Cation–cation interactions between NpO2+ and UO22+ at different temperatures and ionic strengths

Abstract

Cation–cation interactions between NpO₂⁺ and UO₂²⁺ were studied at different temperatures (283.15 K to 358.15 K) and different ionic strengths (3–4.5 mol dm⁻³) by spectrophotometry and microcalorimetry. The cation–cation complex between NpO₂⁺ and UO₂²⁺ was weak and became stronger as the temperature was increased from 283.15 K to 358.15 K. The molar enthalpy of complexation was directly determined for the first time by microcalorimetry to be (4.2 ± 1.6) kJ mol⁻¹ at 298.15 K, in good agreement with the trend in the stability constant at different temperatures. The small and positive enthalpy and entropy of complexation support the argument that the cation–cation complex between NpO₂⁺ and UO₂²⁺ is of inner-sphere type. At each temperature, the stability constants of the cation–cation complex were found to increase as the ionic strength was increased. The specific ion interaction theory (SIT) was used to obtain the stability constants at infinite dilution and variable temperatures.