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

522 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS is important and shows the reasons why end-points (b) and (d), at which most of the indicator is in the aqueous phase and nearly all the surfactant in the organic phase, are unsatisfactory with commercial matehals. End- point (a) is rarely used because it gives no warning of its approach. It also has the disadvantage that trace impurities in the indicator, particularly of the oxidation products in methylene blue, may impart a bluish colour to the aqueous layer well before the end-point. The same impurities also interfere in end-point (e) and from our experience we recommend the use of this end-point only with the artionic indicators which are purer and more stable than the cationic ones. This leaves end-points (c) and (f) as the best for cationic indicators such as methylene blue. It might be mentioned at this point that methylene blue seems to be practically the only cationic indicator that is used for the titration and despite the limitations it seems to be preferred to anionic indicators in most commercial analytical laboratories in the U.K. and abroad. A wide choice of artionic indicators of the sulphonphthalein, sulphonic acid, and fluorescein classes is available, but the vast majority of workers appear to follow closely the technique of the early workers, Barr, Oliver, and Stubbins, and to use bromophenol blue. Standardisation At the end of the titration in cases (a) and (e) the whole of the indicator is in combination with surfactant, and a blank correction, which is constant, calcu!ab!e, reproducible and readily determined, must be applied. In cases (c) and (f) only part of the indicator remains combined with surfactant and the necessary correction depends upon the proportion involved, and this depends in turn upon the relative volumes of aqueous and chloroform layers. Experimental determinations of the blank or of a correction factor have been described by several writers, but for routine analysis it is sufficient to standardise the titrant under similar conditions to those of a determination, thus eliminating the correction. The standard substance in this approach must be of a similar composition to that being determined, which means in many cases that it must contain a mixture of isomers or of homologues as do the commercial materials, and its composition can therefore only be established by other analytical methods, principally by extraction or ion- exchange methods. For na!kyl sulphates, it is possible to prepare the pure matehals, and these can be used as standards in this field. Miscellaneous Factors After the choice of indicator, of end-point and of standard, that of the titrant is the most important. Cetyltrimethylammonium chloride and cety!- pyridinium bromide seem to be most frequently used for anionic detergents.