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

Abstract

Thermodynamic properties of gluconic acid (HGH 4(aq)) complexation with Nd(III) have been studied in acidic solutions at 25 °C and 1.0 M NaClO 4 by potentiometry, spectrophotometry and calorimetry. Three Nd(III)/gluconate complexes were identified in solutions of p C H 2.5–4.5. The formation constants and the complexation enthalpies are: log β 1 = 2.55 ± 0.05 and Δ H 1 ° = − ( 8.22 ± 0.23 ) kJ mo l − 1 for Nd(GH 4) 2+, log β 2 = 4.45 ± 0.05 and Δ H 2 ° = − ( 15.3 ± 0.4 ) kJ mo l − 1 for Nd(GH 4) 2 +, and log β 3 = 5.60 ± 0.15 and Δ H 3 ° = − ( 24.6 ± 0.6 ) kJ mo l − 1 for Nd(GH 4) 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 Ca 2+ and NpO 2 + as the 1:1 stability constants are linearly correlated with the effective cationic charges of the metal cations.