 + I2 = Na2S4O6 + 2NaI.

[0036] The electrolysis current and the titration end time are recorded using a coulometric titrator, and the content of S2O3 2- in the solution can be calculated according to the amount of electricity consumed during electrolysis:

[0037] The amount of electricity consumed during electrolysis Q = electrolysis current i x electrolysis time t, and the electrolysis time is the titration end time;

[0038] At this time, the amount of electricity Q 标准 of standard coulometric titration = i1 x t1;

[0039] The amount of substance n 2- of S2O3 标准 in the sodium thiosulfate standard solution is: 标准 Q / z / F = c 标准 x V 标准1 :

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

[0041]

Sample solution preparation

[0042] Specifically, first, an oxidizing agent solution and a sodium hydroxide solution are added to the zinc-nickel electroplating solution. Under alkaline conditions, nickel ions will produce light green Ni(OH)2 precipitate: Ni 2+ + OH - = Ni(OH)2↓. Under the action of the added oxidizing agent 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 high-nickel hydroxide Ni(OH)3 precipitate. Taking potassium persulfate as an example, the precipitate conversion process is as follows: Filter and thoroughly wash with water to remove the oxidizing agent solution in the solution to obtain high-nickel hydroxide precipitate. Then, KI solution and sulfuric acid are added to the black high-nickel hydroxide precipitate to make the pH of the solution acidic. At this time, Ni(OH)3 oxidizes KI to generate I2, obtaining a sample solution containing nickel ions and I2 (the amount of substance is n 碘 ).

[0043]

Sample coulometric titration

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

[0045] The sample solution, sodium thiosulfate standard solution (n 标 , corresponding to V 标准2 ) and KI electrolyte are added into the electrolytic cup to immerse the electrode with the minimum volume, and pre-stirring for about 10 seconds as the test solution for sample coulometric titration. The residual amount n 测 of sodium thiosulfate after reaction is determined by coulometric titration. 标 According to n 测 -n 碘 =2n 镍 =n 镍 , the molar concentration c - of nickel in the zinc-nickel electroplating solution can be calculated.

[0046] During the sample coulometric titration process, there are two sources of I2, one is I2 obtained by oxidizing KI electrolyte with constant current electrolysis, electrode reaction: 2I 2- -2e→I2, which acts as a titrant; the other is I2 generated by Ni(OH)3 oxidizing KI solution in the sample solution, which acts as an indirect determination of the content of nickel.

[0047] The two sources of I2 and the excess S2O3 2- in the sodium thiosulfate standard solution in the test solution undergo redox reaction quantitatively, and in neutral or weakly acidic solution, the titration reaction is as follows: 2Na2S2O3+I2=Na2S4O6+2NaI. Using the coulometric titrator to record the electrolysis current and the titration end time, the content of S2O3 2- in the solution can be calculated according to the amount of electricity consumed during electrolysis:

[0048] Since I2 generated by Ni(OH)3 oxidizing KI in the sample solution will consume part of S2O3 2- , when the sample solution is determined, part of the I2 oxidized by S2O3 样品溶液中由Ni(OH)3氧化KI生成的I2消耗的 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 sodium thiosulfate standard solution, the titration end time of sample coulometric titration will be shorter.

[0049] At this time 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 =i1×t2; Q 样品溶液中由电解KI生成的I2消耗的 =i2×t 测定 ,

[0051] We can get:

[0052]

[0053]

[0054] The molar concentration c of nickel in the final available zinc-nickel electroplating solution 镍 The calculation formula is as follows:

[0055]

[0056] Wherein, t2 is the theoretical titration end point time of the second part of 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 of sodium thiosulfate standard solution and the second part of sodium thiosulfate standard solution are the same (V 标准1 = V 标准2 ), the corresponding theoretical titration end point times of the two are also the same (t1=t2) under the condition that the electrolytic current is unchanged, and at this time, C 标准 *V 标准1 >C 镍 *V 样品 .

