Mixed ternary ion associate formation between xanthene dye, cinchona-alkaloid and quaternary ammonium ion and its application to the determination of trace amounts of quaternary ammonium salts in pharmaceuticals.

Xanthene dyes such as eosin and tetraiodofluorescein form 1:2 ion associates with quaternary ammonium ions at pH 7-9; however, the development of color in the organic solvent is poor. When a quaternary ammonium salt is added in the extractable 1:2 associate formed between a xanthene dye and a cinchona-alkaloid in the neutral media, a mixed ternary ion associate (xanthene dye:cinchona-alkaloid:quaternary ammonium ion=1:1:1) is formed. Its extractability is enhanced due to the more bulky associate formation. The ion association caused by addition of cinchona-alkaloids is unique. Of the cinchona-alkaloids, quinidine and cinchonidine with tetraiodofluorescein show excellent effects on sensitivity and extractability. Eosin and tetraiodofluorescein are useful as ion association reagents in the ion associate formation. The apparent molar absorptivities are about 1x10(5) l mol(-1) cm(-1) and the calibration range for quaternary ammonium salt is from 2.5x10(-7) to 1.5x10(-6) M. The proposed method is applicable to the selective and sensitive determination of cetylpyridinium chloride, berberine chloride and benzethonium chloride in pharmaceuticals.