Ionic Equilibria and the pH Dependence of Solubility

Part of the Biotechnology: Pharmaceutical Aspects book series (PHARMASP, volume VI)


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).


Weak Acid Free Acid Aqueous Solubility Drug Substance Ionic Equilibrium 
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© Springer 2007

Authors and Affiliations

    • 1
  1. 1.Center for Pharmaceutical PhysicsMilfordNJ

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