THERMODYNAMIC PARAMETERS OF LANTHANOID(III) COMPLEXATION WITH -PROPYLIDENEHYDRAZINO)-5,6-DIPHENYL-1,2,4-TRIAZINE (DCPT) .2.

The complex formation equilibria between lanthanoid(III) and DCPT ions have been studied by potentiometry in 75% (v/v) dioxane-water mixture and 0.10 M KNO3 as constant ionic medium, at 10, 20, 30 and 40 degree s C. A series of mononuclear protic [Ln(HL)](2+), [Ln(HL)(2)](+), [Ln( HL)(3)], [LnL(HL)(2)](-), [LnL(2)(HL)](2-) and non-protic [LnL(3)](3-) complex species were found in solutions and their formation constants , ethalpies and entropies were determined. It was established that the stepwise Gibbs energies of the protonated complexes varied in the seq uence - Delta G(Ln(HL)) > - Delta G(Ln(HL)2) > - Delta G(Ln(HL)3), and the stepwise enthalpies the order was - Delta H-Ln(HL > - Delta H(Ln( HL)2) > - Delta H(Ln(HL)3), and the entropy markedly increased in the order Delta S-Ln(HL) > Delta S(Ln(HL)2) > Delta S(Ln(HL)3). The result s are discussed in relation to changes in the metal-water and metal-li gand bond strength. The variation of the thermodynamic parameters as a function of ionic potential of lanthanide elements is discussed in te rms of the different hydration numbers of lighter and heavier lanthani de(III) ions.