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2-3
Trace Oxygen Analyzer Operational Theory 2
Teledyne Analytical Instruments
At the top end of the cell is a diffusion membrane of Teflon, whose
thickness is very accurately controlled. Beneath the diffusion membrane
lies the oxygen sensing element—the cathode—with a surface area almost
4 cm
2
. The cathode has many perforations to ensure sufficient wetting of
the upper surface with electrolyte, and it is plated with an inert metal.
The anode structure is below the cathode. It is made of lead and has a
proprietary design which is meant to maximize the amount of metal avail-
able for chemical reaction.
At the rear of the cell, just below the anode structure, is a flexible
membrane designed to accommodate the internal volume changes that
occur throughout the life of the cell. This flexibility assures that the sens-
ing membrane remains in its proper position, keeping the electrical output
constant.
The entire space between the diffusion membrane, above the cathode,
and the flexible rear membrane, beneath the anode, is filled with electro-
lyte. Cathode and anode are submerged in this common pool. They each
have a conductor connecting them to one of the external contact rings on
the contact plate, which is on the bottom of the cell.
2.2.3 Electrochemical Reactions
The sample gas diffuses through the Teflon membrane. Any oxygen
in the sample gas is reduced on the surface of the cathode by the following
HALF REACTION:
O
2
+ 2H
2
O + 4e
––
––
–
→ 4OH
––
––
–
(cathode)
(Four electrons combine with one oxygen molecule—in the presence
of water from the electrolyte—to produce four hydroxyl ions.)
When the oxygen is reduced at the cathode, lead is simultaneously
oxidized at the anode by the following HALF REACTION:
2Pb + 4OH
––
––
–
→ 2Pb
+2
+ 2H
2
O + 4e
––
––
–
(anode)
(Two electrons are transferred for each atom of lead that is oxidized.
TWO ANODE REACTIONS balance one cathode reaction to transfer four
electrons.)
The electrons released at the surface of the anode flow to the cathode
surface when an external electrical path is provided. The current is propor-
tional to the amount of oxygen reaching the cathode. It is measured and
used to determine the oxygen concentration in the gas mixture.
The overall reaction for the fuel cell is the SUM of the half reactions
above, or: