Electrochemical Behavior and Determination of Salicylic Acid at Carbon-fiber Electrodes

The loss of electrode activity (referred to as electrode fouling) caused by an electropolymerized passive film of salicylic acid (SA), a major hydrolysis product of aspirin, significantly decreases the accuracy and the precision of indirect electrochemical detection of aspirin. However, the electrode fouling problem for electrochemical determination of SA and ways to remove the fouling film have been sparsely discussed. In addition, the electrochemical behavior of SA on a carbon-fiber electrode (CFE), widely used for the study of neurochemicals in vivo and in vitro, has not yet been studied. Herein, we report the electrochemical behavior of SA at CFEs using cyclic voltammetry and differential pulse voltammetry. The irreversible and pH dependent electrochemical oxidation of SA produced dihydroxybenzoic acids, SA dimeric products and poly-SA films that strongly adhered to the electrode surface and passivated further reactions. The electrode fouling caused by the SA films on a CFE decreased electrode sensitivity and reproducibility for the indirect determination of aspirin. In this study, the fouling effects were characterized with Fe(CN)64−/3−. Interestingly, we found that the poly-SA films on CFE can be removed by a strong base, undergoing the intramolecular base catalyzed hydrolysis of salicylate esters. This non-destructive cleaning method of poly-SA films on the electrode improved the linear dynamic range of SA as well as the accuracy and precision of SA measurements. This new cleaning method of CFEs was further applied to the indirect determination of aspirin in commercially available pharmaceutical tablets.