Method for determining zinc content in zinc-nickel plating solution

Abstract

The application provides a method for determining the zinc content in a zinc-nickel plating solution. The method comprises: adding a first potassium ferricyanide-sulfuric acid supporting electrolyte and a first zinc standard solution into an electrolytic cup, performing first coulometric titration, and obtaining the actual determination time t of the standard solution 标 ; adding a second potassium ferricyanide-sulfuric acid supporting electrolyte, a second zinc standard solution and a pretreated sample into the electrolytic cup, performing second coulometric titration, and obtaining the actual determination time t of the sample to be measured 样 ; then calculating the zinc molar concentration c in the zinc-nickel plating solution by using formula (I) and formula (II) 样 The application establishes a coulometric titration analysis method for zinc ions in a zinc-nickel plating solution. [Fe(CN)6] 4‑ is generated by electrolysis, and a quantitative precipitation reaction of [Fe(CN)6] 2+ and Zn is utilized. The improved standard addition method based on a zinc standard solution can quickly, accurately and on-line determine the zinc content in an alkaline zinc-nickel plating solution.

Description

TECHNICAL FIELD

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

[0002] Zinc-nickel alloy electroplating is a steel anode type protective coating, and the corrosion resistance of zinc-nickel alloy coating is higher than that of zinc plating layer, and it also has the advantages of good paintability, weldability, formability and low hydrogen brittleness. Thus it has been widely used in the automobile, aerospace, light industry, household appliance and other industries, especially for replacing the higher toxic cadmium plating process, and good social and environmental benefits have been achieved. In recent years of research and application, the industry has accumulated rich experience in using alkaline zinc-nickel alloy electroplating process, but there are still some problems in plating solution maintenance, such as complex composition of alkaline zinc-nickel plating solution, difficulty in plating solution analysis, etc., which need further research.

[0003] The technical solutions disclosed in application No. 201510332653.4 "New method for determining the content of zinc in alkaline zinc-nickel alloy plating solution" and application No. 201711499436.X "Rapid analysis method for the content of zinc in alkaline zinc-nickel alloy plating solution" both use sample pretreatment methods to separate or mask interfering substances. Under suitable acid-base conditions, the chemical volumetric method based on EDTA complexation reaction principle is used, but it is not easy to realize rapid and accurate online determination of the content of zinc in the plating solution. Application No. 201310319864.5 "Method for determining the content of zinc and nickel in zinc-nickel alloy electroplating solution" uses spectrophotometry to determine the content of nickel and complexometric titration to determine the total amount of zinc and nickel, and indirectly obtains the content of zinc, which increases the determination steps and introduces more errors into the results. SUMMARY

[0004] The main purpose of the present application is to provide a method for determining the content of zinc in zinc-nickel plating solution, so as to solve the problem that it is difficult to quickly and accurately determine the content of zinc in alkaline zinc-nickel plating solution in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a method for determining the content of zinc in zinc-nickel plating solution is provided, which uses a coulometric titration reactor for determination; uses potassium ferricyanide-sulfuric acid electrolyte as supporting electrolyte, which is equally divided into two parts, namely first potassium ferricyanide-sulfuric acid supporting electrolyte and second potassium ferricyanide-sulfuric acid supporting electrolyte; uses zinc standard solution as standard solution, which is equally divided into two parts, namely first zinc standard solution and second zinc standard solution; the determination method comprises the following steps: step S1, adding the first potassium ferricyanide-sulfuric acid supporting electrolyte into the electrolytic cup of the coulometric titration reactor, then adding the first zinc standard solution, and performing first pre-stirring until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then performing first coulometric titration to obtain the actual determination time t 标; step S2, taking the zinc-nickel plating solution, adjusting pH≤3 to obtain a pretreated sample; step S3, adding a second potassium ferricyanide-sulfuric acid supporting electrolyte into an electrolytic cup of the coulometric titration reactor, then adding a second zinc standard solution and the pretreated sample, and performing a second pre-stirring until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then performing a second coulometric titration to obtain an actual measurement time t of the sample to be measured 样 ; step S4, using formula (I) to calculate the measurement coefficient I / zF of the first and second coulometric titrations respectively, and when the difference between the two is less than or equal to ±0.01, using formula (II) to calculate the molar concentration c of zinc in the zinc-nickel plating solution 样 :

[0006]

[0007] wherein I is the electrolysis current, in mA; z is the conversion coefficient, 2 / 3; F is the Faraday constant, 96485 C / mol; c0 is the measured molar concentration of zinc in the solution, in M; V0 is the volume of the solution measured, in mL; t ep is the electrolysis time corresponding to the end point, in s;

[0008]

[0009] wherein V 标 is the volume of the first zinc standard solution, in mL; c 标 is the molar concentration of zinc in the first zinc standard solution, in M; V 样 is the volume of the pretreated sample, in mL.

