PbO2-based graphite-epoxy electrode for potentiometric determination of acids and bases in aqueous and aqueous-ethanolic media

The construction and analytical evaluation of a PbO2-based graphite-epoxy electrode sensitive to H3O+, based on incorporation of lead(IV) oxide in a graphite-epoxy matrix, are described. The data obtained from a variety of acid-base titrations in aqueous and aqueous-ethanolic media were compared with those obtained by use of a glass electrode under the same conditions. The proposed electrode provides a linear response in the pH range from 1 to 11 with a slope of -58.7+/-0.3 mV pH(-1) and -60.8+/-0.2 mV pH(-1) in aqueous and ethanolic media, respectively. The response time was less than 15 s and the lifetime of the electrode was at least eight months (ca. 5000 determinations) and its performance is good in pH determination and end-point detection in potentiometric acid-base titrations in both aqueous and aqueous-ethanolic media.

Amperometric determination of xanthine and hypoxanthine at carbon electrodes. Effect of surface activity and the instrumental parameters on the sensitivity and the limit of detection

The performance of active graphite and carbon fiber surfaces produced by different mechanical/electrochemical methods of surface activation has been investigated in the amperometric determinations of xanthine and hypoxanthine under physiologically relevant conditions. The electrodes showed better limits of detection (LOD) when used with differential techniques with a capability of discriminating the analytical signal from the background. Square wave voltammetry and cyclic voltammetry showed the most sensitive response. Electrochemically activated carbon fiber ultramicroelectrodes showed the highest sensitivity (58 A M(-1) cm(-2)) and the LOD in the 200 nM range was observed at the rough pyrolytic graphite electrodes by square wave voltammetry. The results demonstrate the feasibility of the development of new electroanalytical methods for the determination of oxypurines in biological samples.