Potential Determining Ion

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

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

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

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

The ability of an ion to preferentially dissolve (as a result of unequal activities) over its counterion is classified as the Potential Determining Ion. This in turn results in the remaining particle possessing either a net positive/negative surface charge.One way in which surface charge can develop is by adsorption of an ion where the solid acts as an electrode. (e.g., H⁺ and OH⁻ on the surfaces of clays).

In clay-aqueous systems the potential of the surface is determined by the activity of ions which react with the mineral surface. Frequently this is the hydrogen ion H⁺ in which case the pertinent activity is determined by the pH.

The simultaneous adsorption of protons and hydroxyls as well as other potential determining cations and anions, leads to the concept of zero point of charge or ZPC, where the total charge from the cations and anions at the surface is equal to zero.

The charge must be zero and this does not necessarily mean the number of cations versus anions in the solution are equal. For clay minerals the potential determining ions are H⁺ and OH⁻ and complex ions formed by bonding with H⁺ and OH⁻.