On a potentiometric - coulometric method for the quantitative determination of Vanadium

A potentiometric - coulometric method is described which allows to determin e Vanadium concentrations down to a level of 5*10(-5) M. In solutions of 1 to 3 M HClO4, VO2+ is quantitatively reduced to VO2+ by mercury metal, acco rding to reaction (1) VO2+ + Hg + 2H(+) double left right arrow VO2+ + 1/2 Hg-2(2+) + H2O (1) The equivalent amount of Hg-2(2+) produced is titrated either by cathodic r eduction or by anodic oxidation (depending upon the amount) on a mercury el ectrode which, together with an external auxiliary electrode, a constant cu rrent supply and the sample solution, constitute the coulometric circuit. T he solution, a H+ glass membrane and an additional Hg electrode form the ju nctionless electrochemical cell whose emf is measured after each step of cu rrent delivery. From the data emf vs, microfaradays emitted, the initial co ntent of V(V) in the sample can be calculated with an accuracy of a few ten ths of a micromol. In reaction (1) Hg may be reeplaced by Ag. In this case, however, only the oxidation option of Ag is advised, because the cathodic reduction of Ag+ ha s not proved to be quantitative, at the acidity levels adopted in this proc edure.