A spectroscopic study of Cm(iii) complexation with propionate in saline solutions at variable temperatures

Abstract

The complexation of Cm(III) with propionate is studied by time resolved laser fluorescence spectroscopy (TRLFS) in saline solutions (NaCl and CaCl₂) as a function of ionic strength, ligand concentration, and temperature. The molar fractions of the different Cm(III) species are determined by peak deconvolution of the measured fluorescence spectra. By using the specific ion interaction theory (SIT), the stability constants at zero ionic strength (log K⁰_n) of the first and second complexation steps are determined. The stability constants of the mono- and dipropionate complexes increase continuously with increasing temperature between 20–90 °C in both background electrolytes. The log K⁰_n values are linearly correlated to the reciprocal temperature, indicating Δ_rH⁰_m = const. and Δ_rC⁰_p,m = 0. Therefore, the thermodynamic constants (Δ_rH⁰_m, Δ_rS⁰_m, Δ_rG⁰_m) for the formation of the [Cm(Prop)]²⁺ and [Cm(Prop)₂]⁺ complexes are derived from the integrated van't Hoff equation. The results show that both reactions are entropy driven. Furthermore, neither the formation of ternary species including Ca²⁺ nor a complexing effect of Cl⁻ at elevated temperatures is observed under the chosen experimental conditions. Lastly, the ion–ion interaction coefficients of both complexed species with Cl⁻ are derived for the first time.