Thermodynamic parameters including the equilibrium constants and enthalpy of complexation of Np(V) with oxalate at variable temperatures (T = 283–343 K, ionic strength = 1.05 mol kg−1NaClO4) were determined by spectrophotometric and microcalorimetric titrations. The results show that the complexation of Np(V) with oxalate is moderately strong and becomes weaker at higher temperatures. The complexation is exothermic and driven by both enthalpy (negative) and entropy (positive) in the temperature range from 283 K to 343 K. As the temperature is increased, both the enthalpy and entropy of complexation increase (ΔH becomes less negative and ΔS becomes more positive), having opposing effects on the complexation. Because the increase in the enthalpy (ΔH) exceeds that of the entropy term (TΔS), the complexation of Np(V) with oxalate becomes weaker at higher temperatures. The effect of temperature on the complexation is discussed in terms of the energetics of ion solvation and hydrogen bonding involved in the complexation.
