Sensitive determination of quaternary ammonium salts by extraction-spectrophotometry of ion associates with bromophenol blue anion and coextraction

The chloroform extraction behaviour of the ion-associates of a Bromophenol Blue anion (BPB2− :blue) with quaternary ammonium cations (Q+) was examined. During extraction using Q+ with a small number of carbon atoms (Nc) or using Q+ with a large Nc and high concentrations of BPB2−, the addition of a proton to BPB2− gives HBPB− (yellow), which can be extracted as the 1:1 ion-associate Q+ ·HBPB− (ϵ = 2.5 × 104 1 mol−1 cm−1 at λmax = 416 nm) with Q+ into chloroform. At high BPB2− concentrations, the extraction level of the 1:2 ion-associate of one Q+, (Q+)2 ·BPB2− (ϵ = 9.4 × 104 1 mol−1 cm−1 at λmax = 608 nm) was elevated by the presence of other Q+ and the sensitivity and the selectivity for Q+ determination was increased by using co-extraction and a wavelength of 608 nm.