SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN 17 In concluding this account I wish to thank those of my present and past colleagues who have taken active and devoted part in this work. A great number of the experimental data are taken from the above-mentioned M.Sc. thesis of Miss P.M. Martin. Mr. E. Kendrick, M.Sc., and Mr. R. Mayoh, B.Sc., Mr. F. Smith, M.Sc., and Mr. W. F. Maddams, M.Sc., have contributed to the experimental evidence. DISCUSSION Dr. Mitchell asked to be informed of the concentration of liquid paraffin, and the solvent used, in the solution used for spectrophotometric examina- tion. Results obtained in his laboratories showed many samples, apparently of good quality and stability, to give ratios ranging from 20 upwards. While such samples literally complied with it, the standard (not less than 2) did not appear to be entirely satisfactory or should one suspect such samples-- or the results ? Most analysts disliked the sulphuric acid test as troublesome, and would welcome a spectrophotometric test instead. However, we were now told that the two tests measured different things. Analysts certainly would not want both tests. Further, they would want assurance that reliable and consistent results would be obtained, so that different analysts could expect to get reasonable agreement between their figures. Dr. Mitchell suggested that results recently obtained in his own, and other, laboratories had failed to give this assurance, and he did not consider that the test, as now defined, was satisfactory. In particular, he could not support its adoption as an official test. Author: The solvent used for the preparation of solutions of liquid paraffin must be transparent in the wavelength region between 2400 • and 2800 •. Most commercially obtainable solvents such as hexane, cyclo- hexane, heptane, and iso-octane must be purified before they are used for spectrophotometric work. The concentration of the liquid paraffin solution must be adjusted to suit the absorption level of any particular sample of liquid paraffin. The greatest accuracy is obtained when, with a photographic plate instrument both the maximum at 2710 2• and the minimum at 2490 2• are recorded at an optical density of 1.00, and with a photo-electric instrument at an optical density of about 0.4. Whenever the spectrum of a solution is measured, the cell in the reference beam should be filled with the corresponding solvent. When the absorption level of a liquid paraffin sample is sufficiently low that the material can be measured "neat," either in a 1 cm. cell or in an adjustable cell of smaller path length, a similar cell should be placed in the reference beam and a check should be made by filling this cell with freshly distilled water to ensure that reflections from the inside of the cell windows are not significant.

18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS With samples of liquid paraffin from widely different refineries in this country, on the continent of Europe, and from the United States of America, the author never came across any materials showing absorption ratios "from 20 upwards." In fact, there is every likelihood that no larger absorption ratio than 3.98 can exist, since the "Group A" compounds have been identi- fied as alkylated symmetrical octahydrophenanthrene. Liquid paraffin solutions prepared with a solvent of the hexane or iso- octane type must give similar spectra to the "neat" material, provided that there is no leakage from the cell which contains the solution. If there were such leakage it might obscure the cell windows. From a large amount of experimental data we drew the conclusion that the ratio of the absorption intensities at the maximum at 2710 A and the minimum at 2490 A must be at least 2.0, when the absorption intensity at the maximum lies between E(1%, 1 cm.)= 0.100 and E(1%, 1 cm.)---- 0.010, for the material to be stable. This does not exclude the possibility that a material of an absorption ratio of 1.5 stored in a glass bottle in diffused daylight up to twelve months may have remained colourless and odourless. I have never suggested that both tests, the spectrophotometric method and the sulphuric acid test, should be used in future. On the contrary, the whole emphasis of to-night's paper is to counsel the abandoning of the sulphuric acid test and the adoption of the described spectrophotometric quality and stability criterion instead. Samples of liquid paraffin have been exchanged by various laboratories and very close agreement of the results was obtained, even though different types of spectrophotometers, i.e., instruments with photographic plate recording and photo-electric instruments, were applied. Dr. Mitchell: Information supplied since the meeting The high ratios referred to above were obtained in tests conducted on dilute (4 to 5 per cent w/v) solutions of liquid paraffin in n-hexane. Since the meeting, and using experimental details supplied to us by Dr. Schnur- mann, we have found that lower ratios, more of the order of those obtained by Dr. Schnurmann, have been obtained by taking readings on undiluted liquid paraffin. In fact, the ratios have generally been around 1.5, though the samples appeared to be of satisfactory quality and stability. Several had been stored for up to twelve months in diffused daylight, yet they had remained colourless and odourless. Of course, these conditions of storage are much less severe than exposure to direct sunlight. Since Dr. Schnur- mann's test and standard would condemn such samples, we still consider that the test, in its present form, is unsuitable as an official standard.* Dr. Posner: Since it might be possible to produce artifacts in the ratio * Editorial Note. Dr. Mitchell's mild storage stability test has no relation to Dr. Schnurmann's and no useful comparison can be made.