3O4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 4.--pH values of a strong base (sodium hydroxide) and a strong acid (hydrochloric) in anhydrous ethanol and water at various esterified shellac base hair spray after four years of shelf testing. The samples that had the low pH were discolored, and the cans were badly corroded. Figure 8 shows the appearance of the can that contained Sample No. 5. The pH was down to 2.5. The perfume had turned sour and the color of the product was dark brown. Analysis of the product showed only 1.5 p.p.m. iron and 23.8 p.p.m. tin. Figure 9 shows another container with the same product in which the pH remained at 5.9. Here the container is clean and unaffected. The product retained its light color and its odor. Invariably a drop of pH to below 3 results in considerable container corrosion and product deterioration. From periodic analyses of shelf life samples, it appears that pH drop occurs first, followed by corrosion and product deterioration. Conductivity Figure 5.--pH of a weak acid (acetic) and a weak base (NH4OH) in anhydrous ethanol and water at various concentrations.

ANHYDROUS AEROSOL HAIR SPRAY SYSTEMS 305 pH of HAIR SPRAY CONCENTRATES with DIFFERENT RESIN BASES Resin pH Ciba 9.0 National Starch 8.3 Shellac (Esterified) 5.4 Polyvinyl pyrrolidone 6.3 Copolymer V.P/V.A #735 7.6 Figure 6.--Ciba = Ciba No. 325. National Starch = National Starch No. 28-1310. CORROSI ON CHARACTERISTICS OF SHELLAC BASE HAIR SPRAY 4-YEAR SHELF LIFE S&mple No. pH Can Condition 1 5.9 O.K, • •. 6 Det/nned 3 5.8 O.K. 4 1. 9 D•tinned 5 2.2 Detinned 6 5.9 O.K. Figure 7. measurements of these solutions would undoubtedly increase with pH drop. It is of course well established that electrolytic corrosion is a function of the conductivity of the solution. Bates and Schwarzenbach (5) studied 3 series of solutions: HCI, an acetate buffer, and a triethanolamine buffer in which the solvent water was replaced successively by ethanol. The proton (hydrogen ion) activ- ity of an acetate buffer strongly decreases when the solvent water is re- placed by EtOH and the proton activity of a triethanolamine buffer strongly increases thereby. In the case of a dilute solution of HC1, the