Potential Determining Ion

When placed into solution, salts begin to dissolve and form ions. This is not always in equal proportion, due to the preference of an ion to be dissolved in a given solution. The ability of an ion to preferentially dissolve (as a result of unequal activities) over its counterion is classified as the Potential Determining Ion. The properties of this ion are strongly related to the surface potential present on a corresponding solid.

This unequal property between corresponding ions results in a net surface charge. In some cases this arises because one of the ions freely leaves a corresponding solid and the other does not or it is bound to the solid by some other means. Adsorption of an ion to the solid may result in the solid acting as an electrode. (e.g., H⁺ and OH⁻ on the surfaces of clays).

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

${\displaystyle \mathrm {NaCl} _{(s)}\leftrightarrow \mathrm {Na} _{(aq)}^{+}+\mathrm {Cl} _{(aq)}^{-}}$

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

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

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