J. Cosmet. Sci., 54, 113-118 (March/April 2003) Potentiometric/turbidometric titration of antiperspirant actives CLIFFORD T. JOHNSTON, STANLEY L. HEM, ERIC GUENIN, JAIRAJH MATTAI, and JOHN AFFLITO, Departments of Agronomy (C. T.J. ) and Industrial and Physical Pharmacy (S.L.H.), Purdue University, West Lafayette, IN 47907, and Colgate-Palmolive Company, Piscataway, NJ 08854 (E.G., J.M., J,A.). Accepted for publication August 19, 2002. Synopsis A titration procedure that simultaneously monitors the pH and turbidity of an antiperspirant solution during neutralization with sodium hydroxide was developed to characterize antiperspirant actives. Alumi- num chloride, aluminum chlorohydrate (ACH), and aluminum zirconium glycine complex (AZG) gave distinctive pH/turbidity profiles. The activated forms of aluminum chlorohydrate (ACH') and aluminum zirconium glycine complex (AZG') produced more turbidity than the non-activated forms. On an equimolar basis, AZG' produced more turbidity than any of the antiperspirant actives tested. INTRODUCTION Antiperspirants are water-soluble compounds that diffuse into the sweat duct and pro- duce an insoluble phase when exposed to sweat. The precipitate physically plugs the sweat duct (1). This mechanism was first proposed by Relier and Luedders (2) and was supported by other investigators (3-7). The major antiperspirant actives currently in use are ACH and AZG. Teagarden et al. (8) demonstrated that ACH solutions become turbid when diluted with deionized water. The insoluble material was identified by infrared spectroscopy as gibbsite, a polymorph of aluminum hydroxide. Standards for ACH and related compounds, as well as complexes related to AZG, are given in The United States Pharmacopoeia (9). Tests to characterize ACH (10) or AZG (11) have been recently described. However, none of the standards or tests are related to the ability to precipitate in the sweat duct. Thus, a study that combined potentiometric and turbidometric titration was undertaken in an attempt to characterize the precipitation behavior of antiperspirant actives. This approach expands on the standard potentiometric Address all correspondence to Stanley L. Hem. 113

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.