pH-meteric studies have been carried out to determine the dissociation constant of 2-Sulphonic-4-methyl phenyl-hydrazono di-ethylmalonate abbreviated (2-SPHDEM) 1 igand, as well as, the stability constants of its complexes with trivalent lanthanides and some divalent metal ions in various mixed aqueous solvents, viz: ethanol-water, methanol-water and dioxane-wate~ at 25°C and 0.1 M ionic strength. The dissociation constant of ligand decreases (increase in acidity) by increasing the dielectric constant of the solvent. The stability constants of 1:1 and 1:2 chelates with trivalent lanthanides have been determined in 50'; (v/v) ethanol-water solvent. The effect of variation of the dielectric constant of different mixed aqueous solvents on the stability of chelate compounds of divalent metal ions was studied and discussed. The stability of these chelates follows the order: dioxane-water> ethanol-water> methanol-water.
F I ZIDAN ,A SALMAN ,N EL KHOLY ,B A EL SHETARY ,
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