Thermodynamic quantities associated with the interaction of mixed ligand rare earth chelates of EDTA with TPP and ATP

Potentiometric equilibrium measurements have been made at 2, 25, 35 and 45° for the combination of protonated (tetranegative) and fully dissociated pentanegative) anions of TPP and of the tetranegative anions of ATP with 13 rare earth ions in a 1:1:1 ratio of rare earth: EDTA:TPP and rare earth: EDTA: ATP. The enthalpy and entropy changes for the formation of protonated and normal chelates of TPP and for the normal chelates of ATP are calculated from temperature coefficient data. The partial molal entropies (S̅LnXY) and entropy of complexation in the gas phase (ΔSgLNXY) of the mixed ligand rare earth complexes of EDTA with ATP and TPP fall into three groups La---Pr, Pr---Tb and Tb---Tm indicating a change in the hydration number of the lanthanide ions in three respective groups. The enthalpies for the mixed ligand rare earth complex formation (ΔH°) change in an erratic manner both for the ATP and TPP mixed ligand complexes