Solubility is expressed in units of concentration. After reviewing many publications featuring solubility measurement of drugs we adopted the following definitions for the solubility parameters that our instruments measure.
- Intrinsic solubility is the equilibrium solubility of the free acid or free base form of an ionisable compound at a pH where it is fully un-ionised.
- Equilibrium solubility is the concentration of compound in a saturated solution when excess solid is present, and solution and solid are at equilibrium.
- Kinetic Solubility is the concentration of a compound in solution when an induced precipitate first appears.
- Supersaturated solutions contain excess neutral species in solution, which will carry on precipitating until the system reaches equilibrium.