[0010] Further, the potassium ferricyanide-sulfuric acid electrolyte is prepared by the following method: adding a sulfuric acid solution to a potassium ferricyanide electrolyte; preferably, the pH of the potassium ferricyanide electrolyte is 1-6, the molar concentration is 0.05-0.15 M, and the molar concentration of the sulfuric acid solution is 1.5-2.5 M; preferably, the volume ratio of the potassium ferricyanide electrolyte to the sulfuric acid solution is (50-100):(0.05-0.1).

[0011] Further, the molar concentration of the zinc standard solution is 0.05-0.5 M.

[0012] Further, in step S1, the volume ratio of the first potassium ferricyanide-sulfuric acid electrolyte to the first zinc standard solution is (50-100):(0.1-0.2).

[0013] Further, in step S2, the pH of the zinc-nickel plating solution is adjusted using a sulfuric acid solution.

[0014] Further, in step S2, the volume ratio of the zinc-nickel plating solution to the sulfuric acid solution is (0.1-0.25):0.3.

[0015] Further, in step S3, the volume ratio of the second potassium ferricyanide-sulfuric acid electrolyte to the zinc-nickel plating solution is (50-100):(0.1-0.25); and / or the volume ratio of the zinc-nickel plating solution to the second zinc standard solution is (0.1-0.25):(0.08-0.2).

[0016] Further, the first pre-stirring time is 30-60s; and / or the second pre-stirring time is 60-120s.

[0017] Further, the electrolysis current of the coulometric titration reactor is 8-12mA.

[0018] Further, the working electrode of the coulometric titration reactor is a double platinum sheet, the auxiliary electrode is a platinum wire electrode with saturated K2SO4 solution in a glass tube, the indicating electrode is a platinum sheet electrode and a tungsten wire electrode, and the glass tube of the indicating electrode is filled with saturated K2SO4 solution.

[0019] The technical solution of the present application establishes a coulometric titration analysis method for zinc ions in a zinc-nickel plating solution, generates [Fe(CN)6] 4- by electrolysis, and develops a coulometric titration method by using the quantitative precipitation reaction of [Fe(CN)6] 2+ with Zn 样 to quickly, accurately and on-line determine the zinc content in an alkaline zinc-nickel plating solution. Based on the accurate determination of the Zn(II) standard solution, the present application adopts a modified standard addition method with the zinc standard solution as the base, linearly correlates the actual determination time with the concentration of the standard solution, and converts the Zn(II) concentration in the zinc ion standard solution by internal standard conversion to obtain the zinc molar concentration c 样 in the zinc-nickel plating solution. The method can complete the determination in a short time and ensure the accuracy of the analysis results. This analysis method not only significantly improves the detection efficiency, but also provides more reliable data support for the quality control of zinc-nickel plating solution electroplating. DETAILED DESCRIPTION

[0020] 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.

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

[0022] As described in the background of the present application, there is a problem in the prior art that it is difficult to quickly and accurately determine the zinc content in an alkaline zinc-nickel plating solution. In order to solve the above problem, in a typical embodiment of the present application, a method for determining the zinc content in a zinc-nickel plating solution is provided, which is determined using a coulometric titration reactor; a potassium ferricyanide-sulfuric acid electrolyte is used as a supporting electrolyte, which is divided into two parts, namely a first potassium ferricyanide-sulfuric acid electrolyte and a second potassium ferricyanide-sulfuric acid electrolyte; a zinc standard solution is used as a standard solution, which is divided into two parts, namely a first zinc standard solution and a second zinc standard solution; the determination method comprises the following steps:

[0023] Step S1, a first potassium ferricyanide-sulfuric acid supporting electrolyte is added to the electrolytic cup of the coulometric titration reactor, then a first zinc standard solution is added, and a first pre-stirring is performed until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then a first coulometric titration is performed to obtain the actual determination time t 标 of the standard solution; Step S2, a zinc-nickel plating solution is taken, and the pH is adjusted to be less than or equal to 3 to obtain a pretreated sample; Step S3, a second potassium ferricyanide-sulfuric acid supporting electrolyte is added to the electrolytic cup of the coulometric titration reactor, then a second zinc standard solution and the pretreated sample are added, and a second pre-stirring is performed until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then a second coulometric titration is performed to obtain the actual determination time t 样 of the sample to be measured; Step S4, the determination coefficients of the first coulometric titration and the second coulometric titration are calculated using formula (I), and when the difference between the two is less than or equal to ±0.01, the determination coefficient range between parallel samples is less than 2%, then the zinc molar concentration c 样 in the zinc-nickel plating solution is calculated using formula (II):

[0024]

[0025] wherein I is the electrolysis current, in mA; z is the conversion coefficient, 2 / 3; F is the Faraday constant, 96485 C / mol; c0 is the zinc molar concentration of the solution to be determined, in M (i.e. mol / L); V0 is the volume of the solution to be determined, in mL; t ep is the electrolysis time corresponding to the end point, in s;

[0026]

[0027] wherein V 标 is the volume of the first zinc standard solution, in mL; c 标 is the zinc molar concentration of the first zinc standard solution, in M; V 样 is the volume of the pretreated sample, in mL.

