Ionic strength dependence of formation constants. Part 4. Potentiometric study of the system Cu2+-Ni2+-citrate

Abstract

The ternary Cu2+-Ni2+-citrate (cit3−) system was investigated potentiometrically in aqueous solution, at different temperatures, 10≤t≤45°C, and ionic strengths, 0.03≤I≤0.8 mol dm−3, using potassium nitrate or tetraethylammonium bromide as background salt. Since the citrate anion forms weak complexes with potassium, the stability constants here reported differ according to whether the potassium association is considered or not. In the presence of both Cu2+ and Ni2+, the mixed metal species, [CuNi(cit)2H−2]4− is formed with citrate in solution, in addition to the various binary complexes. We have obtained the dependence for all the formation constants on ionic strength and temperature. The previous suggestions concerning a general equation for describing the dependence, log β=f(I), are confirmed; from the study of log β=f(T) we have obtained the values of thermodynamic parameters. The dependence of ΔH on ionic strength is discussed.