Teledyne 3220 - Multi-channel oxygen monitor system - Manual (Page 26)

2 Operational Theory Model 3220
Teledyne Analytical Instruments 12
2.5.2 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)
In this reaction, 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)
In this reaction, 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
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—such as iodine, bromine, chlorine and
fluorine—are present in the sample.
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.
2.5.3 The Effect of Pressure
In order to state the amount of oxygen present in the sample as a
percentage of the gas mixture, it is necessary that the sample diffuse into
the cell under constant pressure.
If the total pressure increases, the rate that oxygen reaches the
cathode through the diffusing membrane will also increase. The electron