Spectrophotometric determination of quaternary ammonium salts by a flow injection method coupled with thermochromism of ion associates

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Abstract
A selective and rapid spectrophotometric method has been established for the determination of cetylpyridinium and benzalkonium chlorides by a flow injection technique coupled with ion-pair extraction and thermochromism of the ion associates. Selectivity can be enhanced because the absorbance of ion association compounds formed between co-existing amines and the anionic tetrabromophenolphthalein ethyl ester disappears at elevated temperatures in the flow cell. Accordingly, interferences due to amine associates can be eliminated by absorbance measurement at 45 °C. The sample throughputs were 60 h–1 for cetylpyridinium and 50 h–1 for benzalkonium. The calibration graphs were linear in the range 5 × 10–7–2 × 10–6 mol dm–3 for both compounds. The relative standard deviations (n= 5) for 1 × 10–6 mol dm–3 cetylpyridinium and benzalkonium at 45 °C were both 2.1%. The proposed method was used for the selective and rapid determination of quaternary ammonium compounds in pharmaceuticals.

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