Formation and stability of zinc(II) and cadmium(II) citrate complexes in aqueous solution at various temperatures.

The citrate complexes of Zn(2+) and Cd(2+) have been investigated by pH titration at I = 0.1 M (KNO(3)) and 10, 25, 35 and 45 degrees . The species found were [Zn(cit)](-), [Zn(cit)H], [Zn(cit)(2)](4-) and [Zn(2)(cit)(2)H(-)(2)](4-), [Cd(cit)](-), [Cd(cit)H], [Cd(cit)(2)](4-) and [Cd(cit)H(-1)](2-). From the dependence of the formation constants on temperature, DeltaH degrees and DeltaS degrees values were calculated. Speciation in the Zn(2+)]- and Cd(2+)-citrate systems is discussed with particular attention to formation of polynuclear species. Some comparisons with literature data are made.