The acidic tin-zinc alloy plating bath contains stannous sulfate, zinc sulfate, citric acid, ammonium sulfate and brightener, etc. It is generally used to prepare steel, copper and zinc-aluminum alloy die-casting Protective coating of parts. Two analytical methods for zinc sulfate in the plating bath are introduced in the commonly used reference books: Method 1: Use ammonium fluoride to mask tin, use xylenol orange as indicator under the condition of pH=5.5, and titrate zinc with EDTA standard solution; Method 2 Use potassium sodium tartrate to mask tin, potassium cyanide to mask copper and other impurities, use chrome black T as an indicator under the condition of pH=10, and titrate zinc with EDTA. The test showed that the citric acid in the plating bath caused the titration end point to change slowly when the method was used, and a part of the zinc ions that formed the complex with citric acid could not be titrated by EDTA, resulting in low measurement results. The use of highly toxic cyanide as a masking agent in method two has been banned. To this end, through experimental research, a new method for the determination of zinc sulfate in acidic tin-zinc alloy plating bath was developed. 1. Analysis method 01 Principle Add hydrogen peroxide to oxidize the divalent tin in the acidic tin-zinc alloy plating solution to tetravalent tin, and oxidize the divalent iron to trivalent iron. Use sodium fluoride to mask tin ions and aluminum impurities, triethanolamine to mask iron impurities, and sodium dimethyldithiocarbamate to mask copper impurities. Under the condition of pH=10, using chrome black T as indicator, titrate zinc with EDTA standard solution. 02 Reagents (1) Hydrogen peroxide: 30% hydrogen peroxide aqueous solution. (2) Sodium fluoride: solid. (3) Triethanolamine solution: 50g/L triethanolamine aqueous solution. (4) Ammonia-ammonium chloride buffer solution: pH=10, ammonium chloride 54g/L, 25%-28% ammonia water 350mL/L. (5) Sodium dimethyldithiocarbamate solution: 10g/L sodium dimethyldithiocarbamate aqueous solution. (6) Chrome black T indicator: 0.2g of chrome black T and 100g of sodium chloride are ground and mixed evenly. (7) EDTA standard solution: 0.05mol/L EDTA-2Na aqueous solution. 03 Determination Pipe 5mL of acidic tin-zinc alloy plating solution into a 300mL conical flask, add 50mL of water, 0.5mL of hydrogen peroxide, 1g of sodium fluoride, 10mL of triethanolamine solution, 10mL of ammonia monoammonium chloride buffer solution, and dimethyl dithio 10 mL of sodium carbamate solution, a little chrome black T indicator, titrate with EDTA standard solution until the test solution turns from purple to blue as the end point. 04 Calculation  Where P(ZnSO4·7H20) is the mass concentration of zinc sulfate heptahydrate in the sample (g/L), 287.6 is the molar mass of zinc sulfate heptahydrate (g/mo1), and c is the concentration of EDTA standard solution (moFL), V is the volume (mL) of the EDTA standard solution consumed during titration, and Vo is the volume of the sample to be tested (5 mL in this method). 2. Discussion 01 Choice of indicator Prepare test solution A with analytical reagents: zinc sulfate heptahydrate 30.00g/L, citric acid monohydrate 80g/L. Pipe 5mL of test solution A in a 300mL conical flask, add 70mL of water, pH=5.5 hexamethylenetetramine-hydrochloric acid buffer solution. 10mL, 3 drops of xylenol orange indicator, titrated with EDTA, the end point changes very slowly, when the test solution turns yellow, P(ZnSO4·7H2O)=27.76g/L, the recovery rate is 92.53%. In the titration process of this method, a small part of the zinc ions coordinated by citric acid cannot form a purple-red complex with the xylenol orange indicator when approaching the end point, resulting in a significantly lower determination result. Pip 5mL of test solution A in a 300mL conical flask, add 70mL of water, 10mL of ammonia monoammonium chloride buffer solution, a little chrome black T indicator, and titrate with EDTA to blue. The end point changes sharply. P(ZnSO4 ·7H20)=29.99g/L, the recovery rate is 99.97%. It can be seen that chrome black T is used as an indicator when EDTA is used to titrate zinc, and citric acid has no effect on the determination. Therefore, this method chooses chrome black T as the indicator. 02 Masking of tin ions Prepare test solution B with analytical reagents: zinc sulfate heptahydrate 30.00g/L, stannous sulfate 20g/L, and citric acid monohydrate 80g/L. Pipette 5mL of test solution B into a 300mL conical flask, add 70mL of water, 1g of sodium fluoride, 0.5mL of hydrogen peroxide, 10mL of triethanolamine solution, 10mL of ammonia monoammonium chloride buffer solution, a little chrome black T indicator, and titrate with EDTA. Obtained P(ZnS04·7H20)=30.02g/L, and the recovery rate was 100.07%. It can be seen that sodium fluoride can completely mask tin ions when using this method to determine zinc. 03 Determination of the amount of sample drawn The concentration of zinc in the acidic tin-zinc alloy plating bath is low. This method draws 5 mL of the sample and can obtain the analysis result of 4 significant digits. 04 Precision and recovery rate Use analytical reagents to prepare acidic tin-zinc alloy plating solution: zinc sulfate heptahydrate 29.00g/L, stannous sulfate 22g/L, citric acid monohydrate 84g/L, and ammonium sulfate 66g/L. Then add 0.5 g/L of ferrous sulfate heptahydrate and aluminum sulfate octahydrate, and 0.05 g/L of copper sulfate pentahydrate. Using this method to determine 6 times in parallel, the obtained P(ZnS04·7H20) were 29.12, 28.96, 28.93, 28.96, 28.88 and 28.93g/L, with an average value of 28.96g/L and a relative average deviation of 0.17%. The recovery rate is 99.59%～100.41% 3. Conclusion The precision and accuracy of this method are high, which solves the problems of slow end-point discoloration and low determination results of the current method. |
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