Factors Affecting Activity Coefficient

The activity coefficient of an ion depends on the ionic strength of the solution.

As indicated by the Debye–Hückel theory, when the ionic strength approaches zero, the activity coefficient approaches unity.

As the ionic strength increases from 0 to 0.1 moles per liter, the activity coefficient value decreases.

However, with an increase in ionic strength above 0.1 moles per liter, the activity coefficient may increase.

The influence of ionic strength on the activity coefficient is associated with the charge of the ion, which also directly influences the activity coefficient.

With an increased charge on the ion, the decrease in the activity coefficient is even more pronounced.

For a particular ionic strength, ions with the same charges have approximately the same activity coefficients.

Another significant factor affecting the activity coefficient of an ion is the ion size parameter—the effective diameter of the hydrated ion.

Smaller ions with a high charge have larger effective diameters and deviate less from ideality than similarly charged ions with smaller diameters do.