Spectrophotometric determination of trace amounts of quaternary ammonium salts in drugs by ion-pair extraction with bromophenol blue and quinine

Abstract

The sensitive and selective method reported here is based on the reaction of quinine cations with bromophenol blue (BPB) to form a 1:1 complex anion, which, with a quaternary ammonium salt at pH 6.7, is extractable into chloroform as an ion pair. Berberine and benzethonium were extracted in large amounts into chloroform with BPB only when quinine, in a pH 6.7 buffered aqueous solution, co-existed with them. In addition, when this showed a strong ion associate ability, then berberine and benzethonium could react in the organic phase with BPB to give a blue product. The absorbance of the ion associate in chloroform was measured at 610 nm. Calibration graphs were linear in the range 0.5 × 10^–6–3.0 × 10^–6M for berberine and benzethonium. Molar absorptivities were 3.75 × 10⁴ l mol^–1 cm^–1 for the berberine associate and 3.14 × 10⁴ l mol^–1 cm^–1 for the benzethonium associate. The blue associate can be used for selective and sensitive spectrophotometric determination of berberine and benzethonium in multi-component drugs.