A potentiometric sensor is a type of chemical sensor that may be used to determine the analytical concentration of some components of the analyte gas or solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

. These sensors measure the electrical potential of an electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 when no current is flowing.

Principle

The signal is measured as the potential difference (voltage) between the working electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 and the reference electrode. The working electrode's potential must depend on the concentration of the analyte in the gas or solution phase. The reference electrode is needed to provide a defined reference potential.

Classification of sensors

Potentiometric solid state

Solid state (electronics)

Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 gas sensors have been generally classified into three broad groups.

• Type I sensors have an electrolyte
  Electrolyte
  In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
  
   containing mobile ions of the chemical species in the gas phase that it is monitoring. The commercial product, YSZ oxygen sensor, is an example of type I.

• Type II sensors do not have mobile ions of the chemical species to be sensed, but an ion related to the target gas can diffuse in the solid electrolyte to allow equilibration with the atmosphere. Therefore, type I and type II sensors have the same design with gas electrodes combined with metal and an electrolyte where oxidized or reduced ions can be electrochemically equilibrated through the electrochemical cell
  Electrochemical cell
  An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...
  
  . In the third type of electrochemical sensor, auxiliary phases are added to the electrodes to enhance the selectivity and stability.

• Type III sensors make the electrode concept even more confusing. With respect to the design of a solid state sensor, the auxiliary phase looks as part of the electrode. But it cannot be an electrode because auxiliary phase materials are not generally good electrical conductor
  Electrical conductor
  In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
  
  . In spite of this confusion, type III design offers more feasibility in terms of designing various sensors with different auxiliary materials and electrolytes.