Trivalent vanadium electrogenerated from tetravalent vanadium at the mercury cathode has been used as a coulometric intermediate.In sodium acetate-hydrochloric acid buffer solution of about pH 3, the current efficiency was found to be near 100%.Milligram quantities of Fe(Ⅲ), Ⅴ(Ⅴ), Cr(Ⅵ), and Mn(Ⅶ) were titrated with electro-generated Ⅴ(Ⅲ), with dead-stop end point detection, the relative errors being less than 1%.Evidently oxidants with potentials higher than that of mercury can be determined coulometrically at the mercury cathode.