HYDROGEN-ION ADSORPTION AT THE RUTILE-WATER INTERFACE TO 250-DEGREES-C

A stirred hydrogen-electrode concentration cell was used to follow hyd rogen 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 t he pHzpc, negative surface charge is screened more efficiently by Naions than positive charge is by Cl- ions and this effect increases wit h temperature. Thus, Na+ moves closer to the rutile surface with incre asing temperature relative to Cl-. Finally, [pHzpc - 1/2pK(w)] is appr oximately constant (-1.1 +/- 0.2) for this rutile sample to 250 degree s C and is also equivalent to the equilibrium constant for an isocoulo mbic form of the ''1pKa'' surface ionization model. Thus, the constanc y of [pHzpc - 1/2pK(w)] may also be useful for extrapolating rutile pH zpc values to higher temperatures and pressures than those studied her e, and may also apply to many other oxide surfaces as well.