2- Measurement of the dissolved oxygen DO [mg/L] or % O2 Saturation

Galvanic method with membrane covered galvanic dissolved oxygen sensor

Sensor closedSensor opendirect measurement in Karlsruher flask,
decrease of oxygen demand

There are a lot of manufactures and also a lot of different sensors for the measurement of dissolved oxygen.

One of them is the galvanic measurement with sensors like the good old Clark electrode.
How does it work?

In bottom of the open body of the sensor you can see a little gold tip. That tip works like a Cathode and make a reduction of the dissolves oxygen (deliver electrons, electron donator).

Cathode: O2 + 2 e− + 2 H2O → H2O2 + 2 OH− and H2O2 + 2 e− → 2 OH−

The small silver ring in the middleof the open sensor head is the Anode. Here we have an oxidation, the equation you can see below (takes electrons, electron acceptor).

Anode: 4 Ag → 4 Ag+ + 4e− and 4 Ag+ + 4Cl− → 4 AgCl

If the Sensor is closed,inside the sensor is an electrolyte like Potassium chloride solution (KCl) between the two electrodes.

In both equation you can see the electrons, this electrons are not stable in liquid water, so they induced two different electromotive force in the metal electrodes.

Galvanometers can measure the current conduction between the two electrodes.

But how comes the dissolved oxygen to the gold tip. Simple, the little gold tip in the electrolyte is covert by a thin plastic membrane (mostly modified PVC). If the concentration of the dissolved oxygen outside the sensor is height, a lot of this oxygen goes through the membrane and concentration of the oxygen inside the sensor goes up. That effect induced a height current conduction between the two electrodes. Low concentration outside contrary shows a low current conduction.

Important is the sensor calibration. The amount of dissolved oxygen is dependent by temperature and air pressure. Modern units have circuits for compensation both.

During this calibration procedure the closed sensor is insert in a calibration tube (inside is a little water soaked sponge) with water saturated air. This is the point for % O2 Saturation.

Problem: The diffusion of oxygen through membrane impoverished the concentration of the oxygen outside the membrane, so the solvent must stirred during the measurement.

Water Quality Index


Note: If dissolved oxygen is greater than 140%, the quality index equals 50.