114 JOURNAL OF COSMETIC SCIENCE titration of aluminum chloride (12) and provides an optical method to determine the onset and extent of precipitation. MATERIALS AND METHODS The antiperspirant actives studied were ACH (Chlorhydrol PDR, Reheis, Berkeley Heights, NJ), ACH' (Reach-101, Microdry, Reheis), AZG (Westchlor ZR 30B, West- wood, Middletown, NY), and AZG' (Westchlor ZR 30B DM XF, Westwood). The apparatus used for the potentiometric/turbidometric titration is shown in Figure 1. Twenty-five milliliters of antiperspirant solution, adjusted to 0.1 M in metal ion (either A1 or A1 plus Zr) with double distilled water, was placed in the reaction vessel. An automatic titrator (Radiometer) added 0.5 N NaOH to the reaction vessel and recorded the pH. The contents of the reaction vessel were circulated through a 1-cm pathlength flow-through optical transmission cell using a peristaltic pump. The flow rate was 100 ml/min. The volume of the flow-through cell and lines was 10 mi. The absorbance was recorded at 600 nm using a X. 19 UV-visible spectrometer (Perkin Elmer). RESULTS AND DISCUSSION The potentiometric/turbidometric titrigraph of 0.1 M A1C13 is shown in Figure 2. Both the pH and turbidity increased slowly until approximately 12 ml of 0.5 N NaOH was added. Both the pH and turbidity increased rapidly at this point. The maximum turbidity occurred when 13.2 ml of base was added. The pH was 7.4 at this point. The Perkin Elmer UV-Visible spectrometer 1 cm pathlength flow cell Radiometer Automatic pH titrator ... burette / titrant .•.• Stirred reaction vessel Peristaltic Pump Figure 1. Schematic of the potentiometric/turbidometric titration apparatus.

TITRATION OF ANTIPERSPIRANT ACTIVES 115 12 10 pH 6 4 Titration curve Turbidity curve 0 5 10 15 20 25 _ 1.5 0.5 N NaOH, ml Figure 2. Potentiometric/turbidometric titrigraph of 25 ml of 0.1 M A1CI• titrated against 0.5 N NaOH. turbidity decreased as more base was added. The precipitate had dissolved when 19.2 ml of the base (pH 11.6) was added. The potentiometric/turbidometric titrigraph is consistent with the properties of alumi- num chloride and the pH-solubility profile of aluminum hydroxide. The relatively constant pH observed until approximately 12 ml of base had been added indicates that the added hydroxyls were coordinated to aluminum ions and thus did not significantly raise the pH. Polymerization of the hydroxy-aluminum species began at this point, and the turbidity increased sharply. The pH also increased sharply at this point, as the added hydroxyls were not completely coordinated to aluminum (13). The dissolution of the precipitate as the titration proceeded above pH 7.4 is believed to be due to the am- photeric nature of aluminum. The minimum solubility of aluminum hydroxide occurs at pH 5.8 (14). Thus, the dissolution of the precipitate when the pH was above 7.4 reflects the conversion of AI(OH 0 to soluble aluminate anion (AI(OH)•). Based on the turbidity curve, the precipitate dissolved after 19.2 ml of base was added. The potentiometric/turbidometric titrigraph of ACH is shown in Figure 3. Formation of a precipitate required much less base than was required for aluminum chloride. The pH increased sharply from the beginning of the titration. The turbidity increased after the addition of approximately 1 ml of 0.5 N NaOH. The maximum turbidity occurred when 4.0 ml of 0.5 N NaOH was added. At this point, the pH was 11.7. The precipitate completely dissolved after 6.3 ml of 0.5 N NaOH was added. This point corresponds to a pH of 12.0. ACH required much less base to initiate precipitation than aluminum chloride because it is partially neutralized (10). Consequently, the starting pH of 0.! M ACH solution