A stirred hydrogen-electrode concentration cell was used to follow hydrogen ion adsorption by the rutile surface in NaCl media (0.01-1.0 m) between 25 and 250°C. The pH of zero net surface charge (pHzpc) decreases to 200°C and then appears to increase. Away from the pHzpc, negative surface charge is screened more efficiently by Na+ ions than positive charge is by Cl- ions and this effect increases with temperature. Thus, Na+ moves closer to the rutile surface with increasing temperature relative to Cl-. Finally, [pHzpc - 1 2pKw] is approximately constant (- 1.1 ± 0.2) for this rutile sample to 250°C and is also equivalent to the equilibrium constant for an isocoulombic form of the "IpKa" surface ionization model. Thus, the constancy of [pHzpc - 1 2pKw] may also be useful for extrapolating rutile pHzpc values to higher temperatures and pressures than those studied here, and may also apply to many other oxide surfaces as well.
ASJC Scopus subject areas
- Geochemistry and Petrology