Single-ion activities based on the electrical double-layer model: an indirect test of the Gouy–Chapman theory

Abstract

Single-ion activity coefficients for NaOH aqueous solutions in the concentration range m= 0.01–1.0 mol kg^–1 have been determined by measuring the potential difference for the cell Hg(σ= const.)|NaOH (m)|HgO|Hg (σ is the Hg surface charge density). The approach rests on the assumption that the electric potential drop across the Hg/solution interface, provided specific adsorption of ions is absent, is independent of the electrolyte concentration once the diffuse-layer contribution is accounted for by the Gouy–Chapman theory. Determinations have been extended from σ= 0 up to σ=–14 µC cm^–2 by measuring the potential of zero charge and integrating capacitance data. The results have shown that at m > 0.3 mol kg^–1 the model breaks down owing to the presumable attack of Hg by the electrolyte. At m < 0.3 mol kg^–1 the activity coefficient of OH^– turns out to be lower than the mean ionic coefficient. Calculations by means of the ‘hydration theory’ of Robinson et al. agree with the data of this work only qualitatively.