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 (šLnY<SUB>2</SUB>) and entropy of complexation in the gas phase (š (LnY<SUB>2</SUB>)) 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 ΔH<SUB>f</SUB><SUP>o</SUP> curves also indicate a non-uniform dehydration of the rare earth ions in 2:1 TPP complexes