Abstract
The percentage of unprotonated aromatic amino grouping (p K a 8.4) in the chloroquine molecule has been determined spectrophotometrically from measurements at 329 nm and 343 nm in suitable buffer solutions from pH 2.20 to 9.00. It is shown that ethanol, glycerol, lactose and dextrose can reduce the extent of protonation of chloroquine over this pH range, and could therefore possibly promote a more rapid absorption of chloroquine from the gastrointestinal tract. Complexation effects in the presence of these compounds cause chloroquine spectral shifts even in 0.01 M hydrochloric acid and in 0.01 M sodium hydroxide solutions. Errors can be minimised in the direct spectrophotometric assay of chloroquine by making measurements at 343 nm in 0.01 M hydrochloric acid. The ionisation of many compounds used in medicine that are weak acids or bases could be subject to effects similar to those shown by chloroquine.
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