Codeine is a weakly basic drug and hence its pH will be given by ph _ èpKw + èpKa + 2log c where c = 1 g in 120 cm3 = 8.33 g dm 3 = 0.02633 mol dm 3.

Drug salts

Because of the limited solubility of many weak acids and weak bases in water, drugs of these types are commonly used as their salts; for example, sodium salicylate is the salt of a weak acid (salicylic acid) and a strong base (sodium hydroxide). The pH of a solution of this type of salt is given by pH _ èpKw + èpKa + 1log c

Alternatively, a salt may be formed between a weak base and a strong acid; for example, ephedrine hydrochloride is the salt of ephedrine and hydrochloric acid. Solutions of such drugs have a pH given by ph _ 2pKa- 2log c

Finally, a salt may be produced by the combination of a weak base and a weak acid, as in the case of codeine phosphate. Solutions of such drugs have a pH given by pH _ 1pKw + 1pKa -2pKb

Notice that this equation does not include a concentration term and hence the pH is independent of concentration for such drugs.

EXAMPLE 3.8 Calculation of the pH of drug salts Calculate the pH of the following solutions

(a) 5% oxycodone hydrochloride (pKa = 8.9, mol. wt. = 405.9).

(b) 600 mg of benzylpenicillin sodium (pKa = 2.76, mol. wt. = 356.4) in 2 cm3 of water for injection.

(c) 100 mg cm 3 chlorphenamine maleate (mol. wt. = 390.8) in water for injection (pKb chlorphenamine = 5.0, pKa maleic acid = 1.9).

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