酸性染料ブロモクレゾールパープルと第四級アンモニウムイオンの変色反応を利用する陰イオン界面活性剤の吸光光度法

水溶液中で第四級アンモニウムイオン(セチルジメチルベンジルアンモニウムイオン=CDMBA+)の酸性染料(ブロモクレゾールパープル:BCP)に及ぼす変色反応を利用する陰イオン界面活性剤(ドデシル硫酸ナトリウム:Na・LS)の吸光光度定量法を開発した.CDMBA(+)はpH8付近でBCPと反応し,588nmでの吸光度を低下させる.又CDMBA(+)はLS(-)と安定なイオン会合体を形成する.従って一定量のCDMBA(+)の存在下では,LS(-)量の増加に伴い588nmでの吸光度は増大する.この原理による陰イオン界面活性剤の吸光光度法での検量線は0～1.7×10(-5)Mの濃度範囲でベールの法則に従い,直線の傾きから求めたモル吸光係数は約6×10(4)l mol(-1)cm(-1)であった.本法は溶媒抽出法に比べ,共存イオン特にかさ高い陰イオンの影響が小さい操作の簡便な定量法であり,陰イオン界面活性剤1～2ppmに対する視覚分析法としても用いることができる.

A method for spectrophotometric determination of anionic surfactants with Bromocresol Purple (BCP) and cetyldimethylbenzylammonium ion (CDMBA(+)) was developed. The addition of CDMBA(+) to BCP solution buffered at pH 8 leads to a decrease in the absorbance at 588 nm. When anionic surfactants are added to the solution, the absorbance at 588 nm increases with increasing amounts of anioic surfactants. This increase in absorbance is caused by an ion association of CDMBA(+) with anionic surfactants. On the basis of this principle, the simple spectrophotometric method for the determination of anionic surfactants was developed. The recommended procedure is as follows: Pipet an adequate amount of a sample solution containing anionic surfactants up to 1.7×10(-5) M into a 10-ml volumetric flask. Add 0.5 ml of 4×10(-4) M CDMBA solution, 1 ml of 1.7×10(-4) M BCP solution and 0.5 ml of 0.2 M phosphate buffer solution(pH 8.1). Mix thoroughly and dilute to the mark with distilled water. Measure the absorbance of the solution at 588 nm against the reagent blank. The calibration graph shows linearity up to 1.7×10(-5) M of anionic surfactants and the apparent molar absorptivity is about 6×10(4) l mol(-1) cm(-1). This proposed method is very simple and there is minimal interference from coexisting anions. The method also can be applied to the determination of anionic surfactants by a visual method.