JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS porated into liquid compositions, which must remain clear in the cold, is restricted. In general, fatty acid alkylolamides tend to raise the viscosity of detergent solutions less than they do that of pure water, but in some cases an increased thickening effect can be achieved when polyphosphates are present also. It has been possible here only to indicate the types of fatty acid alkylo- lamides which are available, and to give a brief idea of the purposes for which they are used. The number of possible compounds is very large and the emulsion chemist should find a range of compounds even more extensive and more valuable to him than the wide range of esters of polyhydric alco- hols with fatty acids. The detergent chemist has already come to regard -[atty acid alkylolamides as essential in many of his formulations, and .doubtless much still remains to be discovered. VISIBLE REGION ABSORPTIOMETRY IN COSMETIC ANALYSIS By T. T. GORSUCH, B.Sc., A.R.I.C.* IN A paper in Volume II of the Journal, G. R. Clark •, writing on the use of spectrophotometric methods in cosmetic analysis, dismisses the visible region in one short paragraph. While it must be agreed that its dependence on colour is a limitation, it by no means follows that this restricts its useful- ness to colour matching and the standardisation of dyes and pigments, as is there suggested. Any matehal which can be converted to a coloured deriva- tive is potentially amenable to absorptiometric estimation, and a large number of substances of considerable interest to cosmetic chemists can be estimated by the measurement of the colour or turbidity produced by various general or specific reagents. It must also be borne in mind that the ultra- violet and infra-red regions have difficulties and limitations of their own: by no means all substances will give an absorption in the ultra-voilet while in the infra-red it is more a case of an embarrassment of riches, with interference from solvent absorption becoming a major factor, greatly limiting the number of systems which will give satisfactory results. One great advantage of visible region absorptiometry, as opposed to ultra-violet or infra-red spectrophotometry, lies in the comparative cheapness of the apparatus required: a visible region spectrophotometer costs, perhaps, * County Laboratories, Ltd., Stanmore, Middx. 178

VISIBLE REGION ABSORPTIOMETRY IN COSMETIC ANALYSIS one-third as much as the ultra-violet instrument and is only one-fifteenth of the price of the infra-red equipment. This price differential is of particular importance to the small laboratories where the main concern is control, involving the estimation of substances known to be present, rather than the identification of the components of an unknown, eomple mixture. In many laboratories, considerable use is made of the even cheaper tintometer, but although this instrument is well regarded for many pro- cedures, it is by no means as versatile as the photo-electric absorptiometer, and also is prone to considerable subjective variation, making comparisons between operators and between laboratories difficult: this inherent fault of the tintometer is eliminated by the objective method of measurement employed in the absorptiometer. Practically any determination which can be made using a tintometer can be repeated on an absorptiometer, while many which are outside the scope of the former are readily achieved with the photo-electric instrument. The techniques employed in the visible region are essentially similar to those used in the ultra-violet or the infra-red, and depend in the main upon a comparison of the absorption of the unknown with the absorption of a standard prepared and estimated under exactly similar conditions--the wave-length employed having been previously determined. For a single estimation on an unknown substance it is frequently more convenient to prepare a series of solutions in which increasing quantities of a standard are added to aliquot parts of the unknown. The absorptions of these solutions, after parallel development of the colour, are then plotted against the added standard. Graph I shows the result obtained by adding increasing quantities (1 ml., 2 mls., 3 mls., and 4 mls.) of a standard solution of anhydrous lanolin in benzene (101.6 mgms. in 100 mls.) to 1 ml. portions of a solution of a lanolin-eontaining base, also in benzene. The green colour on which the estimation depends is produced by treating the solution with acetic anhydride and concentrated sulphuric acid, and its density is measured after precisely thirty minutes. The continuous variation of colour intensity with time in this estimation is an illustration of the need for a standardised procedure. The intercept at 0.232 is the absorption of the solution containing the base alone. Graph II shows the same points with the intercept adjusted to zero. From this the concentration of lanolin in the base can be readily calculated. The great merit of this method is that it does not require an extensive investigation into the factors influencing the colour formation, as the calibration curve is obtained within the experiment, under conditions which are the same for the unknown and for the standards. It does, how- ever, require that the line through the experimental points should be straight, that is that Beer's Law should apply, or the transfer of the intercept to zero will be unjustified. 179