Teledyne 3290 - OEM percent oxygen analyzer - Manual (Page 19)

Percent Oxygen Analyzer Operational Theory
Teledyne Analytical Instruments 9
The anode structure is larger than the cathode. It is made of lead and is
designed to maximize the amount of metal available for chemical reaction.
The space between the active elements is filled by a structure
saturated with electrolyte. Cathode and anode are wet by this common
pool. They each have a conductor connecting them, through some
electrical circuitry, to one of the external contacts in the connector
receptacle, which is on the top 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:
O2 + 2H2O + 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 + H2OPbO + 2H + + 2e– (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
proportional 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:
2Pb + O
2
→ 2PbO
These reactions will hold as long as no gaseous components
capable of oxidizing lead are present in the sample. The only likely
components are the halogens—iodine, bromine, chlorine and fluorine.
The output of the fuel cell is limited by (1) the amount of oxygen in
the cell at the time and (2) the amount of stored anode material.
In the absence of oxygen, no current is generated.