Kinetic study and analytical applications of the micellar catalysed reactions of 1-fluoro-2,4-dinitrobenzene with thiols using a fluoride-selective electrode

Abstract

A kinetic study and analytical application of the micellar catalysed reaction of several thiols (thioglycolic, mercaptopropionic, mercaptosuccinic and thiobenzoic acids, thioglycerol and thiophenol) with 1-fluoro-2,4-dinitrobenzene (FDNB) using a fluoride-selective electrode is described. The effect of various types of surfactants, cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), Tween 80 and sodium dodecyl sulfate, on the electrode behaviour and the reaction rate was systematically studied. Cationic surfactants (CTAB, CPC) showed a considerable acceleration of the reaction rate (up to 684 times). Reaction orders (unity for both FDNB and thiol) and experimental stoichiometric reaction-rate constants in the presence of CPC were calculated from initial slope measurements (ΔE/Δt). Calibration graphs for quantitative determinations in micromolar to millimolar concentration ranges were constructed using the fixed-time method (10^ΔE/S–1 versus c). The proposed method allows measurements in highly coloured or turbid solutions. Results obtained for the determination of thioglycolic acid in commercial hair cold-wave suspensions and depilatory cream compare favourably with those given by the reference iodimetric titration method. Automation of the method can be achieved by using flow injection with a fluoride flow-through electrode as detector.