MEMBRANE ELECTRODE

"An ion-selective electrode assembly terminating in an ion permeable (e.g., ion-exchange) membrane sensing element. The membrane separates the internal filling solution (that contains a fixed concentration of the ion to be detected) and the test solution. The potential across the membrane depends on the concentration ratio of the ion in the two solutions. The assembly also contains an internal reference electrode immersed in the filling solution, serving as an electrical contact with a stable potential. The potential of this assembly is then measured against an external reference electrode immersed in the test solution. See also Donnan potential. "