<P> Above a certain voltage, the current plateaus and increasing the potential any further does not result in a higher rate of electrocatalysis of the reaction . At this point, the reaction is diffusion - limited and depends only on the permeability properties of the membrane (which is ideally well characterized, the electrode being calibrated against known standard solutions) and by the oxygen gas concentration, which is the measured quantity . </P> <P> The Clark oxygen electrode laid the basis for the first glucose biosensor (in fact the first biosensor of any type), invented by Clark and Lyons in 1962 . This sensor used a single Clark oxygen electrode coupled with a counter-electrode . As with the Clark electrode, a permselective membrane covers the Pt electrode . Now, however, the membrane is impregnated with immobilized glucose oxidase (GOx). The GOx will consume some of the oxygen as it diffuses towards the PT electrode, incorporating it into H O and gluconic acid . The rate of reaction current is limited by the diffusion of both glucose and oxygen . This diffusion can be well characterized for a membrane for both the oxygen and glucose, leaving as the only variable the oxygen and glucose concentrations on the analyte - side of the glucose membrane, which is the quantity being measured . </P>

The polargraphic (clark) electrode measures partial pressure of o2 by