Precise coulometric determination of acids in cells without liquid junction. Part III. Determination of the silver error by amperostatic anodic stripping

Abstract

The plating and stripping of silver on platinum electrodes have been examined in the context of the determination of the silver error, which arises from the solubility of silver bromide when the deposition of bromide on a silver anode is used as the auxiliary reaction in the coulometric assay of acids. In order to calibrate the stripping method, it is necessary to plate known amounts of silver quantitatively on to platinum-gauze electrodes. Low recoveries are obtained when the platinum is not fully reduced. The oxidation and reduction of platinum electrode surfaces have been briefly examined, and it is demonstrated that oxide forms on an electrode when its potential is allowed to rise beyond 0·8 V, the termination potential in silver stripping. For calibration purposes, plating and stripping in a 0·1 M solution of silver nitrate in 0·1 M perchloric acid was first investigated. Amperostatic and potentiostatic reduction of the platinum electrode are shown to be ineffective, but chemical reduction leads to excellent plating and recoveries, provided great care is taken completely to remove all traces of reductant. Calibration being satisfactory, stripping in 0·1 M perchloric acid, as in an actual acidimetric assay, has been examined and shown to give excellent recoveries. The anodic stripping curves show an extended second wave, which is identified as arising from reduction of oxygen to hydrogen peroxide at the auxiliary stripping electrode, particularly when the latter becomes plated with silver. The hydrogen peroxide is oxidised at the stripping electrode, and the process is cyclic.