520 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS contents is deduced by difference. In a simple case such as the above, the respective merits of the different approaches are readily ascertained, but with more complex mixtures, especially those containing ethylene oxide derivatives, a great deal of work is needed to find the best scheme of separation or even to find a good one. COLORIMETRIC DETERMINATION Co!orimetric methods of determination are not used so much as volumetric, but they are better discussed first because they employ reagents which are used as indicators in volumetric procedures. The most widely used colori- metric method for artionic surfactants is that of Jones n which consists of shaking an aqueous solution of the sample with methylene blue (designated MB. C1) and chloroform. The surface-active agent forms a salt which is chloroform-soluble, while the excess of methylene blue remains in the aqueous layer, e.g. RSOaNa q- MB. C1 = RSOa.MB q- NaC1 water chloroform soluble soluble By spectrophotometric measurement of the blue chloroform extract, or by comparison with standards, the surfactant content of the solution can be determined. Subsequent workers have tried to eliminate interferences to which the method is subject, and a useful procedure for analysing river water and sewage effluents is that of Longwell and Maniece in which a preliminary extraction with methylene blue from alkaline solution is carried out. Methylene blue is readily oxidized to azures which may be present in the reagent when purchased and which interfere with the colorimetric deter- mination as they compete with methylene blue for the anionic surfactant. The azures can be readily removed by a pre-extraction as described by Abbott 1•. Some of the triphenylmethane dyes, such as rosaniline, have also been used, but we have found them to be much inferior to methylene blue. Carboxylic acids are surface active only in neutral or alkaline solution, and methylene blue cannot be used for their determination. The only suitable alternative reagent, discovered after a wide search, is dimidium b•'omide, described by Holness and Stone •. For cationic surfactants, a wide range of reagents is available bromophenol blue is frequently used, though we have found methyl orange •a to be better, as it can be used at lower pH values giving more precise results with primary, secondary and tertiary, besides the quaternary amines. For determining non-ionic surfactants, most co!orimetric methods are based on precipitation procedures and use a conventional co!orimetric or titrimetric estimation of the inorganic reagent in the precipitate or flitrate.

THE ANALYSIS OF SYNTHETIC DETERGENTS 521 Colorimetric procedures that do not require a separation by filtration or centrifuging are adaptations of colorimetric tests, a starch-iodine procedure and a cobalt thiocyanate method having been published. THE 2•.NIONIC-CATIONIC TITRATION Anionic and cationic surfactants, when present together in aqueous solutions, will neutralise the surface-activity of each other. This is the basis of an early technique of anionic-cationic titration in which one species is determined by titrating with a standard solution of a surfactant of opposite type, neutralisation of surface-activity being shown by a sharp rise in surface tension. Another means of end-point detection was based on the colour change of an indicator in the presence of long-chain quaternary compounds, and the change of bromophenol blue from purple to sky-blue was used by Hart!ey and Runnicles. End-point detection A more precise procedure was devised by Epton, and by Barr, Oliver, and Stubbins. They introduced an organic solvent to extract the salt of the indicator with the excess of surfactant of opposite type, and the end-point was denoted by the movement of an indicator ion from one phase to the other. If a cationic indicator such as methylene blue is used in the titration of an anionic surfactant, it first forms a chloroform-soluble salt according to the equation of the previous section. During titration with a cationic surfactant, the free anionic compound, that is to say the acid or sodium salt, reacts first and then the methylene blue salt begins to react causing the indicator to return to the aqueous phase. RSO•.MB q- CS.Br = RSO•.CS q- MB.Br chloroform water soluble soluble The end-point may arbitrarily be taken as (a) the first appearance of blue colour in the aqueous layer, (b) the complete transfer of blue colour to the aqueous layer, or (c) partial transfer to the appearance of equal colour intensities of the two layers. The titration may also be done in the reverse manner, adding the indicator to the cationic surfactant in the titration vessel and titrating with an anionic surfactant. The choice of end points is among (d) the first appearance of blue colour in the chloroform layer, (e) the complete transfer of colour and (f) partial transfer to the chloroform layer to give equal colour intensities of the two layers. Cullum's paper • pointing out the existence of equilibria at the end-point