1. Polymer Membrane Electrodes 
Polymer membrane electrodes consist of various ion-exchange materials in an inert matrix such as PVC, polythene or silicone rubber. After the membrane is formed, it is sealed to the end of a PVC tube. The potential developed at the membrane surface is related to the concentration of the species of interest. Electrodes of this type include potassium, calcium and nitrate.   

2. Solid State Electrodes 
Solid state electrodes utilize relatively insoluble inorganic salts in a membrane. Solid state electrodes exist in homogeneous or heterogeneous forms. In both types, potentials are developed at the membrane surface due to the ion-exchange process. Examples include silver/sulphide, chloride and fluoride.   

3. Gas Sensing Electrodes 
Gas sensing electrodes are available for the measurement of ammonia, carbon dioxide, nitrogen oxide and sulfur dioxide. These electrodes have a gas permeable membrane and an internal buffer solution. The pH of the buffer solution changes as the gas reacts with it. The change is detected by a combination pH sensor within the housing. Due to the construction, gas sensing electrodes do not require an external reference electrode.   

4. Glass Membrane Electrodes 
Glass membrane electrodes are formed by the doping of the silicon dioxide glass matrix with various chemicals. The most common of the glass membrane electrodes is the pH electrode. Glass membrane electrodes are also available for the measurement of sodium ions.   
 

Reference Electrodes 
Ion Selective Electrodes are available as half-cells (mono) or as glass or epoxy combination electrodes. Measurements with half-cell electrodes require the use of an additional reference electrode.(Gas sensing electrodes are combination electrodes and do not require a reference electrode).