ABSTRACT
Partially (or sparingly) soluble salts are salts that only partly dissolve, forming a saturated solution of ions. For these systems an equilibrium exists between the solid salt and the dissolved ions:
The equilibrium constant for this reaction is often called the solubility product, Ksp, and is given by:
since MX is a pure solid (see Topic C1), where and are the activities of the M+ ion (the cation, see Topic E1) and the X−ion (the anion, see Topic E1). A good example would be solid silver chloride, which only partially dissolves into Ag+ and Cl− ions. By substituting for the activity of the ions (see Topics C1 and E2):
where and are the activity coefficients of M+and X− and is the standard concentration of 1 mol dm−3. For sparingly soluble salts, such as silver chloride, which have concentrations much less than 0.001 mol dm−3, there is negligible interaction between ions in solution and the activity coefficients can be approximated to unity. The equation then becomes:
The solubility, s, of the salt is the concentration of dissolved salt in the solution. For a salt MX, as one mole of M+ and X− ions is produced by the dissolution of one mole of salt. Therefore:
which allows the solubility of the salt in water to be determined from Ksp. For sparingly soluble salts containing ions with differing stoichiometries, a similar expression can be obtained. For example for silver sulfide, Ag2S, the solubility equilibrium is:
One mole of silver sulfide dissolves to form one mole of sulfide ions and two moles of silver ions.
