Protonation of D-gluconate and its complexation with Np(V) in acidic to nearly neutral solutions

Thermodynamic properties of the protonation of D-gluconic acid (HGH4(aq)) and its complexation with Np(V) have been studied in acidic to nearly neutral solutions at t=25 °C and I=1 M NaClO4by potentiometry, spectrophotometry and calorimetry. The protonation constant (logKH) and enthalpy (ΔHH) of the carboxylate group are determined to be (3.30±0.10) and −(4.03±0.07) kJ mol-1, respectively. Gluconate forms two Np(V) complexes in nearly neutral solutions. The formation constants and enthalpies of complexation are: log β1= (1.48±0.03) and ΔH1= −(7.42±0.13) kJ mol-1for NpO2(GH4)(aq), log β2=(2.14±0.09) and ΔH2= −(12.08±0.45) kJ mol-1for NpO2(GH4)2-. The thermodynamic parameters indicate that gluconic acid, like isosaccharinic acid and other α-hydroxycarboxylic acids, is a slightly stronger acid and forms stronger complexes with Np(V) than simple monocarboxylic acids.