SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN 7 , allowance for the evanescent odour by describing an acceptable product as "odourless and tasteless when cold" and developing "not more than a faint . odour of petroleum when heated." The spectrophotometric quality predict whether a material will "go or years. The interesting point is paraffins which pass the B.P. Acid and stability criterion enables us to off" within a period of days or months that, in selecting, say, fourteen liquid Test with flying colours, their ultra- TABLE I Comparison of absorption spectrophotometric and B.P. Acid Test data of different stability classes of medicinal liquid paraffin (ordered according to their absorption ratios, i.e., Group I, those having ratios • 2.0, Group II, with ratios between 1.8 and 2-0, Group III, with ratios between 1.2 and 1.8, and Group IV, with ratios 1.2). Group -- I II III IV Absorption at Maximum Minimum :(i%, A E(lo/o, A qoband structure,E{1%,[• 0.0004 [ I cm.)[ 0'0099 I 2719 I 0'0037 I 2500 0'0492 I 2718 I 0'0209 I 2492 0.0133 I 2710 I 0.0060 I 2470 0.0557 2714 0'0253 2506 0'0411 I 2716 0.0189 2492 0'0640 I 2708 I 0.0327 2508 0'0363 I 2705 I 0.0188 2482 0.0769 I 2729 I 0.0422 I 2492 0.1003 I 2713 I 0.0550 I 2501 0.0726 I 2707 I 0'0478 I 2518 0.0897 I 2706 I 0.0601 I 2512 0.0704 2735 I 0.0595 / 2555 0.0954 I 2717 10.0809 I 2530 Ratio of absorption intensities at maximum and minimum 9 at 2700 2.68 2.35 2.22 2-20 2.18 1.96 1.93 1.82 1-82 1.52 1.49 1.18 1.18 B.P. Acid Test Re•d I Yellow Stability 2 years + 7 months q- 6 months q- 6 months q- 5 months q- 6 months q- 2 months 5 months {slight trace of odour) 10 days 2 months 21 days 14 days 33 days 31 days violet absorption spectrum presents a simple means of grouping them into classes of widely different stability. The same order of merit is arrived at by determining the period of daylight exposure before a persistent odour can be noticed. Let us arbitrarily decide upon four groups of stability so that Group I contains all materials with an absorption intensity at 2710 A of E(1%, 1 cm.)% 0.100 and an absorption ratio 2.0, Group II with an absorption ratio between 1.8 and 2.0, Group III with an absorption ratio between 1.2 and 1.8, and Group IV with an absorption ratio 1.2. Five of the fourteen materials with E(1%, 1 cm.)-• 0.100 had absorption ratios 2.0 and a sixth showed no band structure (its absorption intensity at 2700 A was E(1%, 1 cm.)= 0.00049). These six materials, which according to our definition belong to Group I, did not develop an odour during daylight exposures appreciably exceeding 5 months. The four samples belonging

8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS to Group II (see Table I) were stable for periods up to 5 months, and the remaining four (two of Group III and two of Group IV) were stable for anything between a fortnight and four or five weeks. ULTRA-VIOLET IRRADIATION OF LIQUID PARAFFIN The ageing of liquid paraffin can be speeded up considerably by exposing it to ultra-violet irradiation. For this purpose quartz containers must be used so that the ultra-violet light has access to the liquid paraffin. It was found that although the spectra of stable liquid paraffins remained unaffected •vhen these materials were exposed in glass bottles to a mercury vapour discharge for periods of several weeks, this irradiation produced significant changes when the same "stable" materials were contained in quartz vessels (Fig. 4). 0.08 0 07 006 0O5 •00a OO3 002 o Ol ooo 2,400 2,500 2,6'00 2,700 2,800. Ultra-violet irradiation--mercury vapour lamp (125 W.) 45 cm distance. O -- Starting material No. 1. A -- Exposed in glass vessel for 643 hours. B -- Exposed in glass vessel for 1353 hours. C -- Exposed in glass vessel for 230 hours. D -- Exposed in quartz vessel for 643 hours. E -- Exposed in quartz vessel for 1353 hours. F -- Exposed in quartz vessel for 230 hours. Fig. 4. Effect of ultra-violet irradiation on two portions of a medicinal liquid paraffin. No appreciable change was obtained when the •naterial was kept in a glass vessel, whereas there was a significant increase in absorption intensity, particularly in the region of 2490 A, of the material in a quartz vessel.