HYGIENIC MANUFACTURE AND PRESERVATION 775 4.33 For•naldehyde 4.331 Properties Freely miscible with water and alcohol. A powerful antibacterial and antifungal agent. Formalin solution contains 40% of formaldehyde. 4.332 Disadvantages Formaldehyde is a volatile compound with a pungent odour and is liable to form insoluble polymers. It is highly reactive chemically and is irritant to the skin. It should only be used as a preservative in relatively low concentrations, and it is incompatible with ammonia, gelatin and other proteinaceous materials, phenols and oxidizing agents. 4.333 Uses Despite the above disadvantages and the fact that sterilizing activity in the product is of short duration due to its chemical instability, formalin is used quite extensively as a preservative in shampoos. It is recognized generally that formaldehyde up to 0.05% is an acceptable preservative in such products from the toxicological standpoint (1). 4.34 Organic mercurial compounds The relevant compounds comprise the phenylmercuric salts, such as the nitrate, borate and acetate. 4.341 Properties These are colourless, odorless, crystalline powders, sparingly soluble in water (1:500 to 1:1 500). They are efficient antifungal and antibacterial agents. 4.342 Disadvantages The phenylmercuric salts are potentially toxic and may sensitize the skin. Activity may be reduced in the presence of nonionic and anionic emulsifying and suspending agents, protein, and compounds having -SH groupings. They are precipitated in the presence of chloride. Some loss of activity may be experienced at high pH values and also through absorption by closure materials. 4.343 Uses The phenylmercuric salts are specially exempted in the Third Schedule to the Poisons Rules {United Kingdom) provided that they are used {as preservatives) in cosmetic preparations containing not more than 0.01%. Local regulations vary from country to country and should be consulted when necessary. From the toxicological standpoint, the phenylmercuric salts are recommended for use in preparations, such as shampoos which

776 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are applied transiently and then rinsed away (1). Phenylmercuric nitrate may be useful for preserving shampoos having a pH value below 7.0. At higher pH values it is less stable and may be irritating. The phenylmercuric salts are generally used in preparations at levels of 0.001-0.004%. 4.35 Sorbic acid (Hexa-2,•l-dienoic acid) 4.351 Properties Sorbic acid is a white crystalline powder with a mildly acrid odour, slightly soluble in water (1:600). It functions as an antimicrobial preserv- ative, active against moulds and yeasts and to a lesser degree against bacteria. Antimicrobial activity is retained in the presence of nonionic surfactants. 4.352 Disadvantages Sorbic acid is only effective in the form of the undissociated acid and therefore only of use in formulations below pH 6.5-7. In high concen- trations, the sorbates oxidize under the influence of sunlight, which can result in product discoloration. Sorbic acid itself may cause slight itching and erythema, but there is little evidence that it has any adverse cutaneous effect at normal use-levels. 4.353 Uses Sorbic acid is used primarily as an antifungal agent, with an optimum pH about 4.5. Its fungistatic activity is increased by the addition of acids and sodium chloride. It may be used in preparations at 0.1-0.2ø//o. 4.36 Dioxin (6-acetoxy-2, •t-dimethyl-m-dioxane) 4.361 Properties Dioxin occurs as a yellow to light amber liquid, soluble in or miscible with water and many organic solvents. It has a rather strong characteristic odour. Dioxin is active against gram-negative and gram-positive bacteria, yeasts and fungi at concentrations in the range of 0.03-0.1% and is active over a wide pH range. Activity is not reduced in the presence of nonionics. It has good bactericidal and fungicidal properties in proteinaceous cosmetic systems. 4.362 Disadvantages The odour may be objectionable and difficult to mask in some formu- lations. Dioxin may cause some darkening or discoloration of a protein-