The potentiometric response of some carbonaceous electrodes

Abstract

Summary The acid-base sensitivity and sensitivity to metallic ions of unimpregnated and impregnated graphite, glassy carbon, graphite oxide, and graphite fluoride electrodes were studied potentiometrically, voltammetrically, and using i.r. spectroscopy and electron-scanning microscopy. The H + sensitivity of graphite and glassy carbon electrodes is chiefly due to the electrode reaction of adsorbed oxygen and/or atmospheric oxygen present in the solution. There is a small concentration of acid-base surface groups on graphite electrodes (most probably carbonyl groups hydrolyzed to quinone systems), below 0.1 mmole g −1 , with a p K a of about 7.1, which causes rather irreproducible sensitivity to metallic cations. Electropositive metallic ions, e.g. Ag + or Cu 2+ , deposit on the carbon electrodes forming imperfect metallic electrodes. The glassy carbon electrode has a negligible concentration of surface acid-base groups and approaches most closely to an ideal inert redox electrode. The graphite oxide electrode is H + sensitive in the acidic region due to formation of a membrane potential, while, in the pH region of about 4–10, it is sensitive to metallic cations due to the presence of acid-base groups (p K a about 3.8, surface concentration about 0.8 mmol g −1 ), which function as a weakly acidic cation exchanger. In contrast to carbon electrodes, graphite oxide does not respond to redox systems. Graphite fluoride is strongly hydrophobic and responds only to certain anions with rather a low sensitivity.