Teledyne 3010TB - Split architecture trace oxygen analyzer - Manual (Page 65)

Part II: 2-3
Oxygen Analyzers Part II: Analysis Units
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 available
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 sensing mem-
brane 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:
Pb + 2OH –
→ Pb
+2 + H
2
O + 2e – (anode)
(Two electrons are transferred for each atom of lead that is oxidized.
Therefore it takes two of the above anode reactions to balance one cathode
reaction and 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.