HYDROGEN-ION ADSORPTION AT THE RUTILE-WATER INTERFACE TO 250-DEGREES-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 degrees C. The pH of zero net surface charge (pHzpc) decreases to 200 degrees 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 degrees C and is also equivalent to the equilibrium constant for an isocoulombic form of the ''1pKa'' 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.