THE ANALYSIS OF SYNTHETIC DETERGENTS 517 made from the published procedure is the rather obvious simplification of using one marker solution containing all the reference compounds. Another method of identifying amides, applicable both to the nonionic alkanolamides used as additives and to amide sulphonates and carboxylates used as the main anionic surfactants, is to apply paper chromatography after acid hydrolysis. The aidehyde nitroprusside spray is the most useful as it distinguishes primary amines such as monoethanolamine and taurine (khaki spots) from secondary amines such as diethanolamine, N-methyltaur- ine and sarcosine (blue spots), all liberated by hydrolysis of the corresponding fatty acid amide. •)UANTITATIVE ANALYSIS BY SOLVENT EXTRACTIONS The most reliable technique of analysing mixtures of surfactants is a series of extractions, with solvents and ion-exchange resins, before and after hydrolysis, to separate the individual fractions which are then weighed. Each fraction can, if desired, be characterized by further analysis including physical methods such as gas chromatography. A series of separations can be assembled in a variety of ways and therefore it seems best here to discuss the subject under sub-headings of the solvents. The general method for liquid-liquid extractions is to use stoppered separating funnels, and for liquid-solid extractions is simple stirring in a beaker or centrifuge tube, followed by filtration or centrifuging. Light petroleum Typical procedures for petroleum extraction are described in the B.P. and U.S.P. monographs for sodium lauryl sulphate. The sample is dissolved in 50% ethanol, for the unsulphated alcohol is less soluble in this than in an aqueous solution of the sample (owing to reduced miceilar effects) and emulsification difficulties are fewer. Three extractions with petroleum are made. The combined extracts are dried, the solvent is distilled off and the residue is weighed. A certain proportion of free lauryl alcohol remains solubilized in the surfactant solution, as is shown by tests on synthetic mixtures, even after 5 or 10 extractions. The loss depends on the con- centration of surfactants and, for reproducible results, the latter is arbitrarily fixed at about 5% w/v. Besides the question of completeness of extraction there are several other difficulties in arriving at a standard method of high reproducibility. Washing the petroleum extracts is always liable to cause losses, and these must be balanced against the errors due to contaminants. Drying the extracts with sodium sulphate or another desiccant may lead to a loss, but omission of the drying step may cause losses by volatilisation in steam. Removal of the solvent without loss of volatile alcohol is also a problem and, finally, errors associated with weighing the residue in a large

518 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS glass flask, namely, adsorption of moisture on the glass, electrostatic charges, and buoyancy errors, can all be significant. Other uses of light petroleum as an extractant are to separate unsul- phonated oil from alky!ar¾1sulphonates, and to extract fatty acids liberated by acidification of soap solutions. The solvent is also used to extract acids and alcohols liberated by the hydrolysis of amides and esters 7. Ethyl ether Ethyl ether extracts the same compounds as light petroleum, and several others too, particularly alkanolamides. Being a single compound of low boiling point it can be distilled from the extract with less uncertainty than attends the removal of light petroleum, a•d for this reason it is preferred in such determinations as the total fatty alcohol in alkyl sulphates. Dis- advantages of ethyl ether are its higher solubility for water and for hydro- chloric acid. It can only be used with dilute aqueous ethanolic solutions and is therefore not very satisfactory for extracting alcohols and acids from solutions of sulphonates as a moderate ethanol content is needed to reduce miceliar effects. Ethyl ether will extract hydroxy-acids which light petroleum will not. Ethyl ether will also extract alkylarylsulphonic acids from 2N hydro- chloric acid, and this is useful for separating these acids from toluene- and xylene sulphonates 8. Accurate quantitative determinations are limited by the same factors as described for light petroleum, and these also apply in varying degree to the other solvents below. Chloroform This solvent will extract most ethylene oxide derivatives, including those with chains of six or more units which are not extracted with ethyl ether. It will also extract alkylarylsulphonates, and many other surfactants, from neutral solutions. One disadvantage of chloroform is that any ethanol in the aqueous solution must first be expelled, and even in the absence of ethanol, emulsification is often troublesome. Alcohol Extraction with ethanol is used to determine the total organic content of built detergents, and by separately determining unsulphonated matter, additive, chloride, etc. the surfactant content can be found. To ensure ex- traction of small quantities of active material contained within the beads of spray-dried powders, it is necessary to take the residue after a few extrac- tions, dissolve it in a small quantity of water, and reprecipitate with alcohol. As ethanol is miscible with water it is clear that extraction from a solid