The anodic formation of calomel films on mercury electrodes-an ellipsometric-galvanostatic study

Abstract

The mechanism of formation of calomel films on mercury electrodes in 1M potassium chloride solutions has been investigated by combining conventional galvanostatic techniques with a new application of ellipsometry. In this application, the ability of surface films to alter the polarization state of light reflected from metals has been used to monitor optically the state of the mercury surface and to signal the formation of a calomel film on a mercury mirror anode. The cell has been designed for simultaneously obtaining potential-time and ellipsometer intensity-time transients. A potentiostat has been used to maintain the mirror electrode at a potential such that it is kept free of anodically-formed films-this is the optical reference state. The cessation of potentiostatic control of the potential and the imposition of a constant anodic current makes calomel formation possible and ellipsometrically detectable in situ. The essential observation is that the mercury anode attains the 'calomel potential' in a time significantly less than the 'induction time' (τ_i) which is necessary for the formation of a calomel film on the anode. This induction time increases if an inert gas is bubbled during the passage of an anodic current. Further, it depends on the galvanostatic current density; the product iτ_2^1/2 increases linearly with current density at low current densities, but is constant at current densities > ~ 0.1 mA/cm^2. The ellipsometer showed that, even when the anodic current was turned off during film growth, calomel growth continued for some time after which the film thickness became fairly steady over several minutes. Under these conditions (of steady thickness), the refractive index of a particular film has been shown to be that of calomel and its thickness ≈ 35 Å. The experimental results are discussed in terms of a dissolution-precipitation model in which a Hg/KCl interface is charged to the 'calomel potential' by a process involving the formation of mercurous adions and specifically adsorbed chloride ions. The transformation of the polarizable interface into a non-polarizable interface occurs through the passage into solution of chloro-mercurous ions, Hg_2Cl^+. The accumulation of these ions is subject to diffusion away from the electrode and to disproportionation into chloro-mercuric ions, HgCl^+, and other mercuric entities. When the chloro-mercurous ions attain a critical concentration (corresponding to the solubility product of calomel with respect to Hg_2Cl^+) in the vicinity of the anode, a film of calomel is precipitated on to the electrode. On the basis of a diffusion precipitation model, the authors have previously derived an expression for the dependence of iτ^1/2_2 on i by solving the boundary-value problem. The predictions of this equation are in conformity with the plot of iτ^1/2_2 against i experimentally observed for calomel formation.