[0028] The application directly determines the content of zinc in a zinc-nickel plating solution in a potassium ferricyanide-sulfuric acid electrolyte by using a coulometric titration analysis method, and overcomes the error caused by a heterogeneous precipitation determination system by using a zinc standard solution to improve a standard addition method, so that the content of zinc in an alkaline zinc-nickel plating solution can be determined quickly, accurately and on-line. The specific description is as follows.

[0029]

Coulometric titration

[0030] Coulometric titration is also called constant current coulometric analysis, and is collectively referred to as coulometric analysis with constant potential coulometric analysis. The basic basis of coulometric analysis is Faraday's electrolysis law, and is an electric analysis method based on measuring 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 be 100%. In ordinary 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 in electrolysis, this method is actually a volumetric analysis method that uses electrons as the titrant. Its advantages are high sensitivity and good accuracy, does not require the preparation of a standard solution for titration, unstable titrants can be electrolytically generated, and the current and time can be accurately measured.

[0031] Specifically, the application uses a coulometric titrator to perform coulometric titration analysis. The coulometric titrator integrates a terminal control unit, a coulometric titration unit, i.e., a constant current and potential (mV / pH) measurement unit, to ensure high precision and high stability of electrolysis control and potential measurement. The coulometric titrator includes at least three types of electrodes, namely a working electrode (electrolysis cathode), an auxiliary electrode (electrolysis anode), and an indicator electrode. The coulometric titration method for determining the content of zinc requires a small amount of sample, and the accuracy of constant analysis can be achieved by determining a small amount of sample, which is easy to realize on-line determination and does not require the preparation of a titrant. The Faraday's electrolysis law is used to generate the titrant in situ by constant current.

[0032] Titration mode parameter setting: According to the specific needs of the experiment, the titration mode parameters can be reasonably set, covering electrolysis current, minimum electrolysis time, program control of electrolysis time and equilibrium time according to equilibrium potential, end point jump amount, and pre-electrolysis time, etc. These are understood by those skilled in the art and will not be described here.

[0033] Titration process monitoring: During the titration process, the instrument will display the current potential value and electrolysis time in real time, and synchronously draw the titration curve, including the potential vs. electrolysis time curve and the first derivative vs. electrolysis time curve, to realize visual monitoring of the titration process.

[0034] End point determination: According to the extreme value of the first derivative vs. electrolysis time curve, the instrument can automatically and accurately find the titration end point, thereby ensuring the reliability of the analysis results.

[0035]

Potassium ferricyanide-sulfuric acid supporting electrolyte

[0036] The present application utilizes constant current electrolysis of [Fe(CN)6] 3- The reduction of [Fe(CN)6] 4- As a titrant, it reacts quantitatively with Zn 2+ The precipitation is generated to determine Zn 2+ The concentration, whose electrode reaction is as follows:

[0037] Working electrode reaction: [Fe(CN)6] 3- + e - → [Fe(CN)6] 4-

[0038] Titration reaction: 3Zn 2+ + 2K + + 2[Fe(CN)6] 4- = K2Zn3[Fe(CN)6]2(s)

[0039] When certain conditions are met (such as pH of 1-6), the electrolysis product [Fe(CN)6] 4- in the solution can quantitatively react with Zn 2+ Using a coulometric titrator, the content of Zn 2+ in the solution can be calculated according to the amount of electricity consumed in the electrolysis process.

[0040] When the pH of the solution is in the range of 1-3, the generated precipitate all exists in the form of K2Zn3[Fe(CN)6]2, and the gradual increase of pH will gradually generate Zn2[Fe(CN)6].

[0041] According to the above basic principle, the K3[Fe(CN)6] electrolyte solution can be used for coulometric titration to determine zinc ions in the solution. Under the condition of assuming that the current efficiency is 100%, the theoretical determination time t 理 is calculated based on Faraday's law. However, the basic titration reaction is the quantitative precipitation reaction of the electrolysis generated [Fe(CN)6] 4- with Zn 2+ , which is affected by the influence of the precipitation dissolution equilibrium on the current efficiency, and the composition of the reaction product K4[Fe(CN)6] is also affected by the pH of the solution system. The actual determination time will be less than the theoretical determination time, and the use of zinc standard solution to improve the standard addition method can reduce the error caused by the above factors.

[0042] And, using potassium ferricyanide precipitation reaction to determine zinc ions, when the content ratio of zinc and nickel is larger, the selectivity for zinc is better, the interference of nickel ions can be overcome by combining the zinc standard solution to improve the standard addition method, the content of zinc ions can be accurately determined, and the usable pH range of the electrolyte can be further expanded to 1-6.

