2.2:
Ionic Strength: Effects on Chemical Equilibria
Adding 50 mmol of potassium nitrate to a liter of saturated aqueous calcium sulfate solution leads to the concentrations of dissolved calcium and sulfate ions increasing by about 30%.
Along with the counter-ions already present, the ions of opposite charge from the added inert salt—potassium nitrate—contribute to the ionic atmospheres around the primary cation and anion.
In consequence, the net charges of the calcium and sulfate ions decrease, resulting in less attraction between them. So, the solubility equilibrium shifts toward the dissociated ions.
This reduced tendency of cations and anions to associate with each other is reflected in their increased concentration in the solution, enhancing the solubility of the sparingly soluble calcium sulfate.
The higher the ionic strength of a solution, the greater the charge on the ionic atmosphere, and the greater the dissociation of the salt into ions.
This phenomenon is known as the salt, electrolyte, or diverse ion effect.
This effect is more prominent for sparingly soluble salts having a greater charge on the constituting ions.
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.