Amperometric Differential Determination of Ascorbic Acid in Beverages and Vitamin C Tablets Using a Flow Cell Containing an Array of Gold Microelectrodes Modified with Palladium Renato C. Matos, þ Ma ´rcio A. Augelli, þþ Jairo J. Pedrotti, þ Claudimir L. Lago, þ and Lu ´cio Angnes* þ þ Instituto de Quı ´mica, Universidade de Sa ˜o Paulo, Av. Prof. Lineu Prestes, 748, 05508-900-Sa ˜o Paulo, SP, Brasil þþ Faculdade de Engenharia de Guaratingueta ´, UNESP, Guaratingueta ´, SP, Brasil Received: March 16, 1998 Final version: June 5, 1998 Abstract A simple and attractive method for quantification of ascorbic acid (AA) in beers, soda, natural juices and commercial vitamin C tablets was achieved by combining flow injection analysis and amperometric detection. An array of gold microelectrodes electrochemically modified by deposition of palladium was employed as working electrode which was almost unaffected by fouling effects. Ascorbic acid was quantified in beverages and vitamin tablets using amperometric differential measurements. This method is based on three steps involving the flow injection of: 1) the sample plus a standard addition of AA, 2) the pure sample, and 3) the enzymatically-treated sample. The enzymatic treatment was carried out with Cucumis sativus tissue, which is a rich source of ascorbate oxidase, at pH 7. The calibration plots for freshly prepared ascorbic acid standards were very linear in the concentration range of 0.18–1.8 mg L ¹1 with a relative standard deviation (RSD) < 1%, while for real samples the deviations were between 2.7 % to 8.9 %. Keywords: Ascorbic acid, Flow injection analysis, Beverages, Vitamin C, Modified microelectrodes 1. Introduction Ascorbic acid, commonly called vitamin C (in which ascorbic acid is just one of the components [1]) can be found in numerous natural sources and is used in a wide scale as antioxidant agent in foods and drinks. Its determination has been receiving great attention in clinical analysis, mainly in serum blood and urine [2]. In recent years, its quantification in foods and beverages has received increasing importance [3]. Its quantification is based on its reducing properties or capacity to produce colored substances. The literature presents many methods such as volumetric, chromato- graphic, enzymatic, electroanalytical and spectrophotometric [4–14] with this latter being the most used despite the inconvenience of simultaneously determinating dehydroascorbic acid, which is one of its oxidation products. Many methods involving flow injection analysis (FIA) has also been proposed for determination of ascorbic acid [15, 16]. Recently, an indirect electroanalytical method for simultaneous determination of ascorbic and dehydroascorbic acid with very low detection limit was described [17]. Electroanalytical methods for ascorbic acid present many advantages as they avoid previous extraction, have high sensitivity and can be used with success in a colored matrix. As a drawback the decrease of the signal observed for solid electrodes [18] after a certain number of analyses is probably caused by the adsorption of the ascorbic acid oxidation product on the surface of the electrode, demanding periodic cleanup. These difficulties drive some authors to prefer the polarographic methods in batch [19, 20] or associated with flow-injection analysis [21]. The association of high-performance liquid chromatography (HPLC) with electrochemical detection [22, 23] has also gained increasing popularity. Madigan et al. [22] developed a method for ascorbic acid in beers involving HPLC with electrochemical detection, attaining a detection limit of 0.2 mg L ¹1 . Before this work a similar method was proposed for determination of sulfite and ascorbic acid in drinks [23] with a detection limit of 0.1 and 0.5 mg L ¹1 for sulfite and ascorbic acid, respectively. Another tendency in the electrochemical analysis of ascorbic acid is the use of modified electrodes, which favor electrocatalysis and anticipate the oxidation potential of the analyte [24]. Gao et al. [25] determined AA by voltammetry and amperometry using a glassy carbon microelectrode electrochemically modified by codeposition of a film containing copper and the complex heptacyanonitrosyl- ferrate. Greenway et al. [26] made its determination in fruit and vegetable juices by flow-injection with immobilized ascorbate oxidase over an unmodified glassy carbon electrode at þ0.6 V. The electrochemical oxidation of ascorbic acid (I) to dehydroas- corbic acid (II) is a process involving two electrons, according to the reaction shown in Scheme 1. In the present work, we describe a differential amperometric determination of ascorbic acid in drinks and vitamin tablets, exploring an array of gold microelectrodes modified by electro- deposition of palladium. The concentration of ascorbic acid in each sample was calculated based on the difference between the current 887 Electroanalysis 1998, 10, No. 13  WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 1040-0397/98/1310-0887 $ 17.50þ.50/0 Scheme 1.