Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.