Thermodynamic and spectroscopic study on Al3+-polycarboxylate interaction in aqueous solution

A thermodynamic and spectroscopic study on the Al3+ interaction with several polycarboxylate ligands, namely malonate (mal), L-malate (mala), propane-1,2,3-tricarboxylate (tca), butane-1,2,3,4-tetracarboxylate (btc), benzene- 1,2,3,4,5,6-hexacarboxylate (mlt) and ethylenediamine-N,N′-disuccinic acid (EDDS), in aqueous solution is reported. Speciation models and thermodynamic formation parameters have been determined by potentiometric and calorimetric titrations at T = 298.15 K, I= 0.15mol·L−1 in NaCl. Although different speciation models have been found for the six systems, ML formation has been ascertained in all the cases. It has been observed that the stability ofML species significantly changes for the different carboxylates. Potentiometric measurements have been also carried out at I=0.5 and 1 mol·L−1. The dependence of formation constants on ionic strength is reported for all the systems over the range 0.1–1mol·L−1, except for mlt one. As expected for typical hard-hard interactions, the contribution to free energy resulted mainly of entropic origin. Al3+-mala and mal systems have been also investigated by 1H NMR spectroscopy. The stability constants obtained by NMR titrations resulted comparable to the ones already calculated by potentiometry, thus validating the model selected to rationalize the potentiometric findings.