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Charging of Silver Bromide Aqueous Interface: Evaluation of Interfacial Equilibrium Constants from Surface Potential Data

A single crystal silver bromide electrode (SCr-AgBr) was used to measure the inner surface potential (Ψ0) at silver bromide aqueous electrolyte interface as a function of 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 (pBrpzc = 6.9 [H. A. Hoyen and 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 Ψ0(pBr) function was linear and slightly dependent on the ionic strength. The reduction of the slope with respect to the Nernst equation, expressed by the alfa 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 (Kn) and silver (Kp) ions on the corresponding sites at the silver bromide surface were evaluated as lgKn= 3.98 ; lg Kp = 2.48. Symmetrical counterion surface association was assumed and equilibrium constants were obtained as lg K(NO3-) = logK(K+) = 4.30.

silver bromide; surface potential; surface charge; surface equilibrium constants; surface complexation model; single crystal electrode

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