514 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS treat the substance with a coloured reagent of the opposite ionogenic type and shake the aqueous mixture with a non-polar liquid such as chloroform. The reagent should be such that its salts with inorganic ions are not extracted by the solvent, but its salts with surface active agents, containing a hydro- phobic group in each molecule, will be readily extracted the appearance of the coloured molecule in the organic layer will show the presence of a surface active agent. The usual reagent for anionic surfactants is methylene blue, but this can be used only in acid solution and cannot therefore detect the carboxylate group. Dimidium bromide s can be used over a wide range of pH values and can therefore be used for carboxylate groups as well as sulphate and sul- phonate. For cationic surfactants, a wide range of dyes and indicators containing the su!phonate group is available and bromophenol blue seems to be most commonly used. An alternative procedure for detecting anionic or cationic surfactants is to test whether the substance will discharge the colour produced with a known surfactant of opposite type and an appropriate reagent. To test for an anionic surfactant, an aqueous alkaline solution of bromophenol blue plus a trace of a cationic surfactant is shaken with chloroform, and then the sample is added and the mixture again shaken. To test for a cationic surfactant, the material is added to acid methylene blue plus a trace of dodecylbenzene sulphonate plus chloroform. A compound which discharges the colour of the chloroform layers in both anionic and cationic tests is an ampho!ytic surfactant. Most non-ionic detergents are of the polyethanoxy type and these will combine with large anions, such as ferrocyanide, cobaltothiocyanate, molybdophosphate, giving precipitates with the cations present, barium being needed in the case of the last. Another test for ethanoxy groups is due to Rosen a and consists of heating with phosphoric acid and testing for acetaldehyde. The polyhydric alcohol type of non-ionic surfactant also reacts with large anions complex iodides are often used, but hexanitrato- cerate is a simpler though less specific reagent. Linking groups The linking group is best investigated by studying the stability of the molecule towards acid and alkaline hydrolysis. In the case of an anionic surfactant, aliquot parts of a solution are assayed by a colorimetric or titrimetric method (a) without hydrolysis, (b) after refluxing in N alkali for 30 minutes, and (c) after refluxing in 2N sulphuric or hydrochloric acid for 2 hours. The sulphate group itself is essentially stable to alkali and is hydrolyzed in acid solution, esters are completely hydrolyzed in both acid and alkaline solutions, while amides are partly hydrolyzed in both media,

THE ANALYSIS OF SYNTHETIC DETERGENTS 515 the extent being characteristic of the particular am/de. If the sample is a sulphate, or is a non-ionic compound, then more specific tests for the ester and amide links are needed. The hydroxamic test for esters is very useful •, while amides can be detected through the primary or secondary amine produced on hydrolysis. Hydrophobic groups The hydrophobic groups are examined after hydrolysis of the surfactant: mild alkaline hydrolysis is sufficient for carboxylic esters, moderate acid hydrolysis for sulphates without a linking group, prolonged acid hydrolysis for amides, hydrolysis with hydriodic or hydrobromic acid for ethers, and hydrolysis with concentrated phosphoric acid for sulphonates without a linking group. The liberated acid or alcohol may be analyzed for acid value or hydroxyl value, but a much more useful technique is gas chromatography, which is applicable to hydrocarbons also. Aromatic rings and ethylene bonds may be detected without hydrolysis ultra-violet spectroscopy is most useful for the former whilst other physical methods such as infra-red spectro- photometry and mass spectra analysis may also be used. PAPER CHROMATOGRAPHY This technique is one of qualitative analysis, but it is usefu!]y discussed under a separate heading. Over the past few years we have developed a comprehensive scheme of identification of detergent components using paper chromatography. Toluene and xy!ene sulphonates, urea, and alkano!arnines or metals used for neutralization are tested for in addition to the main surfactants which are examined in respect of their hydrophilic groups, linking groups and hydrophobic groups. Full details have recently been given by Drewry 5 and, therefore, the present paper will be restricted to a brief outline of the scheme and an account of the developments that have been made since the former paper was written. Paper chromatography is essentially a separation by partition between the stationary water and moving organic solvents, and the first requirement is an optimum irdtia! water content of the paper. In our laboratory it has been found that washing the paper (Whatman No. 1) in 50% ethanol, and allowing it to dry in the air is sufficient. In other laboratories it may be necessary to experiment with different drying conditions. Initially, a large number of solvents was tried, but a mixture based on tertiary butanol was the only one that gave a uniform development in the presence of surface active components. Later it was found that an ethyl acetate mixture as described by Gaspari• et al 6 gave equivalent results to the butanol solvent, though in a development time of only 2-3 hours instead of 15-20 hours. The solvent also contains a little ammonia and methanol.