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 (HGH(4)(aq)) and its complexation with Np(V) have been studied in acidic to nearly neutral solutions at t = 25 degrees C and I = 1 M NaClO4 by potentiometry, spectrophotometry and calorimetry. The protonation constant (log K-H) and enthalpy (Delta H-H) 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 beta(1) = (1.48 +/- 0.03) and Delta H-1 = - (7.42 +/- 0.13) kJ mol(-1) for NpO2 (GH(4)) (aq), log beta(2) = (2.14 +/- 0.09) and Delta H-2 = - (12.08 +/- 0.45) kJ mol(-1) for NpO2 (GH(4))(2)(-). The thermodynamic parameters indicate that gluconic acid, like isosaccharinic acid and other alpha-hydroxycarboxylic acids, is a slightly stronger acid and forms stronger complexes with Np(V) than simple monocarboxylic acids