SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN 19 absorption maximum/absorption minimum by preferential purification, should not the proposed critical ratio be used with great care ? The need for having the ratio 2 also seems to imply that addition of one component to increase the ratio above the critical value should increase the stability of the oil. Thi• cannot be so, as both components lead to instability of the oil. Since maximum stability would be obtained when both components are absent, a less ambiguous criterion would seem to be the heights of the maximum and minimum above the wavelength axis, without actual refer- ence of these values to one another. Author: The spectrophotometric quality and stability criterion for medicinal liquid paraffin takes into consideration the absorption intensities at two wavelengths. These two wavelengths of 2710/k and 2790/k mark the position of the peak of the absorption band, and of the minimum of the absorption curve for materials which show band structure in this spectral region. A ratio of the absorption intensities of at least two is required for a stable material whose E (1 øfo, 1 cm.) value lies within the range 0.010 and 0 100. For any material which has been purified to such a degree that its spectrum does not show any band structure, the E(1 øfo, lcm.) value at 2710/k must lie below 0 006 for the material to be stable. It is thus clear that the spectrophoto- metric criterion is based on two numerical values: either the absorption ratio in conjunction with the value of the absorption intensity at the maxi- mum (2710 A), or the values of the absorption intensities at the maximum and the minimum. There is the additional precaution necessary of ascer- taining that the position of the minimum has not been displaced to appreci- ably longer wavelengths than 2490/k. There is no suggestion of discriminat- ing between various degrees of stability, but of ascertaining whether or not a material will be stable. Dr. Matalon: I have been very interested by the study that has been made on the nature of the impurities in paraffin oil which cause deterioration on ageing and I would like to know what is the minimum amount required of these impurities to produce an undesirable effect in colour and odour on storage of the white oils." Author.' It depends on the quality of a given stable liquid paraffin what amount of Group B compounds one has to add in order to make it unstable. Experiments were made in which minute amounts of Group B compounds were added to a highly purified liquid paraffin whose E(1 øfo, 1 cm.) value was 0.0025 at 2700 /k. These minute additions had a noticeable effect on the stability of the thus contaminated material. REFERENCES • Pauline Mary Martin, "The Physical Meaning of the British Pharmacopoeia Acid Test." M.Sc. Thesis, London, 1951. 2 R. Schnurmann, Pauline M. Martin and W. F. Maddams, "An ultraviolet spectro- photometric quality and stability criterion for medicinal liquid paraffin." The Journal of Pharmacy and Pharmacology, 8, 298, 1951.

20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS a R. Schnurmann, "l•,tude spectroscopique de la stabilitd photochimique d'huiles de paraffine mddicinales." Journal de Physique, 11i, 503, 1954. ' R. Schnurmann, "The 'Group A' compounds in medicinal liquid paraffin."-- Transactions of the 27th International Congress of Industrial Chemistry, Brussels 1954 THE EFFECT OF VARIATION OF THE WATER AND OIL RATIO ON THE DIFFUSION RATE OF SULPHATHIAZOLE FROM COLD CREAM TYPE OINTMENT BASES AUBERT CORAN* and C. LEE HUYCKi The authors study the effect of varying the phase ratio of cold cream con- raining sulphathiazole. They find that reversal of the external phase is a gradual change and describe two methods for determining the rate of diffusion of the drng from the cream. IT WAS recently established • that cold cream type ointments containing mineral oil, beeswax, borax, and water may be either of the oil-in-water or water-in-oil type, depending upon their oil and water concentration. It was shown •,• that sulphathiazole diffuses more rapidly from oil-in-water type emulsions than from the water-in-oil type. The purpose of this investigation is to show the change in the rate of diffusion of sulphathiazole from this type of base as the water and oil concentrations are changed and to compare two methods of testing the diffusion of sulphathiazo]e from ointment bases. In vitro studies of diffusion rates of drugs from ointment_bases have been &X•..e_nsively conducted by the use of the agar gel method which was developed by Waud and Ramsay.' This method employed an agar 'g• •cø•[•[ihing an i•dicator. The ointment being tested was placed on top of the gel which was contained in a test tube. Measurements of the extent of diffusion was based upon the distance from the top of the gel to the bottom of the indicated area. Other methods 5'6 have shown the extent of diffusion into saline solutions, blood-serum, and water. The method used here was taken from a recent note7 which presented a new method of determining the extent of diffusion of drugs from various ointment bases. This method employed a piece of filter paper moistened with indicator solution. The ointment was placed in the centre of the filter paper in a small hollow cylinder open at both ends. Determinations of the diffusion rates were made by measuring the distance from the outer edge of the ointment to the outer edge of the indicated ring on the filter paper. * Graduate Assistant in Chemistry, St. Louis College of Pharmacy & Allied Sciences, St. Louis 10, Mo. t Professor Industrial Pharmacy, St. Louis College of Pharmacy & Allied Sciences, St. Louis 10, Mo.