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.