Ionic Strength: Effects on Chemical Equilibria

Adding potassium nitrate to a saturated calcium sulfate solution increases the solubility of calcium sulfate by approximately 30%. The inert salt's ions form ionic atmospheres around the primary calcium and sulfate ions, reducing their net charges and weakening the electrostatic attraction between them. This shift toward dissociated ions enhances solubility, a phenomenon known as the salt effect or diverse ion effect.

Le Châtelier's principle cannot predict this equilibrium shift because adding an inert salt like potassium nitrate does not change the concentration of the primary ions directly. Instead, the effect arises from changes in the effective concentration and ionic atmospheres surrounding calcium and sulfate ions, reducing their net charges and altering the equilibrium through activity rather than stoichiometric concentration changes.

Ionic atmospheres form around each primary ion, consisting of oppositely charged ions from both the original salt and the added inert compound. These surrounding ions shield the primary ion's charge, causing its net charge to decrease. The reduced net charge weakens electrostatic attraction between cations and anions, promoting dissociation and increasing solubility of the sparingly soluble salt.

Higher ionic strength produces a greater salt effect because more ions diffuse into the ionic atmosphere surrounding primary ions. This increased ion concentration further reduces the net charge on primary ions, facilitating greater dissociation of the salt. The relationship is direct: as ionic strength increases, the dissociation of sparingly soluble salts becomes more pronounced.

Doubly charged ions, such as those in barium sulfate, experience a more pronounced salt effect than singly charged ions like those in silver chloride when exposed to the same concentration of inert salt. The higher charge density on doubly charged ions creates stronger electrostatic interactions that are more significantly affected by the shielding effect of ionic atmospheres, resulting in greater solubility enhancement.

The salt effect depends critically on ionic strength. As ionic strength increases, more ions populate the ionic atmosphere, progressively lowering the net charge on primary ions and reducing precipitation frequency. This direct relationship means that higher ionic strength solutions promote greater dissociation of sparingly soluble salts, shifting the solubility equilibrium toward dissolved ions.

Ions from inert compounds like potassium nitrate surround primary ions, forming part of their ionic atmospheres. These oppositely charged ions reduce the effective charge on primary cations and anions, decreasing electrostatic attraction between them. This charge reduction shifts the solubility equilibrium toward dissociated ions, increasing the concentration of dissolved calcium and sulfate ions in solution.