[0043]

Zinc standard solution improved standard addition method

[0044] The inventors unexpectedly found during research that in a conventional zinc-nickel plating solution, excessive strong complexing agents (such as polyene polyamines: triethylene tetramine, diethylene triamine, etc.) often exist, which not only make it difficult to apply the chemical volumetric method (indicator determines the end point, and blocks the indicator), but also cause tailing of potential change, lag of titration end point, and large determination error when directly determining the change of solution potential by the coulometric titration method. Therefore, a new analysis and determination method is established in the present application, which balances the complexing agent in the original zinc-nickel plating solution by adding excessive zinc standard solution, so that the sample does not need to be pretreated complicatedly. At the same time, on the basis of accurately determining the content of Zn(II) standard solution, the improved standard addition method is adopted, i.e. the sample addition method with zinc standard solution as the base, to avoid the problems of affecting current efficiency due to the generation of precipitate and affecting product composition due to pH, and to linearly correlate the actual determination time with the concentration of the standard solution, so that the zinc molar concentration in the zinc-nickel plating solution is obtained by internal standard conversion through the concentration of Zn(II) in the zinc ion standard solution under the same environment.

[0045] The conventional standard addition method usually uses the sample to be determined as the matrix (generally in a large amount), and uses the standard solution as the internal standard (generally in a small amount), to determine the content of a specific component in the sample and the spiked sample, so as to eliminate the background interference of the sample. However, the inventors found that when the content of nickel in the zinc-nickel plating solution is high, the nickel will react with [Fe(CN)6] 3- to generate a precipitation reaction, and after the sample is mixed with the electrolyte and before electrolysis occurs, the solution will be precipitated to become a heterogeneous system, resulting in an increase in determination error.

[0046] Therefore, the improved standard addition method is adopted in the present application, the standard zinc solution is used as a large proportion of special matrix, the zinc-nickel plating solution sample to be determined is used as a small proportion of special internal standard, the standard zinc solution is used as the background, and by reducing the proportion of the zinc-nickel plating solution sample to be determined, the interference of nickel ion precipitation on the determination system and the error caused by the precipitation reaction system are reduced as much as possible, so that the content of zinc ions can be accurately determined, and the range of solution acid-base adaptation during analysis and determination can be increased.

[0047] Specifically, in this invention, a first potassium ferricyanide-sulfuric acid supporting electrolyte is first added to the electrolytic vessel of the coulometric titration reactor, followed by the addition of a first zinc standard solution. The mixture is then pre-stirred until the potential value of the coulometric titration reactor stabilizes, specifically with a change of less than or equal to ±10 mV per minute. Then, the first coulometric titration is performed to obtain the actual measurement time t of the standard solution. 标 Next, add a second potassium ferricyanide-sulfuric acid supporting electrolyte to the electrolytic cup of the coulometric titration reactor, then add a second zinc standard solution, a zinc-nickel plating solution with pH ≤ 3 for pretreatment of the sample, and an appropriate amount of sulfuric acid solution (e.g., 6 drops (0.3 mL) 2M). Stir to remove air bubbles (alkaline zinc-nickel plating solutions may contain sodium carbonate, which generates carbon dioxide upon acidification), and then perform a second pre-stirring until the potential value of the coulometric titration reactor stabilizes, specifically, the change is less than or equal to ±10 mV per minute; then perform the second coulometric titration to obtain the actual measurement time t of the sample. 样 .

[0048] Finally, formula (I) was used to calculate the determination coefficients of the first and second coulometric titrations, respectively. When the difference between the two was less than or equal to ±0.01, the range of determination coefficients between parallel samples was less than 2%, proving that the determination system was stable. Then, formula (II) was used to calculate the zinc molar concentration c in the zinc-nickel plating solution. 样 .

[0049] c0 = I·t ep / z·F·V0

[0050] t ep =z·F·V0·c0 / I

[0051] n = V0·c0

[0052] It is derived that This is the determination coefficient of this method, which can ensure the accuracy of metal ion concentration determination.

[0053] t 样 / t 标 =(V 样 c 样 +V 标 c 标 ) / V 标 c 标

[0054] t 样 V 标 c 标 =t 标 (V 样 c 样 +V 标 c 标 )

[0055] t 标 V样 c 样 =t 样 V 标 c 标 -t 标 V 标 c 标

[0056] It is derived that The molar concentration of zinc in the zinc-nickel plating bath, c, is obtained by internal standard conversion using the concentration of Zn(II) in the zinc ion standard solution. 样 Furthermore, it can be converted into zinc mass concentration as needed.

[0057] In summary, this invention establishes a coulometric titration method for zinc ions in zinc-nickel plating solutions. This method involves electrolysis to generate ferrocyanide ions, which are then used to react with Zn... 2+ Based on the quantitative precipitation reaction, a coulometric titration method was developed for the rapid and accurate determination of Zn in zinc-nickel plating solutions. 2+ Based on the accurate determination of Zn(II) standard solution, a modified standard addition method using zinc standard solution as a base was adopted to linearly correlate the actual measurement time with the concentration of the standard solution. Under the same environment, the molar concentration c of zinc in the zinc-nickel plating bath was obtained by internal standard conversion through the concentration of Zn(II) in the zinc ion standard solution. 样 This method can complete the determination in a short time (e.g., 30 minutes) and ensures the accuracy of the analytical results. This analytical method not only significantly improves the detection efficiency, but also provides more reliable data support for the quality control of zinc-nickel plating solutions.