[0059] In summary, considering that there are a large number of interfering ions in the zinc-nickel electroplating solution, the present application establishes a coulometric titration method for quantitative determination of nickel, which indirectly determines nickel in a high concentration of zinc and other metal ions background by using electrogenerated iodine. The present application can effectively and highly selectively determine nickel by using the change of the valence state of the hydroxide of nickel, and is a high-efficiency analysis method for nickel ions Ni 2+ in the zinc-nickel electroplating solution, and is also a cyanide-free coulometric titration method for quantitative determination of nickel. Moreover, the coulometric analysis does not need to prepare a standard solution, and is easy to realize online determination; without preparing a titrant, the Faraday's law of electrolysis is applied, the titrant is generated in situ under constant current, and the accuracy of the analysis result is ensured. The coulometric titration method of the present application for determination of nickel can quickly complete the determination, needs a small amount of sample, and can achieve the accuracy of constant analysis by using a small amount of sample for determination, which can significantly improve the detection efficiency and provide more reliable data support for quality control of zinc-nickel electroplating solution.

[0060] It can be seen that the application utilizes the valence state change characteristics of nickel hydroxide, converts nickel into high-nickel hydroxide directly, generates iodine by reacting with potassium iodide, and indirectly determines the content of nickel by a reduction reaction with sodium thiosulfate solution. The concentration of nickel ions is determined by indirect coulometric titration, which can effectively eliminate the interference of other metal ions such as zinc, realize high-selectivity determination, improve the accuracy and efficiency of nickel content determination, quickly and accurately obtain the content of nickel, provide strong data support for the control and optimization of electroplating process, and at the same time, the method does not need to configure a standard solution, is easy to realize online determination, saves time and cost, solves the problem of accurate determination of the content of nickel in a complex electroplating solution, especially in the presence of a large amount of zinc ions and other metal ions, and the traditional analysis method is difficult to accurately, quickly and selectively determine the content of nickel.

[0061] It should be noted that in the preparation process of the sample solution, the precipitates generated by each reaction are separated by filtration or suction filtration, but not by centrifugal separation, because the inventors unexpectedly found in the research process that a small amount of precipitate will float during centrifugation, and the loss of the filtration method is less, and suction filtration once is enough to complete the recovery of the precipitate, and the conversion of the precipitate will not cause obvious sample filtration loss.

[0062] In a preferred embodiment, the electrolysis current in the standard coulometric titration and the sample coulometric titration is 5-15 mA, and the minimum electrolysis time is 0.5-2 s. By controlling the electrolysis parameters in the above range, the electrolysis speed can be effectively controlled, the current efficiency can be maintained, the determination accuracy can be improved, and the precision of the electrolysis reaction can be considered. 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 determination accuracy. The above range can make the reaction more complete, shorten the analysis time, improve the work efficiency, further ensure the efficiency and accuracy of the electrolysis process, and more effectively selectively determine the concentration of nickel ions in the zinc-nickel alloy plating solution.

[0063] In order to accurately determine the potential of the solution system, and then accurately determine the titration end point, while considering the reaction completion degree and work efficiency, in a preferred embodiment, the equilibrium potential in the standard coulometric titration and the sample coulometric titration is 0.5-2 mV, and the equilibrium time is 1-20 s. By accurately controlling the potential equilibrium conditions of the solution system, the influence of potential fluctuation on the determination result can be further reduced, and the stability, precision and accuracy of the determination result 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 determination result is higher.

[0065] In view of the complexity of the sample composition of the zinc-nickel alloy plating solution, a platinum electrode with good stability and strong corrosion resistance is preferably used as the generating electrode and auxiliary electrode. In a preferred embodiment, independently in the standard coulometric titration and the sample coulometric titration, the positive electrode of the electrolysis electrode 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 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 electrode configuration is more conducive to achieving 100% current efficiency during electrolysis and accurate and stable potential measurement, and can effectively improve the electrolysis efficiency, reduce electrode pollution, and achieve long-term stable operation.

[0066] In order to further promote the titration reaction between the electrolytic titrant and the measured substance after the electrolysis reaction to react quickly and completely in 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 the addition volume of the standard solution, the method can be adapted to the determination of different nickel contents, further improving the application range of the method, and being suitable for the analysis of various concentrations of plating solutions.

