Dissolved Oxygen Sensor: Optical and Polarographic Determination

Dissolved Oxygen Sensor

Optical and Polarographic Sensors for Proper Determination of Dissolved Oxygen

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METTLER TOLEDO offers a comprehensive range of top quality buffers, standards, electrolytes, cleaning and verification solutions for determination of pH, conductivity, ion concentration, ORP and dissolved oxygen. All solutions are available in small volume bottles either in packs or single and single use sachets.

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How to store DO sensors correctly?

All user manuals come with the necessary information about the short and long term storage of the respective probe.

General tips for long term storage:

  • Polarographic DO sensors: with measuring module filled with electrolyte and with green protective cap mounted (dry) to protect from mechanical damage
  • Galvanic DO sensors: with storage vial filled with 10% NaCl
  • Optical DO sensors: in the calibration tube ensuring first that the sponge has been dampened with distilled water or keep the sensor in a beaker filled with distilled water

How does the dissolved oxygen sensor work?

There are three different types of dissolved oxygen sensors:

  • Galvanic             (LE621)
  • Polarographic     (InLab® 605)
  • Optical                (InLab® OptiOx™)

The galvanic and polarographic measuring systems are both electrochemical methods.

The galvanic sensor

The galvanic probe is the simplest, because it produces its own electric current. Two poles made of dissimilar metals cause a self polarization of the sensor when immersed in an electrolyte solution. The poles are usually made of silver and lead and the electrolyte is potassium hydroxide.

The polarization causes a potential that leads to a reduction of the oxygen on the surface of the negative pole, which acts as both catalyst and cathode. The produced current can be measured and it is proportional to the quantity of oxygen reduced and hence to the oxygen available in the measuring chamber. The measuring chamber itself is separated from the sample by a watertight oxygen permeable membrane. To make sure that the dissolved oxygen concentration inside and outside the measuring chamber is equal, constant stirring is indispensable.

The polarographic sensor

It is very similar to the galvanic system, with the exception that the polarization of the poles is effected by an external electronic voltage source. That's why polarographic sensors have to be polarized for 6 hours before use.

Dissolved Oxygen Sensor
polarographic sensor

The optical sensor

As the name suggests, the optical sensors detect and measure the intensity of fluorescent light.

Sensor emitted blue light (1) excites a fluorescent membrane (2), which in turn emits red light waves (3). These light waves form the actual measurement signal, which is captured by a photodiode (4).
Due to quenching effects the intensity and characteristics of this red fluorescence changes with the amount of oxygen present in the sample (5).

This technology is fast, stable and offers highly reproducible results.

Dissolved Oxygen Sensor
optical sensor

Can the InLab® OptiOx™ dissolved oxygen sensor be used with the uPlace™ electrode arm?

Yes, InLab OptiOx™ can mount on the uPlace™ electrode arm using a coated blue aluminum adapter which looks like the Greek letter omega. The adapter can be mounted at the front of the head of uPlace™ electrode arm. The part number is 30246619.

The adapter allows simultaneous use of other sensors such as pH and/or conductivity on the same uPlace™ electrode holder.

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