engleski

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

A single crystal silver chloride electrode (SCr-AgCl) was used to measure the inner surface potential (Ψ 0) at the silver chloride aqueous electrolyte interface as a function of activity of Cl– ions as determined by the Ag/AgCl electrode. Absolute values of the surface potential were calculated from electrode potentials of SCr-AgCl using the value of point of zero charge (pClpzc = 5.2) as the value of point of zero potential. Measurements were performed in potassium nitrate aqueous solutions, as well as in the presence of Li, Na, Cs, Mg, and La nitrates. The Ψ 0(pCl) function was found to be linear within the experimental error and practically the same for all the examined electrolytes and almost independent of ionic strength. The reduction of the slope with respect to the Nernst equation, expressed by the alfa coefficient, was (0.88 ± 0.01) at Ic = 10-1 mol dm-3, (0.87 ± 0.01) at Ic = 10-2 mol dm-3 and (0.84 ± 0.01) at Ic = 10-3 mol dm-3. The results were successfully interpreted by employing the Surface Complexation Model developed originally for metal oxides and adapted for silver chloride. The standard (“ intrinsic” ) equilibrium constants for the binding of chloride (Kn) and silver ions (Kp) on the corresponding sites at the silver chloride surface were evaluated as: lgKn = 2.67 ± 0.05 ; lgKp = 2.07 ± 0.05. Counterion surface association equilibrium constants were also obtained as: K(NO3-) = K(K+) = 2.74 ± 0.05.

Silver chloride; surface potential; surface charge; surface equilibrium constants; surface complexation model; single crystal electrode

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