2.6 : Factors Affecting Activity Coefficient

The extended Debye-Hückel equation indicates that the activity coefficient of an ion in an aqueous solution at 25°C depends on three partially interdependent properties: the ionic strength of the solution, the charge of the ion, and the ion size.

The activity coefficient value for an ion is close to one when the solution has almost zero ionic strength, i.e., when the solution shows close to ideal behavior. As the ionic strength of the solution increases from 0 to 0.1 mol/L, a decrease in the activity coefficient value is observed.

Solutions with an ionic strength above 0.1 mol/L are not well represented by the Debye-Hückel equation. In such solutions, the activity coefficient of an ion may exceed unity. The ionic strength of the solution is closely related to the charge of the ions present in the solution, thereby affecting their activity coefficient. For a particular ionic strength, ions with less charge have activity coefficient values higher than ions with a higher charge. In other words, the deviation from ideality is more pronounced for multiply charged ions than for singly charged ions.

Ions with similar charges often have similar activity coefficients. The dominant factor in the coefficient discrepancies here is the ion size parameter. Ion size describes the effective diameter of a hydrated ion, and even ions with similar charges tend to have different extents of hydration and therefore different ion sizes. Ions with similar charges can be hydrated to different extents, resulting in different effective diameters. These diameters are known as the ion size parameters. Ions with smaller ion size parameters deviate more from ideality, resulting in a lower activity coefficient than those with larger ion size parameters but the same charge.