Title: Surface ionization and complexation models: A comparison of methods for determining model parameters
Abstract: The triple-layer model for the electrical double layer is reviewed briefly with emphasis on the metal oxide-aqueous solution interface. To apply this method the model parameters must be established. Two general methods using acid-base titration data to evaluate the constants have been reported in the literature: a graphical double-extrapolation technique and a numerical optimalization of the adjustable parameters. Comparison of the results of both methods, as obtained for the TiO2KNO3 system, shows a large discrepancy. To apply the graphical method the densities of both the positive and negative sites around the pzc should be small. This condition can only be satisfied if the difference between the two ionization constants, ΔpKi, is large. This assumption cannot be verified on the basis of titration data. A second assumption concerns the extent of complexation; this effect can be minimized by the double-extrapolation technique. For weakly adsorbing ions problems may arise with the determination of the complexation constants due to incomplete complexation. For strong specific adsorption of background electrolyte at the pzc a modification of the graphical method as suggested by Sprycha can be used, provided ΔpKi is large. In this case extra information regarding diffuse layer and specific adsorption is required. The numerical optimalization methods used are essentially nonlinear least-squares procedures for which the root-mean-square difference, Δ, between the measured and the calculated quantity is minimized. Calculations for the TiO2KNO3 system show that it is not possible to find a unique set of parameters on the basis of the available titration data alone. However, a good fit of the experimental results can be obtained if either a small or a large ΔpKi value is chosen as the starting value for the calculation. The fact that no unique solution can be found is related to the large number of parameters to be optimized and the experimental and model inaccuracies. The latter determine the minimum value of Δ which is still relevant. It is concluded that neither the graphical nor the numerical method can give a satisfactory parameter set for the triple-layer model on the basis of titration data alone. Additional information regarding the magnitude of the ΔpKi value is required. Such information can be obtained from ψ0 (pH) measurements. Pseudo-Nernstian behavior of the electrodes occurs for small ΔpKi values, whereas strong deviations form Nernstian behavior occurs when the ΔpKi is large.
Publication Year: 1987
Publication Date: 1987-07-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 104
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