LABORATORY MODELS FOR DEPILATORY PERFORMANCE 373 2-mercapto ethanol. % w/w Neutralizing base of commercial pH Depilation time material (min) Lithium hydroxide 2.7 11.6 3.5 Sodium hydroxide 2.6 11.2 4.0 Potassium hydroxide 3.2 11.1 4.0 Calcium hydroxide 2.4 11.8 5.0 Strontium hydroxide 6.4 11.9 6.0 Barium hydroxide 10.2 11.6 5.0 [Note: No attempt was made to bring these products to a constant pH but only to approximately the right range of pH.] Conclusions. Of the two factors investigated, the most significant one influencing the speed of depilation is the choice of thio compound. In order of effectiveness based on depilation times averaged for all the neutra- lizing bases the results show the following. Thio-compound Average depilation time (min) 2-mercapto ethanol 4.0 Thioglycerol 6.5 Thioglycollic acid 7.5 3-mercapto propionic acid 8.0 2-mercapto propionic acid 11.0 Thiodiglycol 15.0 Thiomalic acid 15.0 The influence of neutralizing base is not so significant as the choice of thio compound, but there is a relationship between speed of depilation and choice of base as follows. Base Average depilation time (min) Sodium 5.7 Lithium 5.7 Potassium 6.5 Barium 6.5 Calcium 7.1 Strontium 8.3 In general monovalent bases would seem to give faster acting depilation than di-valent bases. Based upon these experiments, and on considerations of odour, it was decided to eliminate at this stage certain thio compounds as follows.

374 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Thiomalic acid--slow acting, very bad odour. Thiodiglycol--slow acting, poor odour. 2-mercapto propionic acid--slow acting, poor odour. 3-mercapto propionic acid--moderate speed action, good odour. The choice for retention of thioglycerol and thioglycollic acid was obvious as they were both fast-acting and with reasonable odour 2-mercapto ethanol was also retained because it gave the fastest acting depilation, although it has a very poor odour. From the monovalent bases, sodium hydroxide was selected as the best overall and from the divalent bases calcium hydroxide was selected. Effect of pH Using the basic depilatory cream formula previously described, depila- tory creams were made up containing 5•o w/w of each of the following thio compounds: (a) 2-mercapto ethanol (b) thioglycerol (c) thioglycollic acid. Each cream was neutralized to a range of pH values with (a) sodium hydroxide, and (b) calcium hydroxide the depilation times were deter- mined for each formulation. 2-mercapto ethanol. Depilation time Neutralizing base pH (min) Sodium hydroxide 9 14.0 Sodium hydroxide 10 5.0 Sodium hydroxide 11 3.0 Calcium hydroxide 9 8.0 Calcium hydroxide 10 3.0 Calcium hydroxide 11 3.0 Calcium hydroxide 12 6.0 Thioglycerol. Depilation time Neutralizing base pH (min) Sodium hydroxide 9 15.0 Sodium hydroxide 10 7.0 Sodium hydroxide 11 5.0 Calcium hydroxide 9 10.0 Calcium hydroxide 10 10.0 Calcium hydroxide 11 8.0 Calcium hydroxide 12 6.0