[0058] In a preferred embodiment, the potassium ferricyanide-sulfuric acid electrolyte is prepared by adding a sulfuric acid solution to a potassium ferricyanide electrolyte. Preferably, the pH of the potassium ferricyanide electrolyte is 1-6, the molar concentration is 0.05-0.15M, and the molar concentration of the sulfuric acid solution is 1.5-2.5M. Preferably, the volume ratio of the potassium ferricyanide electrolyte to the sulfuric acid solution is (50-100):(0.05-0.1), which allows for easier control of the electrolyte pH within suitable conditions and further improves the accuracy of the measurement results.

[0059] Accordingly, in a preferred embodiment, the molar concentration of the zinc standard solution is 0.05–0.5 M, which is more suitable for the determination system of the present invention.

[0060] In order to further reduce the influence of the zinc standard solution on the concentration of the supporting electrolyte in the electrolyte, thereby improving the accuracy of zinc determination, in a preferred embodiment, in step S1, the volume ratio of the first potassium ferricyanide-sulfuric acid electrolyte to the first zinc standard solution is (50-100):(0.1-0.2).

[0061] In a preferred embodiment, in step S2, the pH of the zinc-nickel plating solution is adjusted using a sulfuric acid solution, which can quickly adjust the pH while avoiding affecting the composition of the zinc-nickel plating solution, thereby improving the accuracy of the determination results.

[0062] Based on similar reasons, in a preferred embodiment, in step S2, the volume ratio of the zinc-nickel plating solution to the sulfuric acid solution is (0.1-0.25):0.3.

[0063] As described above, the present application uses a modified standard addition method, in which a standard zinc solution is used as a large proportion of a special matrix, and a zinc-nickel plating solution sample to be measured is used as a small proportion of a special internal standard. Specifically, in a preferred embodiment, in step S3, the volume ratio of the second potassium ferricyanide-sulfuric acid electrolyte to the zinc-nickel plating solution is (50-100):(0.1-0.25); and / or the volume ratio of the zinc-nickel plating solution to the second zinc standard solution is (0.1-0.25):(0.08-0.2). Under the above conditions, the interference of nickel ion precipitation with the determination system and the error caused by the precipitation reaction system can be further reduced by controlling the amount of the zinc-nickel plating solution sample added, thereby more accurately determining the content of zinc ions.

[0064] In order to facilitate actual operation, the present application provides an empirical value of the pre-stirring time when the potential value of the coulometric titration reactor is stable. In a preferred embodiment, the first pre-stirring time is 30-60 s; and / or the second pre-stirring time is 60-120 s.

[0065] Based on similar reasons, in a preferred embodiment, the electrolysis current of the coulometric titration reactor is 8-12 mA.

[0066] Considering the complexity of the sample composition, in a preferred embodiment, the working electrode of the coulometric titration reactor is a double platinum sheet, and the auxiliary electrode is a platinum wire electrode with a saturated K2SO4 solution in a glass tube, which has better stability and stronger corrosion resistance. In order to ensure the universality and accuracy of the method, a potential method is selected as the end-point indicating method, and the indicating electrode is a platinum sheet electrode and a tungsten wire electrode, and the glass tube of the indicating electrode is filled with a saturated K2SO4 solution.

[0067] The present application will be further described in detail below in conjunction with specific examples, which should not be construed as limiting the scope of the present application.

[0068] Unless otherwise specified, the instruments and reagents used in the following examples and comparative examples can be self-made or conveniently purchased on the market using conventional methods in the art, and some instruments and reagents are as follows:

[0069] Working electrode (electrolytic anode): double platinum sheet;

[0070] Auxiliary electrode (anolyte): platinum wire electrode with saturated K2SO4 solution in glass tube;

[0071] Indicating electrode: platinum sheet electrode and tungsten wire electrode with saturated K2SO4 solution in glass tube;

[0072] Sulfuric acid solution: 2M, 1.5M, 2.5M;

[0073] Potassium ferricyanide electrolyte: 0.1M, 0.05M, 0.15M, M 329.24g / mol;

[0074] Zinc ion standard solution: 0.2735M, prepared by using high purity zinc sheet and 2M sulfuric acid solution;

[0075] Zinc-nickel plating solution sample: containing zinc 5-15g / L, nickel 0.5-2g / L, sodium hydroxide 115-135g / L, sodium carbonate less than 45g / L, other additives 120-130mL / L. The sample using conventional ICP method of zinc mass concentration correction determination value is 7.86mg / mL.

[0076] Example 1

[0077] Step S1: 1 drop (0.05mL) of 2M sulfuric acid solution, 80mL of 2M potassium ferricyanide electrolyte was added to the electrolytic cup, to immerse the double platinum sheet working electrode as the minimum volume, using 2M sulfuric acid solution to adjust the electrolyte pH at 2-3. Accurately take 100μL of zinc ion Zn(II) standard solution, add to the above electrolyte, pre-stirring for 45 seconds to the potential value is basically stable (change ±10mV), then start the first coulomb titration: electrolytic current is set to 10mA, the minimum electrolysis time and potential equilibrium time is set according to the experimental process needs, the electrolysis time and potential equilibrium time program process setting in this example is shown in Table 1. The actual determination time t 标 of the standard solution is obtained, using formula (I) to calculate the determination coefficient, see Table 2. Parallel determination for 8 times.

