Many drug substances can be classified as being either acids or bases in that they possess the ability to react with other acids or bases that are stronger than themselves. As such, they would also possess the ability to exist as ionic species under certain conditions. The state of ionization of a substance will often profoundly affect its degree of aqueous solubility, as evidenced by the high solubility of sodium benzoate as opposed to the low solubility of benzoic acid. The utility of salt forms as active pharmaceutical ingredients is well known, and represents one of the means to increase the degree of solubility of an otherwise intractable substance (Berge et al. 1977; Stahl and Wermuth 2002).

To develop an understanding as to how the relative acidity or basicity of a drug substance is able to influence its aqueous solubility, the principles of chemical equilibria as applied to weakly ionic substances will first be developed. Subsequent to this, the influence on solubility of the degree of ionization of substances will be considered. However, certain fundamental concepts must be developed first in order for the subsequent sections to be most meaningful.

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