Determination of arsenic by galvanostatic stripping analysis and its application to steels

Arsenic can be determined by galvanostatic stripping analysis with a modified gold-film electrode, prepared by simultaneous electro-deposition of gold and arsenic on a glassy-carbon support. This initially deposited arsenic is stripped from the film before the electrode is used for the analysis, but its presence during the formation of the film apparently leads to uniformly distributed crystallization sites for the subsequent determination of arsenic, so that the precision of the determination is better than that obtained with an unmodified gold-film electrode. The pre-electrolysis is performed potentiostatically in a stirred solution of 7M hydrochloric acid, at a potential from -0.10 to -0.35 V (vs. Ag/AgCl). If the galvanostatic stripping step is performed in quiescent solution after a 6-sec rest-period, at a current of 2-12 muA, then for a 100-sec pre-electrolysis time the calibration curve is linear up to an arsenic concentration of about 2mug ml and the limit of determination is 8 ng ml . In the analysis of steel, the arsenic must first be separated by selective extraction of arsenic(III) bromide into toluene and back-extraction into the supporting electrolyte. A single extraction is virtually 100% quantitative. The relative error of the determination in steels is a few per cent and the results are in good agreement with the certified values for reference materials and with the results obtained by X-ray fluorescence analysis.