A single-unit carbon dioxide-oxygen sensing microelectrode system.

A membrane-covered CO2 microelectrode system is described; it consists of a platinum electrode surronded by a ring-shaped Ag-AgCl electrode, both in contact with an electrolyte layer composed 10 minus 3 M/l quinhydrone in 0.1 N KCl. The resulting oxidation-reduction potential is shown to vary linearly with the logarithm of the PCO2 of the medium. Response time for 95% deflection to a step change in PCO2 of 65 mm Hg is about 1 min i.e. considerably less than in the macroelectrode described previously, but stability is decreased at the same time. Since oxygen in concentrations up to about 21% (air) did not influence the CO2 response and the presence of both CO2 (10.33%) and quinhydrone (10 minus 3 M/l) did not alter the oxygen polarogran, this electorde may be used independently as an oxygen electrode as well. Stability and response time for oxygen were similar to those of the Clark electrode. Possibilities and limitations for in vivo estimation of PCO2 and PO2 are discussed.