A sensor composed of an indicator and a reference electrode in contact with a thin film of solution which is separated from the bulk of the sample solution by a gas-permeable membrane or an air gap. This intermediate solution interacts with the gaseous species (penetrated through the membrane or an air gap) in such a way as to produce a change in a measured constituent (e.g. the $\ce{H^{+}}$ activity) of the intermediate solution. This change is then sensed by the ion-selective electrode and is related to the partial pressure 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 ($\ce{Pt(s)}$|$\ce{H2(g)}$|$\ce{H^{+}(aq)}$ or $\ce{Pt(s)}$|$\ce{Cl2(g)}$|$\ce{Cl^{-}(aq)}$]. These electrodes respond both to the partial pressure of the gas ($\ce{H2}$ or $\ce{Cl2}$) and to the ionic activities ($\ce{H^{+}}$ or $\ce{Cl^{-}}$). The Clark oxygen electrode fits under this classification although, in contrast to other gas sensors, it is an amperometric and not a potentiometric device.