The CO2 conductivity electrode, a fast-responding CO2 microelectrode

A fast-responding CO2 microelectrode system is presented; it consists of a double-lumen polyethylene catheter provided with a stainless steel catheter tip covered by a CO2-permeable membrane, and connected with each lumen separately to a conductivity cell. The catheter is flushed with bidistilled water at constant flow rate and conductivity of inflowing and outflowing water is measured. The change of conductivity in the water leaving the catheter after exposure of the electrode to a medium containing CO2 is related to the PCO2 of the medium. High sensitivity and fast response, both depending on the flow rate of the carrier water, are shown to oppose each other. When cells are placed at a distance of 45 cm from the membrane, response time for 90% deflection is about 10 sec (at flow rates of 5 ml/min), but when located inside the electrode tip at a distance of within 10 mm from the membrane response time is reduced to about 4 sec. Hydrodynamical aspects concerning dispersion of CO2 in the carrier water and reaction kinetics of CO2 hydration are discussed.