Controlled Potential Coulometry of Plutonium

Before operational amplifiers became available, constant current coulometry (CCC) was used more often than controlled potential coulometry (CPC) because the instrumentation was so much simpler. CCC continued to be used for highly precise U determinations in very pure materials (Goode et al. 1967 Malinowski 1967 Merciny et al. 1981). 

Formal potentials of Fe(ll]/Fe(lll) and Pu(lll)/Pu(IV) redox couples, volt versus standard calomel electrode (SCE) 

Chemistry may be combined with CPC to improve selectivity. For example (Jackson et al. 1980) applying the chemistry of the Davies and Townsend titration (seeO Sect. 63.4.4.3), Pu is first reduced electrolytically to Pu(III) in an hydrochloric/sulfamic acid mixture along with iron and other potential interfering species. Iron and a number of the other impurities are reoxidized next without reoxidizing Pu(in). Finally, phosphate is added to lower the redox potential of the Pu(III)/Pu(IV) couple to permit a quantitative electrooxidation of Pu(III) to Pu(IV), without oxidizing chloride ions. The procedure is very selective and is applicable to Pu determinations in the presence of uranium in 10 1 U/Pu molar ratio. 

CPC at a solid electrode has also been applied to determine uranium (Davies et al. 1970 Phillips and Crossley 1978), which is reduced from U(VI) to U(IV) with electrogenerated Fe(II). The approach has been used for sequential coulometry of U and Pu (Angeletti et al. 1969 Davies et al. 1970 Pardon and McGowan 1972 Phillips and Crossley 1978). 

Controlled potential reduction of Pu(VI) to Pu (IV) has also been used to avoid iron interference, but plutonium must be oxidized chemically by AgO, Ce(IV) or fuming HCIO4 prior to its coulometric determination (Shults 1963). 

---