A sensor for measuring sulphur dioxide in flue gases comprises a sensing electrolyte of doped .beta.-alumina or doped NASICON. The dopant serves to increase conductivity at grain boundaries in the sensing element or at the electrode interface, thereby to increase the rate of depolarisation. Preferred dopants are copper silver and nickel.
A process and apparatus for selective detection of NO in fluid media, particularly exhaust gases of internal-combustion engines. This process comprises the steps of positioning a nitrosonium-conducting solid electrolyte between two porous electrodes, acting upon one of the two electrodes with the NO-containing fluid medium, and measuring a voltage difference between the two electrodes as an indication of the NO-concentration in the NO-containing fluid medium.
