Potentiometric studies of the rutile-water interface: Hydrogen-electrode concentration-cell versus glass-electrode titrations

This paper represents a comparison of surface protonation studies of rutile in NaCl media obtained using a conventional glass-electrode autotitrator system from 10 to 50°C, and hydrogen-electrode concentration cells from 25 to 250°C [J. Colloid Interface Sci., 200 (1998) 298]. Experimental conditions were matched as closely as possible between the two techniques, permitting a direct comparison of the results. Values for the pH of zero net proton charge (pH_znpc) of the rutile surface obtained at 10 and 35°C were consistent with the temperature trends observed previously using hydrogen-electrode concentration cells. The pH_znpc of rutile decreases systematically from 5.7 to 4.2 as temperature increases from 10 to 250°C. Moreover, the experimentally determined pH_znpc values agree with independent estimates of the pH at the pristine point of zero charge (pH_znpc) calculated from an extension of the revised Multi-Site Complexation (MUSIC) Model of Hiemstra et al. [J. Colloid Interface Sci., 184 (1996) 680]. Surface protonation curves obtained from the glass-electrode titration results were rationalized using surface protonation constants derived from the MUSIC Model, in conjunction with a Basic Stern representation of the electrical double layer (EDL) structure. Best-fit parameters (Stern layer capacitance values, and electrolyte cation and anion binding constants) are consistent with those obtained from fits to titration curves obtained using hydrogen-electrode concentration cells at 25 and 50°C. Consequently, this comparison demonstrates that independent conventional glass-electrode and hydrogen-electrode concentration cell titrations provide completely compatible results despite the intrinsic differences in the two techniques (pH calibration, equilibration times, stirring rates, gas phase composition, etc).