Potential determining ion

In a colloidal dispersed system, ion dissolution arises, where the dispersed particles exist in equilibrium with their saturated counter part, i.e.

NaCl_(s) → Na⁺_(aq) + Cl^-_(aq)

The behavior of this system is characterised by the components activity coefficients and solubility product, i.e.

$a_{Na^{+}}\cdot a_{Cl^{-}}=K_{sp}$

The ability of an ion to preferentially dissolve (as a result of unequal activities) is classified as the Potential Determining Ion. This in turn results in the remaining particle possessing either a net positive/negative surface charge.