Determination of copper constant current procedure

Discussion, Copper may be deposited from either sulphuric or nitric acid solution, but, usually, a mixture of the two acids is employed. If such a solution is electrolysed with an e.m.f. of 2-3 volts the following reactions occur  

The acid concentration of the solution must not be too great, otherwise the deposition of the copper may be incomplete or the deposit will not adhere satisfactorily to the cathode. The beneficial effect of nitrate ion is due to its depolarising action at the cathode  

The reduction potential of the nitrate ion is lower than the discharge potential of hydrogen, and therefore hydrogen is not liberated. The nitric acid must be free from nitrous acid, as the nitrite ion hinders complete deposition and introduces other complications. The nitrous acid may be removed (a) by boiling the nitric acid before adding it, (b) by the addition of urea to the solution  

The action is rapid, and the acidity of the electrolyte is unaffected. The error due to nitrous acid is increased by the presence of a large amount of iron iron is reduced by the current to the iron(II) state, whereupon the nitric acid is reduced. This error may be minimised by the proper regulation of the pH and by the addition of ammonium nitrate instead of nitric acid, or, best, by the removal of the iron prior to the electrolysis, or by complexation with phosphate or fluoride. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

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