2.2 : Ionic Strength: Effects on Chemical Equilibria

The addition of an inert ionic compound increases the solubility of a sparingly soluble salt. For example, adding potassium nitrate to a saturated solution of calcium sulfate significantly enhances the solubility of calcium sulfate. Le Châtelier's principle cannot predict this shift in the equilibrium. Instead, this could be explained in terms of changes in the effective concentration of the ions in solution in the presence of added inert salt.

In this solution, the primary cation—the calcium ion—is surrounded by the primary anion—the sulfate ion—which in turn is surrounded by calcium ions, forming ionic atmospheres around each ion. Additionally, the primary cation and anion are surrounded by the oppositely charged ions of the added inert salt. The ions from the inert compound in the ionic atmosphere causes the net charge on the primary ions—calcium and sulfate, in this case—to decrease, reducing the frequency of precipitation. This shifts the equilibrium towards the dissociated ion, increasing the solubility of the sparingly soluble salt. This phenomenon is termed the salt effect, electrolyte effect, or diverse ion effect.

The salt effect is highly dependent on the ionic strength of the solution. With an increase in the ionic strength of the solution, more ions diffuse in the ionic atmosphere, causing the net charge on the primary ion to be even lower, facilitating greater dissociation of the salt. Additionally, the charge on the ions constituting the sparingly soluble salt affects the extent of the salt effect. For example, the solubility of doubly charged ions, such as those constituting barium sulfate, is influenced more than the solubility of singly charged ions, such as those constituting silver chloride, by the same concentration of potassium nitrate.