OPEN CIRCUIT POTENTIAL MEASUREMENTS WITH TI/TIO2 ELECTRODES

The open circuit potential (OCP) vs pH response of Ti/TiO2 electrodes prepared by thermal and electrochemical oxidation of metallic titanium was measured in KNO3 aqueous solutions in order to establish whether the slopes of the OCP-pH curves can be used as a measure of the variation of surface potential (psi0) of TiO2 with the pH of the aqueous solution. For comparison purposes, psi0-pH slopes were also evaluated from surface charge-pH data obtained by acid-base potentiometric titrations of TiO2 dispersions. Ti/TiO2 electrodes showed a linear OCP-pH response in the range of pH 5-10 with slopes of -0.039 +/- 0.005 V pH-1. The OCP-pH dependence of Ti/TiO2 electrodes was lower than that predicted thermodynamically. According to the triple-layer model, used to describe the oxide/aqueous solution interface, the OCP-pH slopes obtained for Ti/TiO2 electrodes cannot be identified with the variation of psi0 with the pH. Calculations with this model indicated that neither the single nor the double extrapolation methods used to evaluate the intrinsic ionization constant of oxide surface sites render realistic values of these parameters for the TiO2 surface. The analysis of surface charge-pH data and the use of the triple-layer model to predict experimental values indicated that the psi0-pH slopes of TiO2 surfaces must be greater (in absolute magnitude) than 0.041 V pH-1.