Electrophoresis and titration experiments were used to study the effect of impurities (Zn2+and SO42-) arising as a result of the preparation process, on the pzc and iep of commercial rutile pigment produced by hydrolysis of the sulfate. The presence of these impurities caused the pzc and the iep of the unwashed pigment to be noncoincident; anionic impurities made the pzc move up and the iep move down the pH scale, while cationic impurities had the opposite effect. Successive acid/base/water washings gradually removed the impurities from the surface layers of the oxide and the pzc and the iep of the sulfate base rutile then coincided at pH 5.5. The effect of impurities on the pzc and the iep of commercial chloride base rutile and on hydrous rutile was considered briefly for the purpose of comparison. Hydrous rutile that contained no detectable impurity had an iep/pzc of 6. Heating at high temperatures has been shown to lower the pzc/iep of rutile considerably, but it was found here that although the sulfate base rutile had been heated at 800-1000°C during preparation, the pzc/iep of the clean sample was only 0.5 pH units lower than that of the hydrous rutile. © 1975.
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