In standard ion-selective electrodes the ion-selective membrane is positioned between the sample solution and the internal solution of known, constant ionic composition. A silver/silver chloride wire is placed in this internal solution providing ionic to electronic transduction of the signal. As the composition of the internal solution remains constant the change of potential of the electrode can be attributed solely to the changes at the membrane-sample interface. Functioning of such sensors is well understood and their response can be described with the Nernst equation.