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 the surface of a solid in an aqueous electrolyte solution a shift of the characteristic points, the isoelectric point (iep) and the point of zero charge (pzc), in an opposite direction was described by Lyklema (in: Th.F. Thadros (Ed.), Solid/Liquid Dispersions, Academic Press, London, 1987). In the present article equations are derived describing the dependence of the iep on 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 surface. 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 solutions. The ratio between the amount of specifically bonded cations and anions in the inner Helmholtz plane depends on the concentration of potential determining ions and on the electrolyte concentration. As can be shown this ratio is independent of the electrolyte concentration at the iep. Therefore, this ratio is suitable to decide if there is a predominant cation or anion bining.