ABSTRACT
A phase that separates two other phases to prevent overall mass movement between them, but allows passage with various degrees of restriction of one or several species of the external phases, may be defined as a membrane. The membrane, with asymmetric bathing, may generate a voltage, or the membrane may simply be a barrier that controls access of material to another sensor. The chapter discusses membrane electrodes that respond directly or indirectly to gases. Ion-selective membrane electrodes classified according to the physical state of the substances that form the electrode membrane includes ion-selective electrodes with solid membranes and ion-selective electrodes with liquid membranes. The principal carriers are macrocyclic compounds that form selective complexes with ions in solution and extract those ions preferentially into the membrane to satisfy the electroneutrality condition inside the membrane. The electrodes classified into crystalline electrodes and non-crystalline electrodes.
