When the electrolyte is weak and does not dissociate appreciably, the solubility of the acid in water or acidic solution is S = [HP], which, for phenobarbital is approximately 0.005 mol/lit, in other words, 0.12%
An analogous derivation can be carried out to obtain the equation for the solubility of a weak base as a function of the pH of a solution. The expression is
Where S is the concentration of the drug initially added as the salt and So is molar solubility of the free base in water.
Here pHp is the pH above which the drug begin to
precipitate from solution as the free base
The solubility equation can be written in logarithmic form, beginning with equation 7. By rearrangement, we obtain