[0078] Step S2: take 170μL of zinc-nickel plating solution sample, add 6 drops (0.3mL) of 2M sulfuric acid solution, stir, drain the bubbles, control the pH at 2-3, to obtain the pretreated sample.

[0079] Step S3: 1 drop (0.05 mL) of 2M sulfuric acid solution, 100 mL of 2M potassium ferricyanide electrolyte were added into the electrolytic cup to immerse the double platinum sheet working electrode with the minimum volume, and 2M sulfuric acid solution was used to adjust the electrolyte pH to 2-3. The pretreated sample was added into the electrolyte, and 80 μL of zinc standard solution was accurately added, and pre-stirring was performed for 90 seconds until the potential value was basically stable (change ±10 mV), and then the second coulometric titration was started: the electrolysis current was set to 10 mA, and the minimum electrolysis time and potential balance time were set according to the experimental process, and the setting of the electrolysis time and potential balance time program process in this example is shown in Table 3. The actual determination time t of the sample was obtained 样 .

[0080] Step S4: the determination coefficient was calculated using formula (I), and the c 样 was calculated using formula (II), and was converted into the mass concentration of Zn (II), which is shown in Table 4.

[0081] Example 2

[0082] The difference from Example 1 is that in step S3, 100 μL of zinc standard solution is added.

[0083] Example 3

[0084] The difference from Example 1 is that in step S3, 120 μL of zinc standard solution is added.

[0085] Table 1

[0086] / Total electrolysis phase length (s) Minimum electrolysis time (s) Potential equilibration time (s) Stage 1 100 10 0.5 Stage 2 200 1 1

[0087] Table 2

[0088]

[0089]

[0090] Table 3

[0091] / Total electrolysis phase length (s) Minimum electrolysis time (s) Potential equilibration time (s) Stage 1 200 10 0.5 Stage 2 360 1 1

[0092] Table 4

[0093]

[0094] From Table 2, the precision of the determination method of the zinc content in the zinc-nickel plating solution meets the general determination requirements. From Table 4, the accuracy of the determination method of the zinc content in the zinc-nickel plating solution meets the general determination requirements, the average mass concentration of the zinc-nickel plating solution sample is 7.69 mg / mL, the difference from the corrected determination value of the zinc mass concentration obtained by using the conventional ICP method is only 0.17 mg / mL, and the difference meets the general determination requirements. In summary, the determination method can be used for rapid and accurate determination of the zinc content in the zinc-nickel plating solution.

[0095] Example 4

[0096] Step S1: 1 drop (0.05 mL) of 2M sulfuric acid solution and 80 mL of 2M potassium ferricyanide electrolyte were added to an electrolytic cup to immerse the double platinum sheet working electrode as the minimum volume, and 2M sulfuric acid solution was used to adjust the pH of the electrolyte to 2-3. 200 μL of a zinc ion Zn(II) standard solution was accurately taken and added to the above electrolyte, and pre-stirring was performed for 45 seconds until the potential value was basically stable (change ±10 mV), and then the first coulometric titration was started: the electrolysis current was set to 10 mA, and the minimum electrolysis time and the potential balance time were set according to the experimental process. In this embodiment, the setting process of the electrolysis time and the potential balance time is shown in Table 5. The actual determination time t of the standard solution was obtained 标 , the determination coefficient was calculated using formula (I), and is shown in Table 6. The determination was performed in parallel for 5 times.

[0097] Step S2: 100 μL of the zinc-nickel plating solution sample was taken, 6 drops (0.3 mL) of 2M sulfuric acid solution was added, stirring was performed, air bubbles were discharged, and the pH was controlled to be 2-3 to obtain a pretreated sample.

[0098] Step S3: 1 drop (0.05 mL) of 2M sulfuric acid solution and 70 mL of 2M potassium ferricyanide electrolyte were added to an electrolytic cup to immerse the double platinum sheet working electrode as the minimum volume, and 2M sulfuric acid solution was used to adjust the pH of the electrolyte to 2-3. The above pretreated sample was added to the above electrolyte, and 200 μL of a zinc standard solution was accurately added, pre-stirring was performed for 90 seconds until the potential value was basically stable (change ±10 mV), and then the second coulometric titration was started: the electrolysis current was set to 10 mA, and the minimum electrolysis time and the potential balance time were set according to the experimental process. The actual determination time t of the sample was obtained 样 .

[0099] Step S4: the determination coefficient was calculated using formula (I), and the c 样 was calculated using formula (II), and was converted into the mass concentration of Zn(II), and is shown in Table 7.

