Complexation and thermodynamics of Cm(III) at high temperatures: the formation of [Cm(SO4)(n)](3-2n) (n = 1, 2, 3) complexes at T = 25 to 200 °C

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Output type: Journal article

Author list: Skerencak, Panak, Fanghaenel

Publisher: Royal Society of Chemistry

Publication year: 2013

Volume number: 42

Issue number: 2

Start page: 542

End page: 9

Number of pages: -532

ISSN: 1477-9226

eISSN: 1477-9234

Languages: English-Great Britain (EN-GB)

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Open access status: closed

Abstract

The formation of [Cm(SO(4))(n)](3-2n) complexes (n = 1, 2, 3) in an aquatic solution is studied by time resolved laser fluorescence spectroscopy as a function of the ligand concentration, the ionic strength (NaClO(4)) and the temperature (25 to 200 °C). The experiments are performed in a custom-built high temperature cell for spectroscopic measurements at high pressures and temperatures. The single component spectra of the individual species are identified by slope analysis at every studied temperature and their molar fractions are determined by peak deconvolution of the emission spectra. The results show a strong shift of the chemical equilibrium towards the complexed species at increased temperatures. With the determined speciation, the conditional stepwise stability constants are calculated and extrapolated to zero ionic strength, using the specific ion interaction theory (SIT). The log K(0)(n)(T) values increase by several orders of magnitude in the studied temperature range. The fitting of the temperature dependency of the first and second stability constant (log K(0)(1) and log K(0)(2)) requires an extended van't Hoff equation, taking into account a constant heat capacity of the reaction (Δ(r)C(0)(p,m) = const.). Contrarily, the temperature dependency of the log K(0)(3) is very well described by the linear van't Hoff equation, assuming Δ(r)C(0)(p,m) = 0. Thus, the thermodynamic standard state data (Δ(r)H(0)(m), Δ(r)S(0)(m), Δ(r)C(0)(p,m)) of the stepwise complexation of Cm(III) with SO(4)(2-) are determined. Additionally, the ion interaction coefficients of the stepwise complexation reactions (Δε(n)) are determined as a function of the temperature. The fluorescence lifetimes of Cm(III) are recorded at different sulphate concentrations as a function of the temperature. The results give a strong indication that at T > 100 °C the first excited state of Cm(III) ((6)D'(7/2)) is effectively quenched by a temperature dependent enhancement of the energy transfer from the metal ion to OH vibrations of first shell water molecules.

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