Kinetics and mechanism of electroreduction of ammonia and hydroxyammonia complexes of zinc(II) on a dropping mercury electrode

It is found that the equilibrium potential of the Zn(Hg)/Zn(II) system depe nds on the concentration of ammonia molecules and solution pH. The dependen ce conforms to the literature data on the stability constants for ammonia a nd hydroxyammonia complexes of zinc. Their reduction on a dropping mercury electrode in solutions of pH 9.2-12 and [NH3] = 0.05-2 M yields one irrever sible cathodic wave with a diffusion limiting current. In dilute supporting electrolytes, the plateau of the latter is preceded by a maximum due to ac cumulation of insoluble reduction products on the surface of the mercury dr op. The pH and [NH3] dependences of the half-wave potential of waves that a rt: undistorted by a maximum are analyzed with allowance made for a change in the composition of zinc(II) complexes in the bulk solution. According to the analysis, the slow two-electron electrochemical stage involves complex es Zn(NH3)(2)(2+) that form from complexes present in solution in preceding reversible chemical reactions. The effect the supporting-electrolyte conce ntration has on the electroreduction rate of zinc(II) complexes and the mec hanism of the electrochemical stage is discussed.