On the influence of background electrolyte concentration on the position of the isoelectric point and the point of zero charge

In the case of a specific interaction of background electrolyte ions with t he surface of a solid in an aqueous electrolyte solution a shift of the cha racteristic points, the isoelectric point (iep) and the point of zero charg e (pzc), in an opposite direction was described by Lyklema (in: Th.F. Thadr os (Ed.), Solid/Liquid Dispersions, Academic Press, London, 1987). In the p resent article equations are derived describing the dependence of the iep o n the electrolyte concentration for a specific interaction of one ion only as well as for the case that two ions have specific interactions with the s urface. The considerations are made using the triple layer model and a 1-1 electrolyte. Because of transcendental equations an exact analytic solution is not possible for the pzc. Here an approximation published in Janusz et al. (J. Colloid Interface Sci. 187 (1997) 381) is compared with numerical s olutions. The ratio between the amount of specifically bonded cations and a nions in the inner Helmholtz plane depends on the concentration of potentia l determining ions and on the electrolyte concentration. As can be shown th is ratio is independent of the electrolyte concentration at the iep. Theref ore, this ratio is suitable to decide if there is a predominant cation or a nion bining. (C) 2001 Elsevier Science B.V. All rights reserved.