Hydrogen ion adsorption at the rutile-water interface to 250°C

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 2pK_w] 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 2pK_w] 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.