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Operational Theory
(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:
(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:
(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.
2.2.4 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.
The sensing technology deployed in the Model 3190 is an
electrochemical oxygen sensor which is a partial pressure device. Any
changes in the vent pressure of the unit will affect the pressure of the
sample the sensor sees. This will influence the output of the sensor
according to Dalton's Law of partial pressures. Therefore changes in
barometric pressure should be considered in critical applications.
As the pressure changes, the rate that oxygen reaches the cathode
through the diffusing membrane will also increase. The electron transfer,
Teledyne Analytical Instruments
O → PbO + 2H
Pb + H
2
2Pb + O 2 → 2PbO
Model 3190
+
–
+ 2e
(anode)
10
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