SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN Some liquid paraffins have a long shelf-life, they can be stored for periods of years and yet remain water-white and free from odour and taste. Others assume an unpleasant odour after a short time and eventually go yellow. This raises two questions. There is firstly a need for a yardstick which measures the stability of a liquid paraffin, and then it is desirable to under- stand the mechanism of its ageing reaction. The ultra-violet absorption spectrum helps one in finding the answer to both questions. THE ULTRA-VIOLET ABSORPTION SPECTRUM The spectrum of most liquid paraffins shows pronounced band structure in the near ultra-violet region. Absorption intensity plotted as a function of wavelength decreases steeply between 2300 /k and 2400 /k. At about 2490 A the absorption intensity reaches a minimum value followed by a maximum at about 2710 A, with a subsidiary maximum at 2780 A (Fig. 1). 005 004 003 002 2,a00 2,500 I I 2,600 2,700 2,800 WAVEL[NGTH IN ,•U i Fig. 1. Ultra-violet absorption spectrum of a stable liquid paraffin.

4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The band at 2710 zk is typical of a benzenoid molecule, and it can, in fact, be shown that the spectral intensity distribution in the region between 2400 A and 2800 .• is the result of the superimposition of the spectra of two groups of compounds, "A" and "B," one of benzenoid character and the other with a falling absorption curve on progressing to longer wavelengths. Wide variations are encountered of the content of "Group A" compounds in liquid paraffin which satisfies the B.P. Acid Test. The intensity of the band at 2710/k may reach a value o f E (1%, 1 cm.) = 0.3202 or it may be as low as E(1%, 1 cm.)= 0.0005. For the former a B.P. Acid number of 1.5 Red, 4-5 Yellow was determined, and for the latter 0.9 Red, 2.7 Yellow. In view of these particular figures it is worthy of note that there is no correlation between absorption intensity and B.P. Acid number. To give one or two examples, the spectra were measured of materials for which the absorption intensity at 2710 A. had the values E(1%, 1 cm.) = 0.00049, 0.0053, and 0.0494, whereas the corresponding B.P. Acid numbers were Red Yellow 0.9 2.7 2.3 6.2 0.8 1.7 respectively. By a combination of ultra-violet and infra-red spectrophotometry it has been possible to identify the "Group A" compounds as alkylated symmetrical octahydrophenanthrenes. Their concentration in liquid paraffin can be derived from the ultra-violet spectrum. There is about • per cent of alkylated symmetrical octahydrophenanthrenes in a liquid paraffin which at 2710 A has E (1%, 1 cm.) ----- 0.05. Since the naphthene-paraffin molecules are quite innocu- ous this means that the lower the absorption intensity of liquid paraffin is in the spectral region between 2400/k and 2800 A the better is its quality. As an approximate guide for a good liquid paraffin the absorption intensity at 2710 A must be E(1%, 1 cm.)•0.100. The instability of liquid paraffin is caused by the "Group B" compounds. It is possible to remove from liquid paraffin large proportions of its conten. t of "Group B" compounds. When this is done, the ratio between "Group A" and "Group B" compounds increases and the liquid paraffin assumes greater stability. Since the "Group A" compounds essentially determine the value of the absorption intensity at the peak of the band at 2710 A. and the super- imposition of their spectrum on that of the "Group B" compounds governs the absorption intensity and wavelength position of the minimum in the region of 2490 J•, a stability criterion can be based on the value of the ratio of the absorption intensities at the maximum and the minimum in conjunction with a consideration of the E(1%, 1 cm.) value of the maximum. It can thus be shown that when the peak of the absorption band does not lie higher than