Teledyne Insta Trans - trace and percent oxygen transmitter - Manual (Page 16)

Operational Theory Insta-Trans
Teledyne Analytical Instruments 8
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 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.
2.2.4 The Effect of Pressure
In order to state the amount of oxygen present in the sample in
parts-per-million or 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
transfer, and therefore the external current, will increase, even though
the oxygen concentration of the sample has not changed. It is therefore
important that the sample pressure at the fuel cell (usually vent pressure)
remain relatively constant between calibrations.
2.2.5 Calibration Characteristics
Given that the total pressure of the sample gas on the surface of the
Micro-fuel Cell input is constant, a convenient characteristic of the cell
is that the current produced in an external circuit is directly proportional
to the rate at which oxygen molecules reach the cathode, and this rate is
directly proportional to the concentration of oxygen in the gaseous