THE EVALUATION OF COMMERCIAL ALKYL SULPHATES The flask is connected to a Quickfit condenser and the contents kept just on the boil until the fatty alcohols separate quickly as a clear layer after swirling the flask. The condenser is washed down with hot distilled water, the fatty alcohols separated in a separating funnel and the aqueous layer removed. The fatty alcohols are washed twice with hot 25 per cent sodium chloride solution and then transferred to a weighed flask without the use of solvent, cooled and weighed and retained for subsequent analysis. The small amount of fatty alcohols left in the original flask and in the separating funnel is transferred to a second weighed flask with ether, the solvent is removed and the total weight of fatty alcohols is obtained by adding this weight to the main bulk. In this way the bulk of the fatty alcohols is obtained without contamination by solvent or loss of the more volatile constituents. The result is expressed as a percentage of total fatty alcohols. DETERMINATION OF THE I-IYDROXYL VALUE OF THE FATTY ALCOHOLS The method of Ogg, Porter and Willits * based on esterification with acetic anhydride is satisfactory. Full details of the general application of the method are given by Siggia. 6 The determination is carried out in duplicate and a quantity of between 0-5 gm. and 1 gm. is accurately weighed into each of two 250 mi. conical flasks fitted with glass stoppers. A volume of 5 mls. of acetylating reagent is pipetted into each of the flasks and a further 5 mls. into a third flask for the blank. The stoppers of the flasks are moistened with pyridine and loosely inserted into the flasks, which are then heated on a water bath for one and a half hours. After this time 10 mls. of distilled water are added to each flask and the contents are mixed and allowed to stand for two minutes. The flasks are cooled and 10 mls. of n-butanol added to each, rinsing the stopper and sides of the flasks during the addition. The contents of each flask are titrated with approximately 0.5N alcoholic potassium hydroxide solution using cresol red-thymol blue indicator. The alcoholic potassium hydroxide solution .is standardised with 0.SN hydrochloric acid. Hydroxyl Value = (Blank--Sample) rots. KOH x F x 56.1 Wt. Sample taken. where'F is the normality of the alcoholic potassium hydroxide. The acetylating mixture consists of one part acetic anhydride with three parts of pyridine. The cresol red-thymol blue indicator cons'ists of one part of a 0'1 per cent aqueous solution of cresol red neutralised with 0'IN sodium hydroxide, and three parts of a 0'1 per cent aqueous solution of thymol blue similarly neuiralised. The average molecular weight of the fatty alcohols and hence of the alkyl sulphate is calculated from the hydroxyl value. 125

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS DETERMINATION OF THE ACTIVE DETERGENT The fact that certain cationic surface-active agents react stoichiometric- ally with anionic surface-active agents has been used as a basis for estimating the latter. In the method of Hartley and Runnicles • the anionic material is titrated with the cationic surface-active agent using bromo-phenol blue as an indicator to detect the excess. The colour change from purple to blue is judged rather than observed and is not easy to detect, even after considerable experience. In the method of Barr, Oliver and Stubbings 7, the artionic material is titrated with the cationic surface-active agent using a chloroform and an aqueous phase with bromo-phenol blue indicator, taking the first' indication of blue colour in the chloroform phase as the equivalence point. The actual end point is somewhat indefinite and requires comparison with similar mixtures containing first, insufficient, and second, excess of the cationic surface-active agent. In the method of Epton 8 the artionic material is titrated with the cationic surface-active agent using a chloroform and an aqueous phase with methylene blue indicator and taking the equivalence point when the two layers are visually the same. This method is superior to the other two methods and gives readily reproducible results. A weight of sample is taken to give an approximately 0.002 molar solution when dissolved in one litre of distilled water, and 25 mls. are pipetted into a 100 mi. glass-stoppered measuring cylinder. To this are added 15 mls. of chloroform and 25 mls. of methylene blue indicator solution the cylinder is stoppered, shaken vigorously and allowed to stand for one minute, when it will be seen that the deep blue colour of the indicator is concentrated in the chloroform layer. An 0.002 molar solution of cetyl trimethylammonium bromide is added 1 ml. at a time, shaking vigorously after each addition. The depth of the blue colour in the chloroform layer will diminish while the blue colour of the aqueous layer will increase. When the colours are nearing the same depth the additions are made in lesser amounts until finally an addition of 0.1 mi. will cause the colours to be visually the same when viewed in reflected light. The first titration is taken as a guide and two further titrations are carried out and should agree within 0.1 ml. The result is cal- culated to terms of equivalent weight and, using the average molecular weight of the alkyl sulphate obtained from the previous determination, is expressed as a percentage of alkyl sulphate. Cetyl trimethylammonium bromide is preferred to cetyl pyridinium bromide because of its greater solubility in water, and the material is standardised by precipitation with standard potassium dichromate solution, the excess dichromate being measured iodometrically. g The methylene blue indicator solution is prepared by dissolving 50 gins. of anhydrous sodium sulphate in distilled water, adding 120 mls. of 2N sulphuric acid, 6 mls. of 0.5 per cent aqueous methylene blue solution and diluting to one litre. 126