Determination of single ion activities by relying on the electrical double layer model: NaF aqueous solutions

Abstract

Single ion activity coefficients for aqueous solutions of NaF in the range m = 0.1 up to m = 0.9 mol kg−1 have been derived by measuring the potential difference of the cell Hg (σ = const)/NaF(m)/F−-selective electrode. The approach rests on the assumption that the electric potential drop across the Hg/solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy–Chapman theory, provided specific adsorption of ions is absent. The derived γ− values have been compared with the "real" activity coefficients as determined by Rabinovich and Titov and with the predictions of the hydration theory of Bates and Robinson. It is shown that the latter do not agree with the present experimental observations. Possible reasons for this apparent discrepancy are discussed.