THE ANALYSIS OF SYNTHETIC DETERGENTS The stearyl compounds have the disadvantage of lower solubility, while the myristyl and shorter chain homologues may react incompletely. However, we have found that N-alkyl-N-benzyldimethylammonium chlorides give much sharper end-points than the other two groups of titrant. The stearyl com- pound again is of low solubility, but the lauryl, myristyl and cetyl com- pounds all have similar performances and there is little to choose among them except that the cetyl compound, of lowest solubility, is the most readily purified by crystallisation. Regarding the concentration of the titrant, many workers follow Epton in using titrations of about 10 ml with .004 or .0053{ titrant, but larger titrations with more dilute solutions, e.g. around 20 ml or .0013{ as used by Barr et al normally give more precise results, and are at least as accurate if due regard is paid to the blank. All titration procedures are equally suitable for determining anionic or cationic surfactants, the concentration of the other being known, and it is also immaterial whether the solution of unknown concentration is placed in the titration vessel or in the burette, although the former is usually the most convenient. The single phase titration The earliest work by Hartley and Runnicles used a single-phase titration of aniordc and cationic surfactants, but the end-point with bromophenol blue was not so clear as when the two-phase technique was applied. How- ever, with fluorescent indicators such as eosin •5 and dichlorotetraiodo- fluorescein •6 the end-point may be as sharp as with the two-phase method, and such indicators may repay further study. Their main disadvantage is that large amounts of inorganic salts, and moderate amounts of inactive organic compounds, tend to obscure the end-point. OTHER •/[ETHODS OF ASSAY Methods based on the formation of an amine salt with artionic surfactants, using an ordinary primary amine, have been used for a long time. One procedure •* uses toluidine as the amine, extracts the salt with carbon tetra- chloride, and determines the amine in the extract by addition of ethanol and titration with alkali. A similar procedure •a uses benzidine and separates the salt by filtration. The salt may be weighed before fitration to give an indication of the equivalent weight, which cannot be determined by any volumetric method. For determining cationic surfactants, several precipitants containing large artions may be used, and in a review by Chinnick and Lincoln 19, phosphotungstic acid is recommended. Non-ionic surface active agents with an ethanoxy chain of suitable length for detergency can also be precipitated

524 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS with the heteropoly-acids phosphomolybdic acid is preferred, as the pre- cipitate can readily be analysed to determine the surfactant content which varies with different compounds. Ampholytic surfactants of the amino- carboxylate group are usually analyzed by the methods employed for catiordcs, with special attention to reactions in acid solution. Those of the aminosulphonate group are analyzed by the methods used for anionics. Ampholytics containing a sulphate group are subjected to acid hydrolysis to yield an amine which is analyzed as a true cationic compound. CONCLUSION A great deal of work on new methods of detergent analysis is being undertaken in this country and abroad, but most of this falls into two classes. The first is a thorough examination of the traditional methods in endearours to draw up national and international standards, and the second is concerned with new types of detergents, mainly biodegradable anionic surfactants, and a wide range of ampholytics. Most of this paper has been confined to a discussion of analytical methods that have stood up to the recent scrutiny and that are likely to be issued as standards in the near future. Among these procedures are two that have hitherto survived severe criticism, but which have such inherent defects that they are liable to be superseded in the second generation of standards. The first is the large group of analyses by solvent extraction. Each operation is limited by an equilibrium partition coefficient and, though series containing up to a dozen extraction and washing steps have been recommended, the overall accuracy and precision of many is barely acceptable. With chromatographic pro- cedures on the other hand, using columns of ion-exchange resins, of alumina, cellulose, and silica, separations may involve hundreds of theoretical equi- librium stages with no more than a few minutes' attention of the analyst. Such methods will become widely used in the future, but a great deal of tedious study is needed before results can be accepted as reliable and reproducible, especially among different laboratories. The second procedure is the two-phase cationic-anionic titration using methylene blue and this suffers from the same limitations of equilibrium partition as the first method. The best hope for improvement lies in the synthesis of a cationic indicator designed specially for this application. The ideal indicator will probably contain only one basic group, this being a quaternary nitrogen, and will have an intense colour, preferably blue. Nevertheless, an ideal indicator may not overcome the inherent defects of the competing equilibria, for many investigators seem to ignore the extracta- bility of both anionic and cationic surfactants, in the absence of indicator, in solvents such as chloroform. The factors appear to be more serious in