One of the primary physicochemical considerations in preparing pharmaceutical
solutions is the solubility of the drug in a suitable solvent. Solubility
may be defined as the maximum concentration of a substance that may
be completely dissolved in a given solvent at a given temperature and pressure.
When both solute and solvent are liquids, the term
miscibility rather than solubility may be used to
describe the affinity between the liquids.
The solubility of a substance may be described in a variety of ways.
The USP/NF generally expresses the solubility in terms of the volume of
solvent required to dissolve 1 gram of the drug at a specified temperature
(eg. 1 g ASA in 300 ml H2O, 5 ml ethanol at 25°C). Other references
may use more subjective terms to describe solubility, such as those given
in the following table from Remington's.
||Parts of solvent needed for 1 part solute
|| < 1
| Freely soluble
| Sparingly soluble
| Slightly soluble
| Very slightly soluble
| Practically insoluble or insoluble
|| > 10,000
Liquids which form a homogenous system when mixed in any proportion
are said to be miscible (eg. water
and ethanol). Those in which only certain volume ratios produce homogenous
mixtures are said to be miscible in certain proportions
(eg. water and chloroform). Immiscible
liquids will not produce a homogenous solution in any proportions (eg.
water and olive oil).
The aqueous solubility of all drugs is of interest to us, since it is
only in the form of an aqueous solution that a drug can be absorbed into
the general circulation to exert a therapeutic effect.