Oxygen Analyzer Operational Theory
Teledyne Analytical Instruments 19
controlled with respect to the reference electrode. The counter electrode
is used to carry the current that flow through the sensor. A potentiostat is
typically constructed with several operational amplifiers. The three
electrodes in an electrochemical cell and the operational amplifiers in
the potentiostat constitute a feedback-control loop. The potentiostat
technology has been well accepted in the field of electrochemistry, and
proven effective in eliminating polarization of the reference electrode
and automatic compensating electric resistance in the cell.
In a BDS oxygen sensor, the sensing electrode is a working
electrode that is under precise potential control as discussed above. A
stable sensing electrode potential is very critical for an oxygen sensor to
achieve high stability, low noise and large dynamic range. The reference
electrode in a BDS sensor is a Ag/Ag2O electrode which is well known
for its stable electrode potential and compatibility with the KOH
electrolyte in an oxygen sensor. The counter electrode is made of a
Platinum wire.
The sensing process involves electrochemical reactions inside the
sensor. At the sensing electrode, oxygen is reduced at the controlled
potential:
O2+ 2H2O + 4e- —
> 4OH-(1)
There is no net electrochemical reaction at the reference electrode
since it is connected to the high impedance input of the operation
amplifier.
The electrochemical reaction at the counter electrode is:
4OH- —
> O2 + 2H2
O + 4e-( 2)
It is noteworthy that reaction (2) is reverse of the reaction (1). It is
indicative of a net change of zero inside a BDS sensor throughout the sensing
process. This feature produces a long-term stability for the BDS sensor.
There are two resources of oxygen being reduced at the sensing
electrode: from the sample gas and dissolved oxygen within the
electrolyte. The oxygen molecules in the sample gas diffuse to the
sensing electrode through a diffusion barrier (controlled diffusion) to
produce a current signal which is proportional to the oxygen level in the
sample gas. However, the dissolved oxygen in the electrolyte also
diffuses through the electrolyte. It is reduced at the sensing electrode and
produces a background current. This background current represents the
detection limit of an oxygen sensor.