Sensors 2001, 1, 1-12

The paper discusses the anomalies of the former interpretation of the working mechanism of the ion-selective electrodes. It was thorougly discussed why the Donnan experiment could not be applied as the theoretical background of glass electrodes. It was assumed according to the Donnan interpretation that the measurable potential is produced by the transfer of the primary ion through the membrane. Since the 1960s, the author and coworkers have been investigating the problem of what can be the reason for the potential response of the electrodes. Practical measurements in connection with the response time, surface hindered reactions etc. were interpreted. Furthermore, the energy problems according to the Gibbs theory using two electrode components were investigated. It was established that the electrodes on which chemical reactions may occur with the primary ion have a surface reaction as the chemical basis of the response. For electrodes that work on the principle of the lyotropic series, surface reactions are also involved, but their response is not always Nernstian. The energy demand is covered by the charge separation at the electrodesolution interface.