Determination of the Formation Constant of Some Transition Metal Ion- histidine Complexes in Water and Water-dioxane and Evaluation of their Thermodynamic Parameters

Aims: The formation constants of histidine complexes of Fe (III), Co (II), Ni (II), Cu(II), Zn (II) and Pb (II) at temperatures of 25, 30, 37, 45°C were determined potentiometrically, utilizing modified Bjerrum's method. Potentiometric titrations were carried out in 30% and 70% (v/v); 50% and 50% (v/v) water-dioxane mixtures. Ionic strength of medium was retained at 0.10 M by sodium nitrate. The first and second formation constants were obtained by plotting corrected pH versus concentration of standardized NaOH solution after each addition.

Results: The trend for increasing stability in both aqueous solution and in dioxane-water mixtures is as follows:

kf pb(II)-His < kf Co (II)-His < kf Zn (II)-His < kf Ni(II)-His < kf Cu(II)-His < kf Fe(III)-His

The effects of dioxane and temperature on the formation constants of histidine complexes were examined. Our findings showed that the formation constants of the complexes increased as the dioxane content was raised. The negative values of ΔG° are indication of the spontaneity of the reactions. ΔH° values are positive conveying the complex formation is an endothermic process.

Conclusion: The variations in log of formation constants versus 1/T is linear evidencing that ΔH°s are independent of temperature in temperature range of the study, and increase in entropy are cause of formation of these complexes.