The reactions between mercury ions and silver: dissolution and displacements

The results of polarization measurements for the reduction of mercury (I) and mercury (II) on a rotating platinum electrode, and the oxidation of silver and amalgamated silver at a rotating silver electrode are presented. For mercury (I) and mercury (II) diffusion coefficients were estimated from limiting current measurements as a function of rotation speed and temperature. The Values obtained for the diffusion coefficients at 25°C, and activation energies were:

The kinetics of the reactions between a rotating silver disc and solutions of mercury (I) and mercury (II) are also reported. The mercury (I) reaction is a displacement in which mercury is deposited and silver goes into solution. The kinetics are first order-first order reversible, and can be quantitatively accounted for by assuming that the diffusion of mercury (I) to the surface and silver (I) away from the surface are jointly rate controlling. At temperatures below 35°C, the mercury (II) reaction is a dissolution, controlled by the speed at which mercury (II) ions diffuse to the reacting surface. Above 35°C, the mercury (II)-silver reaction is a displacement, with somewhat irreproducible kinetics. The changeover between the two reactions at 35°C apparently occurs because the displacement reaction is thermodynamically favoured above 35°C.