Teledyne 3000TA-XLEU - Trace oxygen analyzer - Manual (Page 26)

Operational Theory 3000TA-XL-EU
Teledyne Analytical Instruments 10
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 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 are specific to oxygen as long as no gaseous
components capable of oxidizing lead—such as iodine, bromine,
chlorine and fluorine—are present in the sample.)
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