Polarographic reduction of pertechnetate

Abstract

Oxidation states were identified for the products of electrolytic reduction of pertechnetate at the mercury cathode in a variety of media, and for the products of reoxidation upon reversing the cell current. Tast polarography, pulse polarography and triangular-wave voltammetry were employed. The first cathodic wave in acidic phosphate was identified as Tc(VII) → Tc(III) by means of the ratio between it and the Tc(III) → Tc(IV) reoxidation wave obtained on anodic-sweep pulse polarography. This wave was then used as a standard to determine values of n in other media of the same ionic strength. The first wave in acidic media occurred at pH-dependent potential and was found generally to correspond to the formation of Tc(III). The Tc(III) could in some media be reoxidized at the electrode to Tc(IV) or Tc(V). For the second wave in acidic media, which occurred at −0.9 V, diffusion current measurements indicated that n≤7 with a catalytic component preventing determination of the exact value. In alkaline or unbuffered media the first wave occurred at −0.8 V and a second wave was sometimes seen at −1.0 V. The wave heights in alkaline media, while generally proportional to concentration of pertechnetate and varying with drop time in the manner expected for a diffusion current, often corresponded to non-integral values of n. In certain basic media the reduction of pertechnetate led to the formation of an insoluble oxidizable surface film. Under all conditions investigated the reduction of pertechnetate proved to be irreversible.