Charging of silver bromide aqueous interface: Evaluation of interfacial equilibrium constants from surface potential data

Abstract

A single crystal silver bromide electrode (SCr–AgBr) was used to measure the inner surface potential ( Ψ 0) at the silver bromide aqueous electrolyte interface as a function of the activities of Br - and Ag +. Absolute values of the surface potential were calculated from electrode potentials of SCr–AgBr using the value of point of zero charge (pBr pzc = 6.9 [H.A. Hoyen, R.M. Cole, J. Colloid Interface Sci. 41 (1972) 93.]) as the value of point of zero potential. Measurements were performed in potassium nitrate aqueous solutions. The Ψ 0(pBr) function was linear and slightly dependent on the ionic strength. The reduction values of the slope with respect to the Nernst equation, expressed by the α coefficient, were 0.880, 0.935, and 0.950 at ionic strengths of 10 −4, 10 −3, and 10 −2 mol dm −3, respectively. The results were successfully interpreted by employing the surface complexation model, developed originally for metal oxides and adapted for silver halides. The thermodynamic (“intrinsic”) equilibrium constants for binding of bromide ( K n ∘ ) and silver ( K p ∘ ) ions on the corresponding sites at the silver bromide surface were evaluated as lg K n ∘ = 3.98 ; lg K p ∘ = 2.48 . Symmetrical counterion surface association was assumed and equilibrium constants were obtained as lg K NO 3 - ∘ = lg K K + ∘ = 4.30 .