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17. Appendices
17.1 Dissolved Oxygen
17.1.1 Dissolved Oxygen Sensor Fundamentals
The electrode used is the amperometric type of Clark Electrode and is suitable for the measurement
of oxygen pressures in the range 0 to 100 cm of mercury. While the probe actually reads partial
pressure of oxygen, the circuit is calibrated to be read in percentage saturation or parts per million
(Milligrams/litre). The operation of the Clark type probe relies on the diffusion of oxygen through a
suitable membrane into a constant environment of potassium chloride. Solution measurements are
best performed with a reasonable flow past the membrane. At sufficiently high flow rates, the
oxygen current is totally independent of the flow (a few cm/sec is sufficient). The cell must not be
shaken however or unstable readings will result from electrolyte surge bringing new oxygen from
the reservoir to the working cathode surface.
17.1.2 Operating Principle
The Clark oxygen electrode consists of a gold cathode and a silver/silver chloride anode, placed in
an electrolyte solution. This solution is contained behind a plastic membrane. In this case the plastic
is 0.001 inch PTFE (Teflon) sheet. It must be realised that using membranes of very different
thicknesses will result in an error in the temperature compensation that is applied in the instrument
for the membrane permeability. This coefficient is +4.2%/
o
C at 25
o
C for this thickness membrane.
A polarising voltage of about 800 millivolts is applied between the two electrodes. The gold
electrode is placed close to the membrane and because of the polarising voltage, oxygen diffusing
through the membrane will be reduced at the gold electrode.
Equation :
O
2
+ 2H
+
+ 2 electrons
→
H
2
O
2
This reduction process will produce a current through the oxygen electrode. A load resistor
(actually a thermistor in this case) situated in the electrode itself, converts this current into a voltage
proportional to the oxygen partial pressure. The thermistor provided within the body of the
electrode has a temperature coefficient of -4.2%/
O
C. This gives an accurate temperature
compensation for the temperature/permeability effect of the membrane, over a range of about 5 to
45
O
C about a centre value of 25
O
C. Note this compensation is not for the solubility effects. A
separate sensor also built into the tip of the probe achieves this.
17.1.3 Probe Storage
The Oxygen probe should be kept moist when not in use to prevent the thin film of electrolyte
behind the membrane from drying out. To achieve this, the probe can be stored with the tip in water
or in a humid environment.
For long term storage of several weeks or more, remove the membrane and empty out the
electrolyte. Replace the membrane without electrolyte to avoid contamination of the gold and silver
surfaces. When the electrode is stored in this way, the membrane should be replaced and the
electrode refilled before use.