The interaction between ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA or H4Z) and Th(IV) and Zr(IV) ions has been investigated potentiometrically by using a mercury indicator electrode. The formation constants of the chelates ThZ and ZrZ have been evaluated at a constant ionic strength of 0.10 (KNO3) and at 0, 10, 20, 30 and 40°. Enthalpy and entropy changes characterizing the formation of the chelates have been calculated by the least-squares method at 25°. The results are collected as follows:
