354 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In the Barr method, the solution to be titrated is made neutral or slightly alkaline. Otherwise the mechanics of the process are very similar to those of the Epton method. In contrast, however, all the indicator remains in the water layer for practically the entire titration, during which the detergent is being converted into its quaternary salt and extracted by the chloroform, as before. Eventually, when almost all the detergent has reacted, the bromophenol blue begins to react, and the salt formed is extracted by the chloroform. The first trace of blue in the chloroform layer indicates the end point. $. The Cullum Method In a later study of the Barr method if was found 7 that the first appear- ance of blue in the chloroform did not mark the true end point, some detergent remaining unreacted in the water layer. On the addition of further increments of the quaternary, these last traces and the indicator react simultaneously, and both quaternary salts are extracted. At the point where the last trace of colour is removed from the water layer, the tendency for the two layers to emulsify reaches a minimum, and at the same time the emulsion type changes from water-in-chloroform to chloroform-in- water. This is easily observed while the two layers are separating after being shaken. Just before the end point the separation consists of the collapse of a network of hollow bubbles of chloroform through the water layer. Just after the end point, the separation consists of the settling out of solid drops of blue chloroform. The change-over is very sharp, one drop (ca.0.02 ml) of the quaternary solution being sufficient to bring it about. Frequently the water layer becomes quite clear at the change- over point. The actual procedure is as follows: Prepare a solution approximately 0.004 N with respect to the surface agent to be determined. Pipette 25 ml into a 200 ml stoppered bottle. Add, by pipette, 1 ml of 0.1% bromophenol blue. Make just acid with 0.1 N hydrochloric acid and restore the blue colour by the addition of a small quantity, about 0.5 g, of sodium bicarbonate. Add 50 ml of chloroform and titrate with 0.004 N cetyltrimethylammonium halide, or some other suitable long-chain quaternary ammonium salt, which has been standard- ised against pure sodium dodecyl sulphate. Add the quaternary in small portions and shake well after each addition. When the colour changes from purplish blue to clear blue, continue titrating in 0.1 ml increments or single drops, until the water layer is colourless and clear or almost clear. For the greatest precision, use the colour transfer only as a guide and take as end point the change-over from bubbles to drops of chloroform as it

SOME ASPECTS OF THE ANALYSIS OF HOUSEHOLD DETERGENTS 355 separates from the aqueous layer. Subtract the blank obtained on the indicator alone. FACTORS INFLUENCING THE TITRATION These considerations apply to the Cullum method, described above. 1. pH The indicator starts changing to the yellow acid form at pI-I 4.6. Above pH 9 or thereabouts, the quaternary is appreciably converted to the free base, which is extracted by the chloroform without reacting with the deter- gent anions. The useful range of pH is therefore 4.6 to 9, and the procedure described gives a pH of about 8. 2. Indicator Any anionic indicator can be used, but bromopheno[ blue remains in its salt form at lower pit values than most other indicators, and blue seems more easily visible at low intensities than other colours. 8. Solvent Many solvents will work, but chloroform and methylene chloride give little trouble with emulsions and yield nearly stoichiometric results. 4. Titrant The following quaternaries have been tried: I)odecyl, tetradecyl and hexadecyl pyridinium bromides dodecyl, tetradecyl and hexadecyl trimethylammonium bromides cetyl and stearyl pyridinium chlorides cetyl and stearyl benzyl dimethylammonium chlorides cetyl trimethylammonium halides. "Hexadecyl" etc., indicates the pure single homologue "cetyl" etc., indicates the mixture of homologues constituting the usual commercial materials. Primary ammonium salts have also been used with complete success, but the pH range (4.6 to 5-2) in which they are useful is too narrow for practical purposes. The identity of the halide ion is quite immaterial. 5. Soaps Soaps do react quantitatively under suitable conditions, but in this titration their effect is like that of a buffer in an acid-base titration. They must therefore be removed, if present in the detergent under examination.