Square Wave Voltammetric Method for Simultaneous Determination of Bleaching Agents in Skin Cosmetics

Skin pigmentation disorders due to melanin excess, could have different causes. Kojic acid (KA) and hydroquinone (HQ) are depigmenting agents widely associated in pharmaceutical formulations to treat skin pigmentation disorders. An electroanalytical method was developed for simultaneous determination of these bleaching agents in skin whitening cosmetics. Cyclic voltammetry (CV) and square wave voltammetry (SWV) experiments were performed using a potentiostat/galvanostat µAutolab (Type III, Metrohm-Eco Chemie). A conventional three-electrode cell containing a glassy carbon (GCE) as working electrode (A=7,07 mm²), Pt disc (A=12,57 mm²) as counter electrode and Ag/AgCl (saturated aqueous KCl) as reference electrode were employed. Before use and between potential scans, the GCE surface was polished. Kojic acid exhibits a well-defined irreversible oxidation peak at 0.72V and hydroquinone a quasi-reversible redox peaks at 0.08 V and -0.02 V, characterized by adsorptive and diffusional processes, respectively. The influence of the pH was examined over the pH range from 2 until 8 in 0.04 mol L^-1 Britton-Robinson buffer solution, the KA and HQ peaks potential shifts negatively with the increase of the pH value, due to the participation of protons in the oxidations processes. Others buffer solutions were examined at pH 8 and Mcllvaine was chosen to subsequent analytical purposes. The square wave voltammetric parameters were optimized by 2³ Full Factorial Design including a central point assayed in quintuplicate. The statistical treatment of the data showed that the main effects and interaction effect of amplitude vs. frequency were significant at 95% confidence level. The best response is obtained at a= 40 mV, f= 40 s^-1 and ΔE_S= 1mV. Under optimized conditions, the calibration curves were obtained in the concentration ranges of 3.5x10^-5 to 2.5x10^-4 mol L^-1 (R= 0.9996) for kojic acid and 1.5x10^-5 to 1.6x10^-4 mol L^-1 (R= 0.9994) for hydroquinone, with  detection limits of 7.84x10^-6 mol L^-1 and 3.71x10^-6 mol L^-1 for KA and HQ, respectively. The proposed method was validated under parameters: specificity, linearity, range, limit of detection, limit of quantification, precision (repeatability and intermediate precision) and accuracy considering Brazilian Health Surveillance Agency (ANVISA) and International Conference on Harmonisation (ICH) guidelines. The developed and validated method has been successfully applied to the direct simultaneous determination of these compounds in real samples (gel and cream).