TRLFS study on the complexation of Cm(III) with nitrate in the temperature range from 5 to 200 °C

Abstract

The formation of aqueous Cm(III) nitrate complexes is studied in the temperature range from 5 to 200 °C by time resolved laser fluorescence spectroscopy (TRLFS). The experiments are performed in a custom build high pressure and high temperature fluorescence cell. The complex formation is measured at nitrate concentrations ranging from 0.10 to 4.61 mol/kg H₂O. The mono- and dinitrate complexes are quantified by peak deconvolution of the fluorescence spectra and the complexation constants are determined as a function of the temperature. The conditional equilibrium constants are extrapolated to zero ionic strength using the specific ion interaction theory (SIT) and the thermodynamic standard state data (Δ_rH°_m, Δ_rS°_m, Δ_rG°_m, Δ_rC°_p,m) are determined from the temperature dependence of the equilibrium constants at I=0. The equilibrium constants up to 75 °C are well described by the Van´t Hoff equation (Δ_rH°_m independent of T and Δ_rC°_p,m=0). Modelling of the data at higher temperatures requires an extended equation including a term for the heat capacity changes (Δ_rC°_p,m).