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

[0068] In order to fully react and accurately calibrate the sodium thiosulfate standard solution, in a preferred embodiment, 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 is (110-500 μL):(70-90 mL), thereby further improving the accuracy of the determination result of the nickel content in the zinc-nickel alloy plating solution. Such volume ratio can also promote the full contact between the standard solution and the electrolyte, and is conducive to further improving the reaction efficiency.

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

[0070] For the purpose of more fully precipitating zinc and nickel in the zinc-nickel plating solution and more fully oxidizing the precipitate to the target valence (trivalent nickel hydroxide), 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 plating 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 to the feed liquid until the pH is ≥10.

[0071] In a preferred embodiment, the molar concentration of the KI solution is 0.1-0.3 mol / L, the volume ratio of the zinc-nickel plating 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 to the feed liquid until the pH is ≤4. Under the above conditions, the high-nickel hydroxide can be more fully reduced to divalent nickel ions, and ultimately a sample solution containing divalent nickel ions and I2 is obtained, which is more convenient for subsequent coulometric titration and improves the accuracy of the determination results.

[0072]

Technical path of coulometric titration analysis

[0073] In the actual coulometric titration process, the supporting electrolyte (such as KI of the present application) undergoes electrode reaction at the electrode, generates a titrant, reacts with the sample solution to obtain a titration product, and records the electric quantity after the end point indication, and calculates the component concentration according to the formula.

[0074] For example, the specific operation of the coulometric titration analysis of the present application 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] The constant current electrolysis of KI is used, and the obtained I2 is oxidized as a titrant, so that the S2O3 2- concentration can be quantitatively oxidized, and the S2O3

[0079] Working electrode reaction:

[0080] Titration reaction:

[0081] The electrolysis product I2 in the solution and S2O3 2- can be quantitatively reacted, and a coulometric titrator can be used to calculate the content of S2O3 2- in the solution according to the amount of electricity consumed in the electrolysis process.

[0082] 2) Titration mode parameter setting: according to the specific needs of the experiment, the titration mode parameters are reasonably set, covering the electrolysis current, the minimum electrolysis time, the program control of the electrolysis time and the balance time according to the balance potential, the end point jump amount, and the pre-electrolysis time and other key elements.

[0083] 3) Sample processing and determination: the sample solution of the zinc-nickel electroplating solution is accurately quantitatively taken, and appropriate sample pretreatment is performed as needed to facilitate further content determination. Subsequently, the established perfect device is used for coulometric titration analysis.

[0084] 4) Titration process monitoring: during the titration process, the instrument will display the current potential value and the electrolysis time in real time, and synchronously draw the titration curve, including the potential corresponding to the electrolysis time curve and the first derivative corresponding to the electrolysis time curve, to realize the visual monitoring of the titration process.

[0085] 5) End point determination: according to the extreme value of the first derivative corresponding to the electrolysis time curve, the instrument can automatically and accurately find the titration end point, thereby ensuring the reliability of the analysis results.

[0086] 6) Result calculation: the concentration of the sample component to be measured is calculated according to the amount of electricity consumed in the electrolysis, and the following classical formula is used for calculation:

[0087]

[0088] wherein c represents the concentration of the sample component to be measured (mol / L); I represents the electrolysis current (mA); t ep represents the electrolysis time corresponding to the end point (s); V0 represents the sample volume (mL); z represents the conversion coefficient; and F represents the Faraday constant (96485 C / mol). This formula ensures the accuracy of the determination of the metal ion concentration.

[0089] The application will be further described in detail in combination with specific embodiments, and these embodiments cannot be understood as limiting the scope of the application.

[0090]

Coulometric titrator

[0091] Equipped with electrolytic electrode positive electrode (double platinum sheet), electrolytic electrode negative electrode (single platinum wire, glass tube filled with 3 mol / L sulfuric acid); indicating electrode (single platinum sheet), reference electrode (black tungsten wire, glass tube filled with saturated potassium sulfate solution).

