Determination of anionic surfactants in an aqueous medium by differential photometric titration method

微分光度滴定装置を用いて,水溶液での陰イオン界面活性剤を定量する方法について検討した.滴定終点決定の指示薬系としては,酸性染料のテトラブロモフェノールフタレインエチルエステル(TBPE)と非イオン界面活性剤のTriton X-100(TX-100)の混合溶液を用いた.TBPEは非イオン界面活性剤の存在下で,酸性では黄色となり可溶化しているが,滴定剤の第四級アンモニウムイオンと反応するとイオン会合体を形成し,解離型(TBPE-)の青色(λ(max)=610nm)となる.滴定剤には第四級アンモニウム塩のジステアリルジメチルアンモニウム・クロリド(DSDMA(+)・Cl(-))を用いた.本滴定法で10(-6)～10(-3)Mの濃度範囲の陰イオン界面活性剤が定量できる.実際試料の定量結果はオキシエチレン基を持つもの以外の試料において,JIS滴定法(JIS K 3362)及びエチルバイオレット抽出吸光光度法(JIS K 0101)とよく一致した.A simple and rapid automatic photometric titration method was developed for the determination of anionic surfactants in a one-phase aqueous system with tetrabromophenolphthalein ethyl ester (TBPE) as an indicator. In the presence of non-ionic surfactants such as Triton X-100, protonated TBPE reacts with cation (Q(+)) to form an ion associate (Q(+)·TBPE(-)), causing a color change from yellow to blue. The titrant, distearyldimethylammonium ion, was added to an anionic surfactant solution at pH 3.2, and the absorbance changes were monitored by a fiber-optic sensor with a 630 nm interference filter. The end points were read from the inflection points with differential curves. Anionic surfactants at concentrations from 10(-6)10(-3) mol dm(-3) could be determined by the proposed method. The relative standard deviations of various anionic surfactants were 0.30.4%. The proposed method was applied to some practical samples, and the results, except for those of anionic surfactants possessing polyoxyethylenyl groups, were in good agreement with the results of the JIS titration method (Epton method) and a solvent extraction method with Ethyl Violet