Cell for measuring gas levels

An electrochemical cell is described for measuring the amount of a predetermined gas in a gaseous mixture, the cell comprising a working electrode, a counter electrode, an electrolyte, and means for applying across the electrodes a potential that varies over a range that includes values at which oxygen is subject to reduction. Current flow between the electrodes is measured in terms of the predetermined gas in the gaseous mixture and means for inhibiting the reduction of oxygen at the working electrode, the means for inhibiting comprising a dissociable compound dissolved in the electrolyte that provides an anion adsorbable on the working electrode sufficiently strongly to prevent its own displacement by oxygen but not so strongly as to prevent its own displacement by the predetermined gas, wherein the anion is of the general formula BzXz2- where B is boron, X is a halogen, and z is equal to 10 or 12