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 degreesC, and hydrogen-electrode concentration cells from 25 to 250 degreesC [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 degreesC 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 degreesC. 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 degreesC. 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). (C) 2002 Elsevier Science B.V. All rights reserved.