For a metal hydroxide that is only very slightly (sparingly) soluble, when the solid is in a saturated solution, the following equilibrium exists with exchange between ions in the solid and in the solution:
Mnn+(aq) + nOH−(aq)
At this point, the addition of any more hydroxide ions will cause a little more of the metal hydroxide to precipitate.
Since this is an equilibrium reaction we can write :
where [ ] represents the concentration in
Since M(OH)n is a solid, its concentration is a constant. So we can now make use of a new constant, called the Solubility Product,
Since the substance is barely soluble.
will have a very low value.
Calculations using the of solubility product
(These are most difficult for a 1:2 like cadmium hydroxide)