A THERMODYNAMIC TEST OF THE DIFFUSE LAYER THEORY AND ITS IMPLICATIONSON ELECTRODE-KINETICS

The thermodynamic compatibility of the diffuse layer theory with the c hemical potential of ionic species in the solution bulk is introduced. From the electrocapillary equation, a compatibility equation, which r elates the potential drop across the diffuse layer to the chemical pot ential of the ionic species in the solution bulk, is deduced for a bin ary electrolyte. When the Gouy-Chapman (GC) theory is introduced in th e compatibility equation, the latter only becomes an identity if the i onic activity is taken as the ionic concentration, as expected. A comp atible theory of the diffuse layer which has GC as the limit case at d ilute solutions and fulfills the compatibility equation for whatever p hysical description of the ionic activity coefficients is stated. The significance of the compatible theory is that the electrode reaction r ates calculated by means of the electrode kinetic equation with the Fr umkin correction using this theory differ from those calculated using GC. In order to test both theories, we have studied the electroreducti on of peroxodisulfate at an amalgamated copper rotating disk electrode . The experimental results fit our compatible theory better than GC do es.