In the examples we have considered so far, the acidic drugs have donated a single proton. There are several acids, for example citric, phosphoric and tartaric acids, that are capable of donating more than one proton; these compounds are referred to as polyprotic or polybasic acids. Similarly, a polyprotic base is one capable of accepting two or more protons. Many examples of both types of polyprotic drugs can be found in Table 3.6, including the polybasic acids amoxicillin and fluorourocil, and the polyacidic bases pilocarpine, doxoru-bicin and aciclovir. Each stage of the dissociation may be represented by an equilibrium expression and hence each stage has a distinct pKa or pKb value. The dissociation of phosphoric acid, for example, occurs in three stages; thus:
H3PO4 + H2O
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