Ionic solid state internal references

Glass pH electrodes and most other practical ion-selective electrodes (ISEs) are membrane devices. The term electrode is, strictly, a misnomer because the active ion-selective membrane has on one side an analyte (the test solution) and on the other an internal (reference) solution containing an internal reference electrode the internal solution and electrode together provide the electrical contact to the membrane. A solid-state ion-selective electrode is one in which the internal components are replaced by a direct back contact to a metal or semiconductor, i.e., an electronic conductor. This is not a trivial matter, because most ion-selective materials are ionic conductors, and... 

Regarding the reference electrode, the conventional Ag/AgCl/KCl electrode is often adapted to the presence of organic solvents and the different conductivity and ionic mobility in the typical HPLC mobile phases by using a porous ceramic frit for the electric contact or by replacing the KCl internal solution by a low-maintenance gel. Solid-state reference electrodes made of Ag or Pd has also been included in commercial equipment. Finally, auxiliary electrodes are currently made of metal and they are often little more than the stainless steel outlet from the cell. 

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