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ionised impurities from water and in the prolongation of drug action. Purified water may be obtained by passing through two columns containing a strong cation exchanger and a strong anion exchanger, respectively, or a column containing mixed resins. Anionic impurities in the water are replaced by OH0 from the anion exchanger and cations by H+ from the cation-exchange resin. Dissolved salts are thus removed and replaced by H2O molecules, but of course nonionic impurities and colloidal material are not removed. Regeneration of the resins is accomplished using NaOH and HCl for the anion and cation exchangers, respectively.

In the prolongation of drug action, ionexchange resins are employed to form complexes with drug substances, especially basic drugs such as ephedrine, pholcodine (XVII)

and phenyltoloxamine (XVIII) (an isomer of diphenhydramine). Pholtex is a sustained-action liquid utilising a sulfonic acid ionexchange resin with pholcodine and phenyl-toloxamine as resin complexes.

Ion-exchange resin (Amberlite IRP-69 resin) has been used with betaxolol as an ophthalmic suspension. This formulation with 0.25% betaxolol gave virtually identical levels of drug in the aqueous humour as did the 0.5% control.10 The rate of release of basic drugs from cation-exchange resins depends on the diameter of the resin beads, on the degree of crosslinking within the resin and on the pKa of the ionisable resin group. The resin-drug complex may be tableted and administered orally; resin complexes have been used to mask the taste of bitter drugs and to reduce the nausea produced by some irritant drugs. Some

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