Teledyne Insta Trans - trace and percent oxygen transmitter - Manual (Page 15)

Trace Oxygen Transmitter Operational Theory
Teledyne Analytical Instruments 7
Figure 2-2: Cross Section of a Micro-fuel Cell (not to scale)
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
sensing 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 electrolyte. 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: