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Operational Theory 2Operational Theory 2

Operational Theory 2

Teledyne Analytical Instruments

Operational TheoryOperational Theory

Operational Theory

2.1 Sensor2.1 Sensor

2.1 Sensor

The sensor is an open-cathode cell, an electrochemical transducer

specific to oxygen. The cathode of the cell is composed of silver screen

elements with a large surface area. The screen assembly is mounted in an

acrylic block, with the lower edges of the screens immersed in potassium

hydroxide electrolyte. A thin layer of electrolyte is maintained on the sur-

faces of the screens by capillary action. A lead disk is positioned under the

screens and serves as the anode. An exploded view of the cell is showm in

Figure 5-1.

The sample gas stream is passed directly over the cathode screens,

initiating an electrochemical reaction. Four electrons are generated by the

oxidation of the lead anode, and are then used to reduce oxygen at the

cathode. The flow of electrons between the anode and cathode creates an

electric current which is directly proportional to the oxygen concentration in

the sample stream. In the absence of oxygen, no oxidation or reduction

takes place, and no current is produced.

In simplified form, the reaction may be described as follows: oxygen is

reduced at the cathode by the mechanism

4e- +O2 + 2H2O → 4OH-

This cathodic half-reaction occurs simultaneously with the anodic half-

reaction

Pb + 2OH- → PbO + H2O + 2e-

The overall reaction is

O2+ 2Pb → 2PbO

2.2 Humidifier2.2 Humidifier

2.2 Humidifier

It is necessary to maintain a film of electrolyte on the screens of the

electrode assembly. This means that the humidity of the sample as it flows

through the cell must be such that the water vapor pressure of the electro-