Determination of surface charge density of α-alumina by Acid - base titration

Abstract

The surface charge density (s_o) of colloidal alpha alumina suspended in various 1:1 electrolytes was measured using acid-base titration. An autotitrator capable of dispensing accurately 25 ± 0.1 mL of titrant was used. The pH and temperature in the titration cell were monitored using single junction electrodes and platinum resistance thermometers, respectively. A constant supply of nitrogen gas in the cell was used to maintain inert conditions. The whole set up was interfaced with a computer for easy data acquisition. It was observed that the material exhibits a point of zero charge (PZC), this occurred at pH of 7.8 ± 0.1, 7.6 ± 0.2, 8.5 ± 0.1, 8.3 ± 0.1 for NaCl, NaNO₃, CsCl and CsNO₃ systems, respectively. It was also observed that below PZC, s_o increases with increase in electrolyte concentration (C_o) whereas above PZC, s_o decreases with increase in C_o. It was concluded that s_o of this material is a function of pH and C_o and that its polarity can be varied through zero by varying these parameters.

KEY WORDS: Alumina, Surface charge density, Acid-base titration, Point of zero charge

Bull. Chem. Soc. Ethiop. 2007, 21(1), 117-128.