Hach-Lange 5740 sc - Manual (Page 10)

6
General Information
2.2 General Sensor Information
The Galvanic Membrane Dissolved Oxygen Sensor allows aqueous samples to be easily
and accurately analyzed for dissolved oxygen concentration. The system consists of a
controller with an integrated display and a sensor for in-situ measurement.
Optional equipment, such as mounting hardware for the probe, is supplied with
instructions for all user installation tasks. Several mounting options are available, allowing
the probe to be adapted for use in many different applications.
Typical applications include aeration basins, nutrient removal in equalization basins,
aerobic and anaerobic digesters, effluent streams, rivers, lakes, and fish ponds.
2.3 Theory of Operation
This galvanic oxygen sensor operates as a battery generating a voltage. The resulting
voltage is directly proportional to the dissolved oxygen concentration. The cell is
constructed with a fine wire coil cathode wrapped around a lead anode. A salt solution fills
the void between the anode and cathode. The sensor is contained by a cylindrical
membrane held in close proximity to the wire coil.
Oxygen from the solution to be tested enters the cell by diffusion through the membrane
and then across the thin electrolyte layer to the cathode. The oxygen is reduced at the
cathode as shown in reaction 1.
1.
The cathode is at such a negative potential that it reduces all the oxygen that diffuses to its
surface. The lead anode is oxidized to give an overall reaction that produces lead
hydroxide as shown in reaction 2.
2.
The overall result of this reaction is the consumption of the lead anode as current flows,
yielding a very sensitive electrode that can detect changes at the microvolt level.
O
2
2H
2
O 4e
–
+ +
4OH
–
→
2Pb 4OH
–
+ 2Pb OH ( )
2
4e
–
+
→