[0100] Example 5

[0101] The difference from Example 4 is that in step S2, 150 μL of the zinc-nickel plating solution sample is taken, 6 drops (0.3 mL) of 2M sulfuric acid solution are added, it is stirred, the air bubbles are removed, and the pH is controlled at 2-3 to obtain the pretreated sample.

[0102] Example 6

[0103] The difference from Example 4 is that in step S2, 200 μL of the zinc-nickel plating solution sample is taken, 6 drops (0.3 mL) of 2M sulfuric acid solution are added, it is stirred, the air bubbles are removed, and the pH is controlled at 2-3 to obtain the pretreated sample.

[0104] Example 7

[0105] The difference from Example 4 is that in step S2, 250 μL of the zinc-nickel plating solution sample is taken, 6 drops (0.3 mL) of 2M sulfuric acid solution are added, it is stirred, the air bubbles are removed, and the pH is controlled at 2-3 to obtain the pretreated sample.

[0106] Example 8

[0107] Step S1 : 2 drops (0.1 mL) of 1.5M sulfuric acid solution and 100 mL of 0.05M potassium ferricyanide electrolyte are added to the electrolytic cup to immerse the double platinum working electrode to the minimum volume, and 2M sulfuric acid solution is used to adjust the pH of the electrolyte to 2-3. 200 μL of the zinc ion Zn(II) standard solution is accurately taken and added to the above electrolyte, and it is pre-stirred for 30 seconds until the potential value is basically stable (change ±10 mV), and then the first coulometric titration is started: the electrolysis current is set to 8 mA, and the minimum electrolysis time and potential balance time are set according to the experimental process, and the actual measurement time t of the standard solution is obtained. 标 The determination coefficient is calculated using formula (I), as shown in Table 8.

[0108] Step S2: 100 μL of the zinc-nickel plating solution sample is taken, 6 drops (0.3 mL) of 2M sulfuric acid solution are added, it is stirred, the air bubbles are removed, and the pH is controlled at 2-3 to obtain the pretreated sample.

[0109] Step S3: 2 drops (0.1 mL) of 1.5M sulfuric acid solution and 100 mL of 0.05M potassium ferricyanide electrolyte are added to the electrolytic cup to immerse the double platinum working electrode to the minimum volume, and 2M sulfuric acid solution is used to adjust the pH of the electrolyte to 2-3. The above pretreated sample is added to the above electrolyte, and 200 μL of the zinc standard solution is accurately added, and it is pre-stirred for 60 seconds until the potential value is basically stable (change ±10 mV), and then the second coulometric titration is started: the electrolysis current is set to 8 mA, and the minimum electrolysis time and potential balance time are set according to the experimental process, and the actual measurement time t of the sample is obtained. 样 .

[0110] Step S4: Calculate the determination coefficient using formula (I), and calculate c using formula (II) 样 , and convert to Zn(II) mass concentration, see Table 8 for details.

[0111] Example 9

[0112] Step S1: Add 1 drop (0.05 mL) of 2.5M sulfuric acid solution, 50 mL of 0.15M potassium ferricyanide electrolyte into the electrolytic cup, with the double platinum sheet working electrode submerged to the minimum volume, and use 2M sulfuric acid solution to adjust the electrolyte pH to 2-3. Accurately pipette 200 μL of zinc ion Zn(II) standard solution into the above electrolyte, pre-stir for 60 seconds until the potential value is basically stable (change ±10 mV), and then start the first coulometric titration: set the electrolysis current to 12 mA, and set the minimum electrolysis time and potential equilibrium time according to the experimental process needs. The actual determination time t of the standard solution is obtained. 标 , and convert to Zn(II) mass concentration, see Table 8 for details.

[0113] Step S2: Pipette 100 μL of zinc-nickel plating solution sample, add 6 drops (0.3 mL) of 2M sulfuric acid solution, stir, and remove air bubbles, to obtain a pretreated sample with pH controlled at 2-3.

[0114] Step S3: Add 1 drop (0.05 mL) of 2.5M sulfuric acid solution, 50 mL of 0.15M potassium ferricyanide electrolyte into the electrolytic cup, with the double platinum sheet working electrode submerged to the minimum volume, and use 2M sulfuric acid solution to adjust the electrolyte pH to 2-3. Add the above pretreated sample to the above electrolyte, and accurately add 200 μL of zinc standard solution, pre-stir for 120 seconds until the potential value is basically stable (change ±10 mV), and then start the second coulometric titration: set the electrolysis current to 12 mA, and set the minimum electrolysis time and potential equilibrium time according to the experimental process needs. The actual determination time t of the sample is obtained. 样 .

[0115] Step S4: Calculate the determination coefficient using formula (I), and calculate c using formula (II) 样 , and convert to Zn(II) mass concentration, see Table 8 for details.

