The Electrokinetic Potential of Sparingly Soluble Salts

Abstract

Abstract 1) The ζ-potentials of such sparingly soluble salts as barium sulfate, barium chromate, lead (II) sulfate and lead (II) chromate have been measured in water by means of the streaming potential method. Most samples prepared by using either lattice ion in excess have a charge of the same sign as that of the ion. This suggests that the lattice ions occupy many more lattice points on the surface than do other ones, and that it is difficult to remove them by washing. One exception is lead (II) sulfate, in which the ζ-potentials of the two samples are positive. This seems to depend on the surface hydrolysis of lead (II) sulfate. 2) The ζ-potentials vary widely also when the lattice ions are added to water. The effect of both lattice ions are the greatest on lead (II) chromate and the least on lead (II) sulfate. The solubility of the solids is an important factor affecting the variation of ζ-potentials when the lattice ions are added. 3) In order to investigate the effect of the mixed electrolyte solution on the ζ-potential of barium sulfate, sodium citrate, which has a remarkable effect by itself, has been mixed with sodium sulfate or barium nitrate. The resultant ζ-potential curves show a definite minimum, which demonstrates that, although the adsorption of citrate ions excels that of sulfate or barium ions at smaller concentrations, the latter become comparable to the former at larger concentrations. 4) The surface chemical action of triphosphate and orthophosphate ions has been tested. As expected, the higher the polymerization degree of phosphates, the larger the effect on the charge of solid surfaces.