Thorium oxidation-reduction potentials

Physical and Chemical Properties. Some of the physical and chemical properties (i.e., K°w K°<= and Henry s law constant) that are often used in the estimation of environmental fate of organic compounds are not useful or relevant for most inorganic compounds including thorium and its compounds. Relevant data concerning the physical and chemical properties, such as solubility, stability, and oxidation-reduction potential of thorium salts and complexes have been located in the existing literature. 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

All actinides from thorium to californium form tetravalent oxidation states. For the three elements of highest atomic number, however, viz. americium, curium, and berkelium, the hydrated ions are too strongly oxidizing to be stable in aqueous solution [7,10]. Their rates of reduction nevertheless vary widely, in the order Bk + < Am < Cm + < Cf, with Bk" being by far the most resistant species. This is also the order of thermodynamic stability, as indicated by the oxidation potentials of the couples [11]. 

Current-potential curves of the O2 reduction were measured [68] in concentrated KOH on alloy electrodes prepared from silver and small amounts of elements that form oxides of low electron affinity. The reactivity of a silver — 1.7w/o (weight percent) magnesium alloy was better than that of pure silver. In contrast, no improvement was found for foils of silver — Iw/o thorium, silver — Iw/o radium, and silver — 1 w/o barium. 

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