The proton-ligand and metal-ligand stability constants of Pr (III) and Nd (III) metal ions with L1 : 3 (2’-chlorophenyl)-4-benzoyl-5(2-hydroxy phenyl) pyrazoles and L2:3 (2’-chlorophenyl)-4-pyridoyl-5 (2-hydroxy phenyl) pyrazole have been studied pH-metrically at 0.1M ionic strength at 30 ± 0.10C by varying the proportions of ethanol -water mixture. The data obtained were used to study the effect of solvent composition on pK and logK values in terms of solubility restrictions. It is observe that pK and logK values increases as the proportion ethanol in ethanol-water mixture increases.
