A sensor composed of an indicator and a @[email protected] in contact with a @[email protected] of solution which is separated from the bulk of the sample solution by a gas-permeable @[email protected] or an air gap. This intermediate solution interacts with the gaseous species (penetrated through the @[email protected] or an air gap) in such a way as to produce a change in a measured @[email protected] (e.g. the H+ activity) of the intermediate solution. This change is then sensed by the @[email protected] and is related to the @[email protected] of the gaseous species in the sample. [Note: In electrochemical literature the term gas electrode is used for the classical, redox-equilibrium-based gas electrodes as well, such as the hydrogen or the chlorine gas electrodes (Pt (s)|H2 (g) | H+ (aq) or Pt (s) |Cl2 (g) | Cl− (aq)]. These electrodes respond both to the @[email protected] of the gas (H2 or Cl2 ) and to the ionic activities (H+ or Cl−). The Clark oxygen electrode fits under this classification although, in contrast to other gas sensors, it is an amperometric and not a potentiometric device.