Metal chelates of tripolyphosphate with rare earths

Potentiometric equilibrium measurements have been made at 25, 35 and 45° for the combination of protonated(tetranegative) and fully dissociated(pentanegative) tripolyphosphate (TPP) anions with La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Y(III) in a 10:1 ratio of ligand to metal ion. The enthalpy and entropy changes for the formation of 2:1 diprotonated and normal chelates of TPP are calculated from temperature coefficient data. The enthalpies and entropies of formation for the 2:1 diprotonated and normal chelate species show no regular trends with ionic size of lanthanide ion. The partial molal entropies (šLnY2) and entropy of complexation in the gas phase (š (LnY2)) for the 2:1 rare earth chelates of TPP fall into three groups, indicating a change in the hydration number of the lanthanide ions. The breaks in the ΔHfo curves also indicate a non-uniform dehydration of the rare earth ions in 2:1 TPP complexes