Determination of antimony, copper, lead and tin in bearing metal controlled potential procedure

10 DETERMINATION OF ANTIMONY. COPPER, LEAD AND TIN IN BEARING METAL (CONTROLLED-POTENTIAL PROCEDURE) 

The principles of electrolysis with controlled cathode potential have been discussed in Section 12.6, and the details given below serve to illustrate the procedure. In this case the amounts of copper and antimony (which are deposited simultaneously) are small, and so the cathode potential can be set immediately to the limiting value, but with the higher proportion of tin it can be set initially to a value which is more positive than the limiting value so as to speed up the deposition process. 

Separate the copper and antimony by dissolving the deposit in a mixture of 5 mL concentrated nitric acid, 5 mL 40 per cent hydrofluoric acid (CARE), and 10 mL water boil off the nitrogen oxides, dilute to 150 mL, and add dropwise a solution of potassium dichromate until the liquid is distinctly yellow. Deposit the copper by electrolysing the solution at room temperature and limiting the cathode vs S.C.E. potential to —0.36 volt. Evaluate the weight of antimony by difference. 

Calculate the percentages of antimony, copper, lead, and tin in the alloy. 

In a similar determination described by Lingane and Jones,11 an alloy containing copper, bismuth, lead, and tin is dissolved in hydrochloric acid as described above, and then 100 mL of sodium tartrate solution (0.1 M) is added, followed by sufficient sodium hydroxide solution (5M) to adjust the pH to 5.0. After the addition of hydrazinium chloride (4 g), the solution is warmed to 70 °C and then electrolysed. Copper is deposited at —0.3 volt, and then sequentially, bismuth at —0.4 volt, and lead at —0.6 volt all cathode potentials quoted are vs the S.C.E. After deposition of the lead, the solution is acidified with hydrochloric acid and the tin then deposited at a cathode potential of — 0.65 volt vs the S.C.E. 

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