SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN This reduced access of radiation in the region of 2710 • gives the impression as if an incubatory period were required for "stable" liquid paraffins exposed to daylight when their spectra are examined at fortnightly intervals until after a period of 8 to `0 weeks (about 1,400 to 1,500 hours) their ultra-violet absorption also begins to increase. Once the chemical reaction of the "Group B" compounds has been initi- ated, it proceeds slowly even in darkness after the termination of the ultra- violet irradiation and at about twice this reduced rate in diffuse daylight. The "Group B" compounds belong to three classes .' (1) alkylated asym- metrical octahydrophenanthrenes and alkylated tetrahydrophenanthrene, (2) viscous liquids with infra-red absorption bands which are indicative of esters, and (3) petroleum sulphonates. Sub-groups B (2) and B (3) form the bulk of the "Group B" compounds. From the saturation levels which the ultra-violet intensity reaches at 2710 2• and at 24.00 2• as the result of ultra- violet irradiation of medicinal liquid paraffin, its content of "Group A" and "Group B" compounds can be estimated on the assumption that the "Group A" compounds have a value E(1%, 1 cm.) ~ 24 at 2710 J• and the "Group B" compounds E(1%, 1 cm.) ~52 at about 2490 2•. The content of "Group B" compounds is approximately one-tenth of the content of "Group A" molecules in stable materials. SPECTROPHOTOMETRIC EVIDENCE OF DEHYDROGENATION OF NAPHTHENE MOLECULES The experimental factors which affect the reproducibility and repeat- ability of the B.P. Acid Test were examined in detail in an M.Sc. thesis by Miss Martin entitled, "The Physical Meaning of the British Pharmacopeeia Acid Test." There is, in addition to the often discussed influence of slight variations in both the strength of sulphuric acid and the vigour of shaking a sample during test, the difficulty that the result of the B.P. Acid Test will depend upon the age of the sample. For the same material different acid numbers are obtained when it is tested immediately on receipt from the plant, and xvhen it is tested one month later. If the Acid Test were doing the job which it was originally designed to do, then the acid would react as long as unsaturated bodies are present in the oil. Reaction should cease immediately when the last unsaturated molecule has been used up. The ten-minute period for the Acid Test has been arbitrarily arrived at as a convenient testing time. Ninety-six per cent sulphuric acid has been proved to continue its action for ten minutes at varying rates according to the quality of the oil. Ninety-seven per cent acid reacts much more vigorously in some cases, and there is doubt whether this activity indicates the presence of unsaturated bodies since it appears more likely that the strong acid dehydrogenates some of the naphthenes in the oil, thus

14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS producing an additional amount of unsaturated molecules, and hence dis- coloration by virtue of adding to the sulphonated products. There is spectrophotometric support for this view of dehydrogenation of naphthenic hydrocarbons by the acid used for the test. A very highly purified liquid paraffin of an absorption intensity E(1%, 1 cm.)= 0.0021 at 2700 21, had a low acid number •vhen examined with 96 per cent sulphuric acid, and there was very little increase in colour with time. When the test was repeated with 97 per cent sulphuric acid the same was true up to the twelfth minute of the reaction, but then a breakdown occurred and the colour increased considerably during the fourteenth nfinute. Twelve samples, of widely differing stability by spectrophotometric evidence, were subjected to the sulphuric acid test, and it was found that the acid number bore very little relation to the spectrophotometric stability of the liquid paraffin. TABLE 2 COMPARISON OF STABILITY AS ESTIMATED SPECTROPHOTOMETRICALLY WITH ACID NUMBER Sample 96 % 1t2SO 4 -keT7 Yellow 1.3 • 3.3 / 1.s / 3-6 1.0 ,• 2.3 1-4 2-7 2.3 5-5 0.9 2-0 2-1 5.3 1.0 2.3 1-3 3-2 1.8 4.3 97 % It•,SO4 Red Yello•v --5 Ys- 2.3 5.2 2.6 7.4 2-8 7.4 0.9 2.1 2-4 6.7 2.8 7.4 1.3 2-5 3.6 9-8 1.8 4.2 2.1 5.0 2-7 6-6 Spectro- photometric Absorption Ratio 1-75 1-85 2.15 2.57 1'48 1 '69 2'67 1 '56 1.94 Spectro- photometrically Estimated Stability Poor-Good Good Very Good Very Good Excellent Very Poor Poor Excellent Excellent Excellent Poor Good It so happens that three of the very stable materials, E, H, and J, have good acid numbers, whereas Sample I would be dismissed as unstable, cer- tainly when 97 per cent acid had been used for the test. On the whole, good and bad acid numbers are distributed quite haphazardly between stable and unstable oils. Further experimental evidence for dehydrogenation was obtained when the bulk of the "Group A" and "Group B" compounds was removed from nine samples of liquid paraffin, all of which showed after this a very low absorption intensity of the order of E(1%, 1 cm.)= 0.003 at about 2700 A, and in all but two of these samples band structure was completely absent. This means that these oils were free from all but minute traces of contaminating