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Chemical Thermodynamics of Thorium

image of Chemical Thermodynamics of Thorium

This volume is the eleventh in the OECD Nuclear Energy Agency (NEA) “Chemical Thermodynamics” series. It is based on a critical review of the thermodynamic properties of thorium, its solid compounds and aqueous complexes, initiated as part of the NEA Thermochemical Database Project Phase III (TDB III). The database system developed at the OECD/NEA Data Bank ensures consistency not only within the recommended data sets of thorium, but also amongst all the data sets published in the series. This volume will be of particular interest to scientists carrying out performance assessments of deep geological disposal sites for radioactive waste.

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The aqueous Th4+ ion

Nuclear Energy Agency

There is no firm evidence for the stability of any valency state other than Th(IV) in aqueous media. Recently, Klapköte and Schulz [1997KLA/SCH] claimed to have identified Th3+ in the reaction of Th4+ with HN3 in slightly acidic solutions (pH = ca. 3.5), by absorption and ESR spectra. However, as noted in Appendix A, Ionova et al. [1998ION/MAD] have seriously questioned both the value of E?? (Th3+/Th4+) chosen by [1997KLA/SCH] in their thermodynamic analysis and the validity of their conclusions, pointing out that the absorption bands observed by these authors could equally well be due to azide complexes of Th(IV), with or without Cl– ligands. Moreover, the stability of HN3 in dilute aqueous solutions suggests that the proposed reduction of Th4+ is unlikely to be thermodynamically controlled as suggested in [1997KLA/SCH]. Thus there seems no incontrovertible evidence for the existence of any stable ions in aqueous solutions other than Th(IV) and its derivatives.

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