Electrochemical Sensors

An electrochemical sensor consists of a chamber with two or three electrodes and an electrolyte. A membrane, such as a porous PTFE-sheet, prevents the electrolyte from leaving the chamber. Gas, however, can pass through the membrane and reach the electrodes. The electrodes are usually made of platinum or gold. An electrochemical reaction with the gas takes place on the measuring electrode. Electrons are released and diffuse to the counter electrode. The current produced by the electrodes that diffuse from one electrode to the other is proportional to the gas concentration.

 
Electrochemical Sensor

The reference electrode produces a constant voltage between the measuring electrode and the counter electrode. Most gases react only within a small range of the reference voltage. Electrochemical sensors are available for many gases (H2S, HCN, CO, Cl2, SO2, H2, NO, and NO2).
Some typical reactions between the gas and the electrolyte:
Gas
 Measuring electrode Counter electrode
H2S
H2S + 4 H2O —> H2SO4 + 7 H+ + 8e- O2 + 4 H + 4e- —> 2 H2O
HCN
2 HCN —>2 H + 2 CN- O2 + 2 H + 2e- —> H2O
CO
 CO + H2O —> CO2 + 2 H+ + 2e- O2 + 4 H + 4e- —> 2 H2O
Cl2
2 H2O —> O2 + 4 H+ + 4e- 2 H+ + Cl2 + 2e- —> 2 HCl
SO2
SO2 + 2 H2O —> H2SO4 + 2 H+ + 2e- O2 + 4 H + 4e- —> 2 H2O
H2
 H2 —> 2 H+ + 2e- O2 + 4 H + 2e- —> H2O
NO
 NO+ 2 H2O —> HNO3+ 3 H+ + 3e- O2 + 4 H + 4e- —> 2 H2O
NO2
2 H2O —> O2 + 4 H+ + 4e- NO2 + 2 H + 2e- —> NO + H2O
Depending on the measured gas, different catalysts, electrodes, electrolytes, and reference voltages are used to get the best selectivity and to avoid cross-sensitivity.


Electrochemical sensors are mainly used to monitor the ambient air for one substance. The cross-sensitivity is, in most cases, too low to use the electrochemical sensor to measure several compounds at the same time.

Electrochemical Sensors for Oxygen
Electrochemical sensors are often used to measure oxygen. These sensors are set up similarly to galvanic cells. They have 2 electrodes, made of different materials, and an electrolyte. Usually gold and lead electrodes are found in the oxygen sensor. The oxygen diffuses through a PTFE-membrane to reach the electrodes.
Oxygen Sensor

Oxygen sensor
The voltage that passes between the measuring electrode and the counter electrode is proportional to the measured gas concentration.
Electrochemical reaction:

  • Measuring electrode: 4 OH- + 2 Pb —> 2 PbO + 2 H2O + 4 e-
  • Counter electrode: O2 + 2 H2O + 4 e- —> 4OH-
    • There are oxygen sensors that measure partial pressure and those that directly measure the % volume. GfG oxygen sensors directly measure the % volume.

    % volume sensors measure the percentage of the gas in the atmosphere, which in clean, dry air is 20.9% volume of oxygen. Partial pressure is defined as the pressure the gas will have if no other gas is in the mixture.

    The concentration of the gas is independent from the absolute pressure. The partial pressure changes in proportion to the absolute pressure. For clean, dry air at an absolute pressure of 2 atm, the following applies:
    % Volume of O2 Partial Pressure of O2
    20.93 Vol% 0.419 atm
    The output signal of a sensor measuring partial pressure changes with the absolute pressure. The sensor must be calibrated if it is used in higher or lower altitudes (i.e. mountains or mines). On the other hand, the signal of a % volume measuring sensor is unaffected by altitude changes. The oxygen % volume does not change.