THE ANALYSIS OF SYNTHETIC DETERGENTS 513 THE ANALYSIS OF SYNTHETIC DETERGENTS W. B. SMITH* A lecture delivered before the Society on 15th May 1963. The subject is introduced with a classification o• surface active agents that are used in all types o• detergents. This is •ollowed by a review o• the older qualitative tests and then an outline o• a new l•aper chromatographic l•rocedure. Quantitative analysis, confined to the determination o• the active constituents, is described under headings o• solvent extraction, colorimetric determination, anionic-cationic titration, and miscellaneous methods. THE WORD detergent nowadays suggests the packet of spray-dried powder used for domestic wash'rag purposes, but liquid products used in the same field may also come to mind. For the purposes of this paper other cleansing materials, namely shampoos and toothpastes, will be regarded as detergents soap is excluded as not failing within the definition of synthetic. However, only the organic surface active ingredients of the detergents will be considered. Most surface active agents which are used as detergents have molecules which are essentially linear and contain at one end groups having an affinity for water (hydrophilic groups), and at the other end groups which are anti- pathic to water (hydrophobic groups). Surface active agents are classed according to whether the active species is an anton, a cation, a non-ionizing group or an ampholytic group. An ampholytic group is one which may act as either anionic or cationic depending on the circumstances, principally oa the pH value of the solution. Hydrophobic groups may be classed under the headings of carboxylic acids (mainly naturally occurring acids), alcohols, hydrocarbons (mainly synthetic hydrocarbons derived from petroleum), and others (polyoxy propylene chains). Between the hydrophilic and the hydro- phobic group, the molecule may conta'm a linking group which may be an ether, ester, or amide. The listing and classification of possible structural groups is an essential prerequisite to the construction of a scheme of quali- tative analysis, and we still find the classification • drawn up five years ago to be a useful starting po'mt. The best systematic procedure is to identify first the hydrophilic group, then the linking group, if any, and lastly the hydrophobic group. QUALITATIVE ANALYSIS Hydrophilic groups One of the best tests for anionic and cationic active compounds is tc• *Marchon Products, Ltd., Whitehaven, Cumberland.

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,