[0092]

Calibration of sodium thiosulfate standard solution

[0093] 1 drop (about 0.05 mL) of H2SO4 solution (2 mol / L), 80 mL of KI solution (0.2 mol / L) was added to the electrolytic cup to immerse the electrode electrolytic double platinum sheet as the minimum volume, adjust the pH so that the pH of the KI electrolyte is about 3.

[0094] Accurately take 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.1 mol / L), add to the above KI electrolyte (volume ratio 110 μL:80 mL), pre-stir for 10 seconds, standard coulometric titration (electrolytic 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 , multiple determinations, the 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 precision of the results of the KI supporting electrolyte coulometric titration method is high, which meets the determination requirements.

[0096] Table 2

[0097]

[0098]

[0099] Example 1 Determination of nickel standard solution

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

[0101] Step S2, accurately take 0.100 mL (V 样品= 0.100 mL) nickel standard solution (theoretical molar concentration 0.3060 mol / L) into an electrolytic cup, adding sodium hydroxide solution (molar concentration 2 mol / L) into the nickel standard solution to make the pH of the feed liquid ≥10, adding oxidant solution (potassium persulfate solution, molar concentration 0.1 mol / L, volume ratio of nickel standard solution to oxidant solution 0.1:1), removing the oxidant solution in the solution after filtering and sufficiently washing with water to obtain a high-nickel hydroxide precipitate; adding 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) into the high-nickel hydroxide precipitate to make the pH of the feed liquid ≤3 to obtain a sample solution.

[0102] Step S3, adding 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte into the sample solution to immerse the electrode as the minimum volume, controlling the pH at about 3, pre-stirring for 10 seconds, and performing sample coulometric titration (electrolysis current 10 mA, minimum electrolysis time 1 s, equilibrium potential 1 mV, and equilibrium time 10 s); the electric quantity value Q measured by the coulometric titrator is 1882 mC, t = 188.2 s, and the molar concentration c of nickel in the nickel standard solution is calculated according to formulas (2) and (3). 测定 测定 镍

[0103] According to formula (3), the following is obtained:

[0104] 0.500 mL (V 标准2 = 0.500 mL) of the sodium thiosulfate standard solution is c 标准 * V 标准2 = i2*t2 = 4830 mC, t2 = 483.0 s;

[0105] According to formula (2), the following is obtained:

[0106] The relative error of the determination result is 0.2%, and the relative error = |measured value - theoretical value| ÷ theoretical value x 100%.

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

[0108] Step S1, determination of the actual molar concentration c 标准 of the sodium thiosulfate standard solution is shown in the calibration part of the sodium thiosulfate standard solution.

[0109] Step S2, accurately taking 0.200 mL (V 样品 ​​​= 0.200 mL) zinc-nickel standard solution (theoretical concentration of zinc is 0.1368 mol / L, and theoretical concentration of nickel is 0.1530 mol / L) into an electrolytic cup, adding sodium hydroxide solution (molar concentration is 2 mol / L) into the zinc-nickel standard solution until the pH of the feed liquid is greater than or equal to 10, adding 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), removing the oxidant solution in the solution after filtering and sufficiently water washing, and obtaining high-nickel hydroxide precipitate; adding 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) into the high-nickel hydroxide precipitate until the pH of the feed liquid is less than or equal to 3, and obtaining sample solution.

[0110] Step S3, adding 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte into the sample solution to immerse the electrode with the minimum volume, controlling the pH to be about 3, pre-stirring for 10 seconds, and performing 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); t 测定 = 186.0 s, and the molar concentration c 镍 of nickel in the zinc-nickel standard solution is calculated according to formulas (2) and (3).

[0111] According to formula (3), the following can be obtained:

[0112] 0.500 mL (V 标准2 = 0.500 mL) sodium thiosulfate standard solution, the theoretical electric quantity value Q of the sodium thiosulfate standard solution is c 标准 * V 标准2 = i2*t2 = 4830 mC, and t2 = 483.0 s.

[0113] Substituting into formula (2) can obtain:

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

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

[0116] Step S1, the determination of the actual molar concentration c 标准 of the sodium thiosulfate standard solution is shown in the calibration part of the sodium thiosulfate standard solution.

[0117] Step S2, accurately taking 1.500 mL (V 样品= 1.500 mL) zinc-nickel plating solution (theoretical concentration of nickel 0.02167 mol / L) into an electrolytic cup, adding sodium hydroxide solution (molar concentration 2 mol / L) into the zinc-nickel plating solution to make the pH of the solution ≥ 10, adding oxidant solution (sodium persulfate solution, molar concentration 2 mol / L, volume ratio of zinc-nickel plating solution to oxidant solution 1.5:1), removing the oxidant solution in the solution after filtration and sufficient water washing, to obtain high-nickel hydroxide precipitate; adding KI solution (molar concentration 0.2 mol / L, volume ratio of zinc-nickel plating solution to KI solution 1.5:10) and sulfuric acid (molar concentration 2 mol / L) into the high-nickel hydroxide precipitate to make the pH of the solution ≤ 3, to obtain sample solution.

[0118] Step S3, adding 0.500 mL of sodium thiosulfate standard solution and 80 mL of KI electrolyte into the sample solution to immerse the electrode with the minimum volume, controlling the pH at about 3, pre-stirring for 10 seconds, performing sample coulometric titration (electrolysis current 10 mA, minimum electrolysis time 1 s, equilibrium potential 1 mV, equilibrium time 10 s), t 测定 = 168.6 s, and calculating the molar concentration c 镍 of nickel in the zinc-nickel plating solution according to formulas (2) and (3).

[0119] According to formula (3), we have:

[0120] 0.500 mL (V 标准2 = 0.500 mL) of sodium thiosulfate standard solution, the theoretical electric quantity value Q = c 标准 * V 标准2 = i2t2 = 4830 mC, t2 = 483.0 s.

[0121] Substituting into formula (2), we have:

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

[0123] Example 4

[0124] Step S1, 2 drops (about 0.10 mL) of H2SO4 solution (2 mol / L), 70 mL of KI solution (0.2 mol / L) were added into the electrolytic cup to immerse the electrode electrolysis double platinum sheet as the minimum volume, the pH was adjusted so that the KI electrolyte pH was about 3. 110 μL of sodium thiosulfate standard solution (theoretical concentration 0.1 mol / L) was accurately pipetted into the above KI electrolyte (volume ratio 110 μL:80 mL), pre-stirring for 10 seconds, standard library coulometric titration was carried out (electrolysis current 10 mA, minimum electrolysis time 1 s, equilibrium potential 1 mV, equilibrium time 10 s), and the actual molar concentration c of the sodium thiosulfate standard solution was calculated according to formula (1) 标准 .

[0125] Step S2, 1.500 mL (V 样品 =1.500 mL) of zinc-nickel electroplating solution (theoretical concentration of nickel 0.02167 mol / L) was accurately pipetted into the electrolytic cup, sodium hydroxide solution (molar concentration 2 mol / L) was added into the zinc-nickel electroplating solution to make the pH of the solution ≥10, oxidant solution (sodium persulfate solution, molar concentration 2 mol / L, volume ratio of zinc-nickel electroplating solution to oxidant solution 1.5:1) was added, and after filtration and sufficient water washing, the oxidant solution in the solution was removed to obtain high-nickel hydroxide precipitate; KI solution (molar concentration 0.2 mol / L, volume ratio of zinc-nickel electroplating solution to KI solution 1.5:2) and sulfuric acid (molar concentration 2 mol / L) were added into the high-nickel hydroxide precipitate to make the pH of the solution ≤3 to obtain sample solution.

