Electrochemical Studies on Eriochrome Violet B and Its Lanthanide Complexes. I. Behaviour of Eriochrome Violet B and Its Lanthanide Complexes at the Dropping Mercury Electrode: D.C., A.C., and Cathode Ray Polarography

The polarographic behaviour of Eriochrome Violet B (5-sulpho-2-hydroxybenzene-azo-2-naphthol) and its lanthanide complexes has been investigated in some detail. Earlier workers found the dye to be reduced at the dropping mercury electrode in (I two-electron step to a stable hydrazo compound. The present work shows, however, that four electrons are involved in the reduction, and that the amines are the final reduction products. To explain the apparently anomalous polarographic behaviour of Eriochrome Violet B, a mechanism involving the disproportionation of an unstable hydrazo intermediate is proposed for the reduction. A diffusion current constant of 5 77 � 0.09 and a diffusion coefficient of 0.57 X cm2 sec-1 were found for the free dye in an ammonia buffer at 26 �C. The lanthanides combine with Eriochrome Violet B in a 1 : 2 ratio. These complexes produce discrete reduction steps which are separated from the free-dye step by a potential difference (ΔE) which depends on the atomic number of the lanthanide. In an ammonia buffer of pH 9.5 the ΔE values vary from 45 mV for lanthanum to 234 mV for lutetium.