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2.3 Theory of Operation
8
This galvanic oxygen sensor operates as a battery generating a voltage. The resulting
voltage is directly proportional to the dissolved oxygen concentration. The cell is
constructed with a fine wire coil cathode wrapped around a lead anode. A salt solution fills
the void between the anode and cathode. The sensor is contained by a cylindrical
membrane held in close proximity to the wire coil.
Oxygen from the solution to be tested enters the cell by diffusion through the membrane
and then across the thin electrolyte layer to the cathode. The oxygen is reduced at the
cathode as shown in reaction 1.
–
→
1.
O
2H
O
4e
+
+
2
2
The cathode is at such a negative potential that it reduces all the oxygen that diffuses to its
surface. The lead anode is oxidized to give an overall reaction that produces lead
hydroxide as shown in reaction 2.
–
→
2.
2Pb
+
4OH
2Pb OH
The overall result of this reaction is the consumption of the lead anode as current flows,
yielding a very sensitive electrode that can detect changes at the microvolt level.
–
4OH
–
(
)
+
4e
2
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