Journal of Electroanalytical Chemistry 515 (2001) 45 – 51 Carbon paste electrode spiked with ferrocene carboxylic acid and its application to the electrocatalytic determination of ascorbic acid Jahan-Bakhsh Raoof *, Reza Ojani, Abolfazl Kiani Electroanalytical Chemical Research Laboratory, Department of Chemistry, Faculty of Basic Science, Mazandaran Uniersity, Babolsar, Iran Received 28 April 2001; received in revised form 5 June 2001; accepted 24 June 2001 Abstract A carbon paste electrode spiked with ferrocene carboxylic acid (FCAMCPE) was constructed by incorporation of ferrocene carboxylic acid in a graphite powder – paraffin oil matrix. It has been shown by direct current cyclic voltammetry and double step chronoamperometry that this electrode can catalyze the oxidation of ascorbic acid in aqueous buffered solution. It has been found that under optimum conditions (pH 5) the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 248 mV less positive than at an unmodified carbon paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range 3.48 ×10 -5 – 0.49 ×10 -3 M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (3 ) was determined as 1.08 ×10 -5 M. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon paste electrode; Ferrocene carboxylic acid; Ascorbic acid; Cyclic voltammetry; Double step chronoamperometry www.elsevier.com/locate/jelechem 1. Introduction Ascorbic acid is known for its reductive properties and for its use on a wide scale as an antioxidant agent in foods and drinks; it is also important for therapeutic purposes. Its analytical determination is generally based on its reducing properties or on its capacity to produce col- ored substances. In the literature, several methods using volumetric, amperometric, titrimetric, voltammetric and spectrophotometric techniques [1–8] have been re- ported with the last being the most commonly used, despite the inconvenience of simultaneous determina- tion of dehydroascorbic acid, which is one of its oxida- tion products. The selective amperometric determination of ascorbic acid is currently of much interest. In samples with low pH, it is almost impossible to determine this compound electrochemically by direct oxidation on a conventional electrode because of fouling by the oxidation products. A carbon electrode, when subjected to an appropriate pretreatment procedure, exhibits a minimal propensity for surface fouling. However, the kinetics of electron transfer are quite sluggish. This last characteristic of electrochemical irreversibility means that ascorbic acid can be oxidized only at potentials considerably re- moved from its standard redox potential. Therefore, some chemically modified electrodes with various active mediators immobilized at the electrode surface for the mediated oxidation of ascorbic acid in acidic solution have been used [9–11]. In most cases the electrode substrates were glassy carbon (GC), Pt, Au and graphite. However, some authors have emphasized the instability of the attached or adsorbed materials on the electrodes as a problem arising in the utilization of chemically modified electrodes [11,12]. It seems that the incorporation of electrocatalysts into the electrode ma- trix can, even partly, help to solve this problem, and carbon paste electrodes spiked with catalyst may be suitable for this purpose. * Corresponding author. Fax: +98-11252-42029. E-mail address: jahan-37@umcc.ac.ir (J.-B. Raoof). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(01)00642-8