Electrochemical oxidation of terbium(III) in aqueous media: influence of supporting electrolyte on oxidation potential and stability

Apart from the generally dominant trivalent oxidation state, several lanthanides can occur in divalent or tetravalent states as well. Changing the valence state alters the chemical properties and, therefore, gives the opportunity to ease intragroup lanthanide separations. The hydrated terbium(III) ion has a highly positive reduction potential (E0 = + 3.1 V vs. SHE), but it can be oxidized to its tetravalent state via ozonolysis or electrolysis, and stabilized in highly concentrated carbonate solutions. In this study, terbium(III) is electrochemically oxidized to terbium(IV) in aqueous carbonate, nitrate, and periodate media. Spectroscopic and elemental analysis were done to characterize terbium(IV) and to gain insight in the stability of these complexes in the aqueous electrolytes of carbonate, nitrate, and periodate. In all these electrolytes, oxidation and stabilization were achieved at different pH, terbium concentration, salt concentration and applied potentials. The most promising electrolyte was found to be carbonate, since it allowed to attain more concentrated and more stable terbium(IV) solutions.