Riken Keiki GD-3EC, Technical Manual

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11. Appendix 11-1 Detection principle
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11-1-5 Electrochemical type
<Sensor overview>
This sensor type detects a gas concentration in the form of a current generated when the detection target gas is
electrolyzed on electrodes maintained at constant potential. This is the most effective gas sensor for detecting
toxic gas. Gases can be selectively detected by selecting a specific potential.
<Sensor structure and principle>

Structure
The sensor consists of an electrode (working electrode) formed of a precious metal catalyst on a gas
permeable membrane, a reference electrode, and a counter electrode, all enclosed in a plastic container
filled with electrolyte solution.

Principle
A potentiostat circuit is used to maintain a constant potential between the working electrode and reference
electrode. The detection target gas passes through the gas permeable membrane and undergoes
electrolysis reaction on reaching the working electrode. A side reaction also occurs at the counter electrode
so that the same amount of electric current flows through it as that produced at the working electrode.
Example: In the case of hydrogen sulfide (H
2
S), the following chemical reaction occurs:
Working electrode: H
2
S + 4H
2
O → H
2
SO
4
+ 8H
+
+ 8e
-
(e
-
is an electron)
Counter electrode: 2O
2
+ 8H
+
+ 8e
-
→ 4H
2
O
The current generated here is proportional to the gas concentration. The gas concentration can therefore be
determined by measuring the current flowing through the working electrode
<Structure diagram>
Electrolyte
Reference
electrode
Catalyst
Working electrode
Gas permeable
membrane
Detection
target gas
Potentiostat circuit
Container
Counter
electrode