engleski

Equilibrium at Metal Oxide Aqueous Interface: Surface Potential Measurements

Determination of surface potentials at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single crystal electrode (SCrE) is described (1). It is proposed to determine the pristine point of zero charge pHeln and the isoelectric point pHiep, and use these data to set the scale of surface potentials (2). The value of pHeln can be obtained at a sufficiently low ionic strength where pHeln coincides with both pHiep and the point of zero charge pHpzc. Measurements of surface potential by means of SCrE provide data which in combination with data obtained by classical techniques (surface charge measurements and electrophoresis) enable accurate interpretation of the interfacial equilibrium, since the assumption of the constancy of some parameters is avoided (3). It is shown that values of thermodynamic equilibrium constants of surface protonation and deprotonation are constant, while the value of the capacitance of the inner layer depends on pH, i.e. on the surface potential. Association of counterions with surface charged groups is interpreted by means of two parameters: thermodynamic equilibrium constant of association with oppositely charged surface groups and electrical capacitance of the inner Helmholtz layer. Measurements of surface potential of different crystal planes provide information on surface heterogeneity. For that purpose the equipment (Fig 1.) for measurements of surface potential of different crystal planes is constructed. The effects of pH, ionic strength and different counterions on surface potential were examined.

surface potential; surface charge; single crystal electrode; surface complexation model; electrical interfacial layer

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