Complexes of some trivalent lanthanides with ethylcyanopyridylhydrazonoacetate and their hydrazides: stability constants and thermodynamic functions

Abstract The stability constants of trivalent lanthanide (La, Ce, Nd, Sm, Gd, Dy and Er) complexes of ethylcyanopyridylhydrazonoacetate (EC-3-PyHA & EC-4-PyHA) and their hydrazides (C-3-PyHAH and C-4-PyHAH) have been determined pH-metrically at different temperatures (25, 30 and 35 ° C) in 10% (v/v) ethanol-aqueous medium at a constant ionic strength of 0.1 M (KNO 3 ). It was found that the values of the stability constants of the complexes gradually increase from La 3+ to Er 3+ and increase with increasing temperature. Conductometric titrations of metal-ligand mixtures confirm the formation of 1:2 chelates with all ligands. Thermodynamic functions of the complexes formed show that chelate formation is an endothermic process (positive values of Δ H ) and proceeds spontaneously (negative values of Δ G ). The entropy changes Δ S have positive values, showing that complex formation is a favourable process.