[0126] Step S3, 0.500 mL of sodium thiosulfate standard solution and 70 mL of KI electrolyte were added into the sample solution to immerse the electrode as the minimum volume, the pH was controlled at about 3, pre-stirring for 10 seconds, sample coulometric titration was carried out (electrolysis current 10 mA, minimum electrolysis time 1 s, equilibrium potential 1 mV, equilibrium time 10 s), and the molar concentration c of nickel in the zinc-nickel electroplating solution was calculated according to formula (2) and (3) 镍 , 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 was 0.02172 mol / L, the relative error of the determination result was 0.2%, and the experimental error (relative range) of two determinations was 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, 110 μL of the sodium thiosulfate standard solution (theoretical concentration 0.05 mol / L) is accurately pipetted into the KI electrolyte solution (volume ratio 110 μL:90 mL), pre-stirring for 10 seconds, standard library coulometric titration is performed (electrolysis current 5 mA, minimum electrolysis time 2 s, equilibrium potential 0.5 mV, and equilibrium time 20 s), and the actual molar concentration c of the sodium thiosulfate standard solution is calculated according to formula (1). 标准 .

[0134] Step S2, 1.500 mL (V 样品 =1.500 mL) of the zinc-nickel plating solution (theoretical concentration of nickel 0.02167 mol / L) is accurately pipetted into the electrolytic cup, sodium hydroxide solution (molar concentration 1.5 mol / L) is added into the zinc-nickel plating solution until the pH of the solution is ≥10, oxidant solution (sodium persulfate solution, molar concentration 1.5 mol / L, volume ratio of the zinc-nickel plating solution to the oxidant solution 1.5:1.5) is added, the oxidant solution is removed after filtration and sufficient water washing, and high-nickel hydroxide precipitate is obtained; KI solution (molar concentration 0.1 mol / L, volume ratio of the zinc-nickel plating solution to the KI solution 1.5:10) and sulfuric acid (molar concentration 1.5 mol / L) are added into the high-nickel hydroxide precipitate until the pH of the solution is ≤3, and the sample solution is obtained.

[0135] Step S3, 0.400 mL of the sodium thiosulfate standard solution and 70 mL of the KI electrolyte solution are added into the sample solution, the electrode is immersed to the minimum volume, the pH is controlled at about 3, pre-stirring for 10 seconds, sample coulometric titration is performed (electrolysis current 5 mA, minimum electrolysis time 2 s, equilibrium potential 0.5 mV, and equilibrium time 20 s), and the molar concentration c of nickel in the zinc-nickel plating solution is calculated according to formula (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) into the above KI electrolyte (volume ratio 110 μL:70 mL), pre-stir for 10 seconds, and perform standard coulometric titration (electrolysis current 15 mA, minimum electrolysis time 0.5 s, equilibrium potential 2 mV, and equilibrium time 2 s), and 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 =1.500 mL) of zinc-nickel electroplating solution (theoretical concentration of nickel 0.02167 mol / L) into an electrolytic cup, add sodium hydroxide solution (molar concentration 2.5 mol / L) to the electrolyte until the pH of the electrolyte is ≥10, add oxidant solution (sodium persulfate solution, molar concentration 2.5 mol / L, volume ratio of zinc-nickel electroplating solution to oxidant solution 1.5:0.5), filter and sufficiently wash with water to remove the oxidant solution from the electrolyte, and obtain a high-nickel hydroxide precipitate; add KI solution (molar concentration 0.3 mol / L, volume ratio of zinc-nickel electroplating solution to KI solution 1.5:1.5) and sulfuric acid (molar concentration 2.5 mol / L) to the high-nickel hydroxide precipitate until the pH of the electrolyte is ≤3, and obtain a sample solution.

[0141] Step S3, add 0.600 mL of the sodium thiosulfate standard solution and 90 mL of KI electrolyte to the sample solution, with the electrode immersed to the minimum volume, control the pH at about 3, pre-stir for 10 seconds, and perform sample coulometric titration (electrolysis current 15 mA, minimum electrolysis time 0.5 s, equilibrium potential 2 mV, and equilibrium time 2 s), and calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formula (2) and (3) 镍 .

[0142] Comparative Example 1

[0143] The content of nickel in the zinc-nickel electroplating solution (theoretical concentration of nickel 0.02167 mol / L) was determined using the dimethylglyoxime gravimetric method (standard GB / T 223.25-1994 "Chemical analysis method for steel and alloy-dimethylglyoxime gravimetric method for determining the content of nickel"). The measured concentration of nickel was 0.01897 mol / L, and the relative error of the determination result was 12.5%.