[0116] Table 5

[0117] / Total electrolysis phase length (s) Minimum electrolysis time (s) Potential equilibration time (s) Stage 1 260 10 0.5 Stage 2 400 1 1

[0118] Table 6

[0119]

[0120]

[0121] Table 7

[0122]

[0123] Table 8

[0124]

[0125] From the above, the embodiments of the present application generate [Fe(CN)6] 4- by electrolysis, and develop a coulometric titration method by using the quantitative precipitation reaction thereof with Zn 2+ , so as to quickly, accurately and on-line determine the zinc content in alkaline zinc-nickel plating solution. On the basis of accurately determining the Zn(II) standard solution, the modified standard addition method is adopted with the zinc standard solution as the base, the actual determination time is linearly related to the concentration of the standard solution, the Zn(II) concentration in the zinc ion standard solution is converted by the internal standard, and the zinc molar concentration c 样 in the zinc-nickel plating solution is obtained. The method can complete the determination within 30 minutes, and ensures the accuracy of the analysis result, and significantly improves the detection efficiency.

[0126] 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. made 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 measuring the zinc content of a zinc-nickel plating solution, characterized by, The determination is performed using a coulometric titration reactor; a potassium ferricyanide-sulfuric acid electrolyte is used as a supporting electrolyte, and is divided into two parts, namely a first potassium ferricyanide-sulfuric acid electrolyte and a second potassium ferricyanide-sulfuric acid electrolyte; A zinc standard solution is used as a standard solution, and is divided into two parts, namely a first zinc standard solution and a second zinc standard solution; the determination method comprises the following steps: Step S1, adding the first potassium ferricyanide-sulfuric acid supporting electrolyte into the electrolytic cup of the coulometric titration reactor, then adding the first zinc standard solution, carrying out first pre-stirring until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then carrying out first coulometric titration to obtain the actual measurement time t of the standard solution 标 ; In step S2, the zinc-nickel plating solution is taken, and the pH is adjusted to be less than or equal to 3, so as to obtain a pretreated sample; Step S3, adding the second potassium ferricyanide-sulfuric acid supporting electrolyte into the electrolytic cup of the coulometric titration reactor, then adding the second zinc standard solution, the pretreated sample, and performing a second pre-stirring until the potential value of the coulometric titration reactor changes by less than or equal to ±10 mV per minute; then performing a second coulometric titration to obtain the actual measurement time t of the sample to be measured 样 ; Step S4, using formula (I) to calculate the determination coefficient I / zF of the first and second coulometric titration respectively, when the difference is less than or equal to ±0.01, using formula (II) to calculate the zinc molar concentration c in the zinc-nickel plating solution 样 : （I）； wherein I is the electrolysis current in mA; z is the conversion factor, 2 / 3; F is the Faraday constant, 96485 C / mol; c0is the molar concentration of zinc of the solution determined, in M; V0is the volume of the solution determined, in mL; t ep is the electrolysis time in s for the corresponding end point. (II); wherein V 标 is the volume of the first zinc standard solution in mL; c 标 is the zinc molarity of the first zinc standard solution in M; V 样 is the volume of the pretreated sample in mL.

2. The assay method according to claim 1, characterized by The potassium ferricyanide-sulfuric acid electrolyte is prepared by adding a sulfuric acid solution into a potassium ferricyanide electrolyte.

3. The assay method according to claim 2, characterized in that, The pH of the potassium ferricyanide electrolyte is 1-6, and the molar concentration is 0.05-0.15 M; the molar concentration of the sulfuric acid solution is 1.5-2.5 M.

4. The assay method according to claim 2, characterized by, The volume ratio of the potassium ferricyanide electrolyte to the sulfuric acid solution is (50-100):(0.05-0.1).

5. The assay method according to claim 1 or 2, characterized by, The molar concentration of the zinc standard solution is 0.05-0.5 M.

6. The assay method according to claim 1 or 2, characterized by, In step S1, the volume ratio of the first potassium ferricyanide-sulfuric acid electrolyte to the first zinc standard solution is (50-100):(0.1-0.2).

7. The assay method according to claim 1 or 2, characterized by, In step S2, the pH of the zinc-nickel plating solution is adjusted by using a sulfuric acid solution.

8. The assay method according to claim 1 or 2, characterized by, In step S2, the volume ratio of the zinc-nickel plating solution to the sulfuric acid solution is (0.1-0.25):0.

3.

9. The assay method according to claim 1 or 2, characterized by, In step S3, the volume ratio of the second potassium ferricyanide-sulfuric acid electrolyte to the zinc-nickel plating solution is (50-100):(0.1-0.25); and / or The volume ratio of the zinc-nickel plating solution to the second zinc standard solution is (0.1-0.25):(0.08-0.2).

10. The assay method according to claim 1 or 2, characterized by, The first pre-stirring time is 30-60 s; and / or the second pre-stirring time is 60-120 s.

11. The assay method according to claim 1 or 2, characterized by, The electrolytic current of the coulometric titration reactor is 8-12 mA.

12. The assay method according to claim 1 or 2, characterized by, The working electrode of the coulometric titration reactor is a double platinum sheet, the auxiliary electrode is a platinum wire electrode with a saturated K2SO4 solution filled in a glass tube, the indicating electrode is a platinum sheet electrode and a tungsten wire electrode, and the glass tube of the indicating electrode is filled with a saturated K2SO4 solution.

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