The proton-ligand formation constant of N-methylisatin-β-amidinohydrazone (β-MIAG) and the metal-ligand formation constant of its complexes with UO₂(II), Cu(II), Ni(II), Co(II), Pb(II), Zn(II), Cd(II) and Mn(II) have been determined at a definite ionic strength, μ = 0.1 M NaCIO₄, and at temperature 30±0.5°C, in aqueous organic solvents of different compositions. The variance ofpK_a and logβ_n with the inverse of the dielectric constant or the mole fraction of the solvent has also been studied. ThepK_a and logβ_n increases with the decrease of dielectric constant and the increase of the mole fraction of organic solvent of the media. In all the systems, the order of logβ_n values of M(II)-β-MIAG chelates is in accordance with Irving-William’s series.

Potentiometric investigations on metal complexes of various bivalent metal ions, viz. UO₂(II), Cu(II), Ni(II), Co(II), Cd(II), Pb(II), Zn(II) and Mn(II) with benzilmonol(lepidy!)hydrazone (BLH) have been carried out at different ionic strengths and at different temperatures in order to determine stability constants of the complexes. Thermodynamic parameters ΔC, ΔH and ΔS have also been evaluated from temperature coefficient data. The effect of varying the dielectric constant of the medium on the stability constants of complexes has also been investigated at 30±0.5°C and μ = 0.1MNaCl. Thermodynamic stability constants and thermodynamic stabilization energies for the first transition metals have also been discussed.

Complexation reactions of bivalent metal ions and ethyl-2,3-dioxobutyrate-2p-bromophenylhydrazone (EDOB-2p-BPH) have been studied potentiometrically in 50% (v/v) ethanol/water medium at different ionic strengths with respect to NaClO₄ and at different temperatures and their stability constants determined. The method of Bjerrum and Calvin as modified by Irving and Rossotti has been used to determine then andpL values. S_min values which have the same significance as ‘T²’ have also been calculated. The thermodynamic stability constants and standard free energy change (ΔG) have also been calculated. ΔG values are negative in all cases indicating that the reactions are spontaneous. The ligand field stabilization energy (δH) has also been calculated for the 3d transition metals.