A sensor composed of an indicator and a @R05229@ in contact with a @T06345@ of solution which is separated from the bulk of the sample solution by a gas-permeable @MT06878@ or an air gap. This intermediate solution interacts with the gaseous species (penetrated through the @MT06878@ or an air gap) in such a way as to produce a change in a measured @C01281@ (e.g. the H+ activity) of the intermediate solution. This change is then sensed by the @I03244@ and is related to the @P04420@ 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 @P04420@ 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.