Ethylenediamine and Hydroxyethylenediamine Complexes of Zinc(II): A Potentiometric Study of Composition and Stability

The equilibrium potential of saturated zinc amalgam is studied as a function of concentration of free ethylenediamine molecules, [en], in the region [en] 0.001–1 M in solutions of pH 9.5, 10.5, and 11.5. At the concentration of zinc(II) ions 2 × 10^–3 M and [en] = 1 M only simple trisethylenediamine complexes of zinc(II) form in all the solutions. At smaller [en] and pH 9.5 and 10.5, complexes Zn(en)₂²⁺ and Zn(en)₂OH⁺ are also present; these are complemented at pH 11.5 by Zn(en)₂(OH)₂ at [en] 0.005–0.1 M. Stability constants for these complexes are calculated.