Thermodynamic quantities associated with the interaction of tripolyphosphoric acid with metal ions

Abstract

Potentiometric equilibrium measurements have been made at 2°, 25', 35' and 45°C for the combination of protonated (tetranegative) and fully dissociated (pentanegative) anions of tripolyphosphoric acid (TPP) with eight bivalent metal ions at a 1:1 molar ratio of ligand to metal ion. The enthalpy and entropy changes for the formation of protonated and normal chelate species are calculated from the temperature coefficient data. The enthalpy terms for the formation of the protonated and normal chelate species of TPP are relatively small, the entropy appears to be the principal driving force for the formation of the protonated and the normal metal chelate species in solution. A regular trend is observed in the entropy of formation ΔS_f°, which increases with the number of negative donor groups on the ligand and the reciprocal square of an effective ionic radius of the cation. This trend is also followed by a large group of ligands like ATP, ADP, AMP, EDTA and NTA that form predominantly ionic complexes in solution. For a given ligand, the constant that corrects the pauling ionic radius to the effective radius is found to increase linearly with the electronegativity of the metal ions.