Determination of surface potentials from the electrode potentials of a single-crystal electrode

Abstract

Determination of surface potentials Ψ 0 at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single-crystal electrode (SCrE) is described. The proposed method is based on the surface complexation model and evaluates the surface potential at the isoelectric point, i.e., at pH iep. This value is used for calculation of Ψ 0 values from the measured electrode potentials. Both 1-p K and 2-p K models produced the same result so that the procedure does not depend on the assumed mechanism of the surface charging. It is proposed to determine the pristine point of zero charge pH eln and the isoelectric point pH iep, and use these data to set the scale of surface potentials. The value of pH eln can be obtained at a sufficiently low ionic strength where pH pzc coincides with pH iep. The method is demonstrated on the example of the anatase single-crystal electrode. From the shifts of pH iep and pH pzc with respect to the pristine point of zero charge pH eln it was concluded that Cl − ions exhibit higher affinity for association with positively charged surface groups than ClO − 4 ions. Also, preferential surface association of Na + cations compared to both anions was detected. The slopes of the Ψ 0 ( pH ) functions were found to be significantly lower in magnitude with respect to the Nernst equation, which is due to the high degree of counterion association at the surface caused by their relatively high concentration.