Determination of Zinc Pyrithione in Hair Care Products on Metal Oxides Modified Carbon Electrodes

Abstract

A simple electroanalytical method for the determination of zinc pyrithione (ZPT) in commercial cosmetic products has been developed using a metal oxide modi®ed carbon paste electrode. Ingredients in the cosmetic products did not show any interference in the determination of zinc pyrithione. The electrocatalytic behavior in relation to the properties of metal oxides and the mechanism of the oxidation process was investigated using cyclic voltammetry, linear sweep voltammetry , chronoamperometry. and differential pulse voltammetry. Comparison with results obtained from high performance liquid chromatography shows good agreement. The zinc and sodium pyrithiones (Omadine) are broad±spec-tum antimicrobial agents, effective against both bacteria and fungi [1]. They were used as cosmetic preservatives in the late 1960's and the Procter and Gamble Co. discovered the antidandruff properties of zinc pyrithione (zinc 2-pyridinethiol-Noxide or ZPT). ZPT has been established as one of the most effective antidandruff ingredients for use in shampoo, conditioner, rinse and hairdressing formulations through various clinical studies [2]. In 1981 the Food and Drug Administration (FDA) reported that 29 formulations registered with the agency contained ZPT and 7 contained the sodium salt [3]. The Economic Community (EEC) Council Directive allows ZPT to be used in cosmetic products as a preservative in shampoos and conditioners as a antidandruff agent at a maximum concentration of 0.5 % and 1.0 %, respectively [4]. The toxicity of pyrithione salts by various routes of exposure has been studied extensively in several species of animals and has been described previously [5±16].Several analytical methods for pyrithiones have appeared in the literature [17±30]. In general, they involve titration with Ti(III) ion [17], thin-layer chromatography [4, 18±19], highperformance liquid chromatography [20±26] and polarography [27±30] procedures. Titrimetric methods suffer from a lack of selectivity for pyrthione and interferences in hair care products. Quantitative TLC analysis requires the use of a spectrodensitometry and ZPT decomposes in sunlight on TLC plates. Sodium pyrithione is dif®cult to quantitate by means of TLC because it reacts with metals in the TLC plate. The normal-phase HPLC conditions in the on-line Cu(II) complex formation technique can damage the analytical column in time. The direct reverse-phase HPLC analysis of ZPT is dif®cult owing to the interaction with the reverse-phase packing materials or stainlesssteel compounds of the liquid chromatography even if Zn(II) is added to the mobile phase.Although polarography offers greater speci®city it is limited to the determination of 2-pyridinethiol and 2-pyridinethiol-N-oxide. ZPT is substantially insoluble in water and is present in aqueousbased products as a dispersion of ®ne solid particles. However, ZPT is soluble in 1 M hydrochloric acid or alkaline solution. ZPT dissolved in 1 M hydrochloric acid showed a degradation of 10 % immediately, 25 % after 24 hours and 93 % af...