POTENTIOMETRIC TITRATIONS OF RUTILE SUSPENSIONS TO 250-DEGREES-C

A stirred hydrogen electrode concentration cell was used to conduct po tentiometric titrations of rutile suspensions from 25 to 250 degrees C in NaCl and tetramethylammonium chloride media (0.03 to 1.1 m). Hydro thermal pretreatment of the rutile improved titration reproducibility, decreased titration hysteresis, and facilitated determination of the point of zero net proton charge (pHznpc). These pHznpc values are 5.4, 5.1, 4.7, 4.4, 4.3 (+/-0.2 pH units), and 4.2 (+/-0.3 pH units) at 25 , 50, 100, 150, 200, and 250 degrees C, respectively. The difference b etween these pHznpc values and pK(w) (the neutral pH of water) is rath er constant between 25 and 250 degrees C (-1.45 +/- 0.2). This constan cy is useful for predictive purposes and, more fundamentally, may refl ect similarities between the hydration behavior of surface hydroxyl gr oups and water. A three-layer, 1pKa surface complexation model with th ree adjustable parameters (two capacitance values and one counterion b inding constant) adequately described all titration data. The most app arent trend in these data for pH values greater than the pHznpc was th e increase in proton release (negative surface charge) with increasing temperature. This reflects more efficient screening by Na+ relative t o Cl-. Replacing Na+ with the larger tetramethylammonium cation for so me conditions resulted in decreased proton release due to the less eff icient screening of negative surface charge by this larger cation. (C) 1998 Academic Press.