VOLTAMIUETRIC DETERMIXATION OF IODIDE AXD BROUIDE -AT THE ROTATING PYROLYTIC GKAPHITE ELECTRODE

Since halogens present in admixture are difficult to separate, it is desirable to be able to determine directly one halogen in the presence of others. The problem, however, of determining very low concentrations of a halide (except fluoride), especially in the presence of large excesses of other halides, has only been resolved with some difficulty_ Excellent reviews with many references on the determination of the halogens aregivenby KOLTHOFFANDSTEKGER~, and ARBISTRONG,GILLAKD ROLF~. Typical of the methods described is the well-known procedure of VAN DER MEULES~ for determiningbromidein the presence of chloride on a micro scale In this method bromide is oxidized to bromate with hypochlonte and, after removal of excess oxidant, the bromate is determin ed iodometrically; iodide, if present, is determined as bromide and a correctio‘n must be applied by determining iodide alone. Iodide can be determined in the presence of bromide and chloride by oxidation to iodate with bromine; after removal of excess o.xidant, the iodate is determined iodometrically- (cf- refs. I and 5). Polarography at the dropping mercury electrode has been used to determine the halogens in terms of the anodic waves that mercury yields in their presencelv”. However, the half-wave potentials for the waves due to chloride, bromide and iodide are so close together that the analysis of mixtures is difficult, if not impossible, especially when one of the halogens is present in excess. Polarography has been used extensively for the determination of bromide and iodide; these are first oxidized to bromate and iodate. ELVING AND 3x1~~4 showed that the anodic decomposition potentials of aqueous solutions of chloride, bromide and iodide were well separated at a rotating waximpregnated spectroscopic graphite electrode. It thus seemed probable that a method for the differential determination of the halogens might be possible in terms of their differential oxidation at a graphite indicating electrode. This possibility was investigated and methods were developed for the determination of iodide and of bromide_