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

THE ANALYSIS OF SYNTHETIC DETERGENTS 519 is the only suitable technique. However, butanol and higher molecular weight alcohols are immiscible with water, while isopropanol is immiscible with a concentrated aqueous solution of sodium carbonate, and these can be used in liquid-liquid extractions. Ion-exchange resins The use of ion-exchange resins differs from the solvent extraction techniques, but it is usefully considered with them as, along with extractions, it can be built into a composite analytical scheme of separations. Ion- exchange resins provide the only simple means of separating anionic, cationic, and non-ionic surfactants. Though simple in principle, the practical use of ion-exchange resins with surface-active solutions involves several complicat- ing factors such as the polarity of the solvent, usually an aqueous alcoholic medium, the swelling and shrinking of the resin, and hydrolysis of the sur- factant on the resin or during elution. A great deal of work on the subject has been done by P. Voogt, among others, but only a little of this has yet been published ø'•ø. Comprehensive scheme of analysis The number of combinations of different surfactants that may be present in a commercial detergent is infinite, and no efficient general scheme of separation can be drawn up the method of analysis must be chosen to deal with the particular types of ingredient known or expected to be present. One decision to be made in dealing with several components is whether to extract them one at a time by the successive application of specific techniques or whether to proceed by division and sub-division, e.g. with six components, first separate two or three from the others, then proceed separately with each group. The latter technique is more complicated, but errors are smaller. Another decision is whether to separate each component in a reasonably pure form, or whether to extract two more more ingredients together and deduce the contents by difference. The case of a simple detergent containing free oil, ethanolamide, and alkylarylsulphonate, together with inorganic salts and water may be taken as au example. Scheme 1 is to extract the free oil with light petroleum from a 50% aqueous ethanolic solution of the sample, then the alkanolamide with ethyl ether after dilution to 20-30% ethanol, and finally the alkylarylsulphonate with chloroform from the residual aqueous solution, or with ethanol from a dried residue. Scheme 2 is to extract the first two components together, using ethyl ether, then separate these later and to extract the alkylarylsulphonate from the aqueous layer. Scheme $ involves extraction of free oil with light petroleum, free oil plus a!kanolamide with ethyl ether, and the total of the organic compounds with ethanol, all on separate samples and the individual