10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS may be adequately preserved at the start. However, over a period of storage the aqueous phase may become denuded of preservation to a point below the minimum concentration for preservation. On Dr. Carrington's reference to preservative effect of propylene glycol, Barr, M., and Tice, L.F., [J. Am. Pharm. Assoc. Sci. Ed. 46 217 (1057)1 showed that, for a range of bacteria and moulds, growth in aqueous solutions of sorbitol was inhibited by the presence of 10-30% of propylene glycol, and by 10-50% glycerol, depending on the organism and on the concentration of the sorbitol in solution. MR. T. P. McLAUGHLI½: According to Table 1, castor oil should be a powerful inactivator. Is this in fact so ? THF. LF. CTURF. R: Difficulty of preservation in the presence of an aqueous phase may account to some extent for the fact that castor oil has found little use in emulsions. MR. T. P. MCLAUGHLI½: Benzoic acid is more effective in acid conditions than are the benzoates. Yet the unionised form should be more soluble in the oil phase and accordingly less active. THF. LF. CTURF. R: Comment on this would have to await determination of partition coefficients and the minimum safe concentration of benzoic acid in the water phase. MR. M. G. •)F. NAVARRF.: Benzoic acid complexes strongly with nonionics and so is not of great value. On the other hand acid conditions in a system may tend to inhibit some bacterial and mould growth. METHODS FOR DETERMINATION OF MONO- ESTER CONTENT OF PROPYLENE GLYCOL MONOSTEARATE NON SELF-EMULSIFYING C. H. JOHNS, B.Sc., Ph.D., and W. P. PEPPER, M.Sc., F.R.I,C.* Delivered at the Summer Conference of the Society on 25th August 1960. Anal•ical methods tot •he determination oI Mono-es•er con•en• Prop•rlene Glycol Monos[eara[e are discussed. Two al[ernafive me[hods analysis are •11y described. •NTRODUCTION I• COSM•.TIC formulations, it is well known that the mono-ester content of the various poly-alcohol esters of fatty acids used has a profound effect upon the nature of the finished product. With propylene glycol mono- *Chesebrough-Pond's Ltd., London, N.W.10o

10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS may be adequately preserved at the start. However, over a period of storage the aqueous phase may become denuded of preservation to a point below the minimum concentration for preservation. On Dr. Carrington's reference to preservative effect of propylene glycol, Barr, M., and Tice, L.F., [J. Am. Pharm. Assoc. Sci. Ed. 46 217 (1057)1 showed that, for a range of bacteria and moulds, growth in aqueous solutions of sorbitol was inhibited by the presence of 10-30% of propylene glycol, and by 10-50% glycerol, depending on the organism and on the concentration of the sorbitol in solution. MR. T. P. McLAUGHLI½: According to Table 1, castor oil should be a powerful inactivator. Is this in fact so ? THF. LF. CTURF. R: Difficulty of preservation in the presence of an aqueous phase may account to some extent for the fact that castor oil has found little use in emulsions. MR. T. P. MCLAUGHLI½: Benzoic acid is more effective in acid conditions than are the benzoates. Yet the unionised form should be more soluble in the oil phase and accordingly less active. THF. LF. CTURF. R: Comment on this would have to await determination of partition coefficients and the minimum safe concentration of benzoic acid in the water phase. MR. M. G. •)F. NAVARRF.: Benzoic acid complexes strongly with nonionics and so is not of great value. On the other hand acid conditions in a system may tend to inhibit some bacterial and mould growth. METHODS FOR DETERMINATION OF MONO- ESTER CONTENT OF PROPYLENE GLYCOL MONOSTEARATE NON SELF-EMULSIFYING C. H. JOHNS, B.Sc., Ph.D., and W. P. PEPPER, M.Sc., F.R.I,C.* Delivered at the Summer Conference of the Society on 25th August 1960. Anal•ical methods tot •he determination oI Mono-es•er con•en• Prop•rlene Glycol Monos[eara[e are discussed. Two al[ernafive me[hods analysis are •11y described. •NTRODUCTION I• COSM•.TIC formulations, it is well known that the mono-ester content of the various poly-alcohol esters of fatty acids used has a profound effect upon the nature of the finished product. With propylene glycol mono- *Chesebrough-Pond's Ltd., London, N.W.10o