A SPECTROPHOTOMETRIC QUALITY AND STABILITY CRITERION FOR MEDICINAL LIQUID PARAFFIN A Lecture delivered to the Society on October 13th, 1955 ROBERT SCHNURMANN, Ph.D.* Liquid Paraffin which passes the "British Pharmacopoeia" Acid Test con- rains "impurities" for which the "acid number" is no yardstick. On the con•rarlr, the acid used for the test dehydrogenates naphthene molecules when extremely "pure" liquid paraffins are examined, and the subsequent sulphona- tion of the dehlrdrogenated molecules may lead to the rejection of a material of high quality. A direct and unambiguous measure of the quality of liquid paraffin is its ultraviolet absorption spectrum. From its shape, and in particu- lar from the absorption intensities at 2710 A and at 2490 A, the "stability" of the material can be assessed. The photochemical degradation of liquid paraffin was studied. Of two groups o2 compounds which can be isolated from medicinal grade material, one absorbs mainly radiation in the region of 2710 A. It is these molecules which, when excited by the absorbed radiation, trigger-of[ an oxidation reaction of molecules belonging to the other group. In order to ensure the "stabilitlr" of liquid paraffin the proportion of its content o2 molecules of this latter group must be kept below the limit which can be derived from its absorption spectrum. INTRODUCTION IN a discussion of the quality and stability of medicinal liquid paraffin it is well to rernember that the manufacture of white oils had its beginnings towards the end of the last century, when petroleum refining methods were in their infancy and very little was known of the molecular constituents of petroleum distillation fractions. Petroleurn is essentially a mixture of paraffinic, naphthenic, and aromatic hydrocarbons. The most reactive group among these three is that of the aromatic molecules. When the medical world began to prescribe as a laxative a highly refined distillation fraction of petroleum, the need arose for a specification which would ensure that the material admissible for medicinal purposes would be practically free from aromatic hydrocarbons. The obvious way of trying to ensure the absence of appreciable amounts of unsaturated hydrocarbons was by colorimetric examination of the reaction product of the refined oil with sulphuric acid. Thus, a sulphuric acid test was developed and introduced into the various Pharmacopoeias in order to make certain that the refining process had successfully removed the aromatic constituents of the oil. * Physics Department, Manchester Oil Refinery Ltd., Manchester, 17.

2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Hydroaromatic molecules are more difficult to sulphonate than aromatic hydrocarbons, and they are also more difficult to remove. Even with modern methods of refining, these partly unsaturated hydrocarbons tend to remain in the oil when it has been treated to medicinal liquid paraffin quality. For these highly refined materials the British Pharmacopoeia Acid Test loses the significance which it had in earlier days, and its results bear not necessarily any direct relation to the amount of residual carbonisable bodies. A very sensitive method of assessing quantitatively the "impurities" left in medicinal liquid paraffin is based on its ultra-violet absorption spectrum. The near ultra-violet absorption spectrum of liquid paraffin shows sufficient structure to identify the chemical nature of the remaining impurities and to formulate a definite quality and stability criterion. In this latter respect the spectro- photometric method goes beyond the aspirations of the sulphuric acid test. Two groups ("A" and "B") of compounds can be isolated from medicinal liquid paraffin which fulfils the conditions laid down in the British Pharma- copoeia. The "Group A" compounds, which are alkylated symmetrical octahydrophenanthrenes, absorb radiation mainly in the region of 2710 A. The 3, transfer the absorbed energy to the "Group B" compounds, which are -a mixture of alkylated asymmetrical octahydrophenanthrenes, alkylated tetrahydrophenanthrene, and sulphonates. In this way the excitation of the "Group A" compounds triggers off a chemical reaction of the "Group B" compounds. •UALITY AND STABILITY OF LIQUID PARAFFIN Medicinal liquid paraffin must be water-white and free from odour and taste. A petroleum distillation cut of suitable viscosity may still contain some aromatic or hydroaromatic molecules even though it has been treated to tt•e extent of making it water-white. Sulphuric acid reacts with unsatur- ated hydrocarbons, and the amount of sulphonates formed when using given proportions of oil and acid is a measure of the concentration of unsaturated molecules in the oil, as long as the acid employed in the test does not react with any naphthene-paraffin molecules. This is the basis of the B.P. Acid Test, which stipulates that an acceptable liquid paraffin must not reach more discoloration than 2.5 Red, 6-5 Yellow on the Lovibond scale as a result of 10 minutes' reaction with a similar volume of 96 per cent sulphuric acid at boiling water temperature. It can be shown by spectrophotometric evidence that when in an oil the mnount of unsaturated hydrocarbons becomes very small, the acid applied for the B.P. test dehydrogenates some of the naphthenic molecules and then sulphonates them. This may lead to the rejection by the B.P. Acid Test of a material which by spectrophotometric evidence has a very high degree of purity.