Complexation of gluconic acid with Nd(III) in acidic solutions: A thermodynamic study

Thermodynamic properties of gluconic acid (HGH4(aq)) complexation with Nd(III) have been studied in acidic solutions at 25 °C and 1.0 M NaClO4 by potentiometry, spectrophotometry and calorimetry. Three Nd(III)/gluconate complexes were identified in solutions of pCH 2.5–4.5. The formation constants and the complexation enthalpies are: log β1 = 2.55 ± 0.05 and ΔH1°=−(8.22±0.23) kJ mol−1 for Nd(GH4)2+, log β2 = 4.45 ± 0.05 and ΔH2°=−(15.3±0.4) kJ mol−1 for Nd(GH4)2+, and log β3 = 5.60 ± 0.15 and ΔH3°=−(24.6±0.6) kJ mol−1 for Nd(GH4)3(aq). The thermodynamic parameters indicate that gluconic acid, like other α-hydroxycarboxylic acids, forms stronger complexes with Nd(III) than simple monocarboxylic acids, likely by the participation of the α-hydroxyl group during complexation. The same type of binding appears to occur with Ca2+ and NpO2+ as the 1:1 stability constants are linearly correlated with the effective cationic charges of the metal cations.