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

[0145] Table 4

[0146]

[0147] From the above, compared with the comparative examples, each embodiment of the present application uses the valence change of nickel hydroxide, combines the redox reaction with the indirect coulometric titration method, obtains a cyanide-free coulometric titration method for quantitatively determining the nickel content in a zinc-nickel electroplating solution, indirectly determines nickel in a high-concentration zinc metal ion background by using electrogenerated iodine, realizes high selectivity and high precision in the determination of the nickel content. Moreover, the coulometric analysis does not need to prepare a standard solution, is easy to realize online determination; does not need to prepare a titrant, uses the Faraday's law of electrolysis, in-situ electrogenerates the titrant I2 under a constant current, and guarantees the accuracy of the analysis results. The coulometric titration method of the present application for determining nickel can quickly complete the determination, needs a small amount of sample, and can achieve the accuracy of constant analysis by using a small amount of sample for determination, and 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 application, the comprehensive effect is better.

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

Claims

1. A method for analyzing nickel in a zinc-nickel electroplating solution, characterized in that, Includes 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. The KI electrolyte with a pH of 2.5~3.5 was divided into two parts, namely the first part of the KI electrolyte and the second part of the KI electrolyte; The first portion of sodium thiosulfate standard solution and the first portion of KI electrolyte were added to an electrolytic cup for standard coulometric titration. The actual molar concentration c of the sodium thiosulfate standard solution was calculated according to formula (1). 标准 ; （1）； Where i1 is the electrolytic 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 This refers to the volume of the first portion of sodium thiosulfate standard solution, in mL; Step S2: Add oxidant solution and sodium hydroxide solution to zinc-nickel electroplating solution, wash with water to obtain high nickel hydroxide precipitate; add KI solution and sulfuric acid to the high nickel hydroxide precipitate to obtain sample solution; Step S3: Add the sample solution, the second portion of sodium thiosulfate standard solution, and the second portion of KI electrolyte to the electrolytic cup, and perform sample coulometric titration. Calculate the molar concentration c of nickel in the zinc-nickel electroplating solution according to formula (2). 镍 ; （2）； Among them, V 标准2 V represents the volume of the second portion of the sodium thiosulfate standard solution, in mL. 样品 t represents the volume of the sample solution, in mL. 测定 t1 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; （3）； Where i2 is the electrolytic current of the sample coulometric titration, in mA.

2. The analytical 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 analytical 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 0.5~2 mV and the equilibrium time is 1~20 s.

4. The analytical method according to claim 1 or 2, characterized in that, Independently, in the standard coulometric titration and the sample coulometric titration, The positive electrode used in the electrolysis is a double platinum sheet, and the negative electrode is a single platinum wire. 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 analytical method according to claim 1 or 2, characterized in that, The theoretical molar concentration of the sodium thiosulfate standard solution is 0.05~0.2 mol / L.

6. The analytical method according to claim 1 or 2, characterized in that, The molar concentration of the KI electrolyte is 0.05~0.5 mol / L.

7. The analytical method according to claim 1 or 2, characterized in that, In the standard coulometric titration, the volume ratio of the first portion of sodium thiosulfate standard solution to the first portion of KI electrolyte is (110~500μL):(70~90mL).

8. The analytical method according to claim 1 or 2, characterized in that, In the sample coulometric titration, the volume ratio of the zinc-nickel electroplating solution, the second part sodium thiosulfate standard solution, and the second part KI electrolyte is 1.5 mL: (0.4~0.6 mL): (70~90 mL).

9. The analytical method according to claim 1 or 2, characterized in that, The oxidizing agent solution is a sodium persulfate solution or a potassium persulfate solution, wherein 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 oxidizing agent solution is 1.5:(0.5~1.5); and / or The sodium hydroxide solution has a molar concentration of 1.5~2.5 mol / L, and the sodium hydroxide solution is added until the pH of the feed solution is ≥10.

10. The analytical method according to claim 1 or 2, 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 sulfuric acid has a molar concentration of 1.5~2.5 mol / L, and the sulfuric acid is added until the pH of the feed solution is ≤4.

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