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I wish I knew how to measure dissolved oxygen in liquids!

Just like other types of liquid analysis, you have more than one way to measure dissolved oxygen (DO) in your solution. Which would suit you best will depend on your application.

DO amperometric sensors can measure oxygen in your solution from trace levels up to high concentrations. However, they require regular upkeep. Optical sensors, on the other hand, have low needs and can still give fast yet stable values.

Let’s review how each method works.

Amperometric sensors

Amperometric probes usually consist of two electrodes: the gold cathode, also known as the working electrode, and the silver anode, called the counter electrode.

The two electrodes sit in an electrolytic liquid within a reaction chamber, separated from the medium by a membrane. This membrane allows DO to diffuse into the sensor.

Image of measure dissolved oxygen in liquids
Image courtesy of emerson.com

To measure the liquid’s DO, apply a direct current to both electrodes. The difference in the partial pressure between the inner and outer membrane walls leads to a diffusion of oxygen through this membrane into the reaction chamber.

Once inside the chamber, the oxygen reduces at the cathode to hydroxide ions (OH). This reduction generates a current flow proportional to the amount of transformed oxygen. With this current, the transmitter can calculate the amount of DO.

At the anode, the silver will oxidize, producing a layer of silver bromide or silver chloride, depending on your electrolyte. And this layer reduces the effective voltage at the anode. Therefore you need to clean it regularly to maintain accuracy. Some sensors have a second silver anode to reduce upkeep.

Optical sensors

Optical sensors use the fluorescence quenching method to measure DO. The sensor has a LED, a photodiode, and a separating section, covered by an oxygen-permeable layer. The partial pressure at this layer matches the partial oxygen pressure of the medium.

The permeable layer has marker molecules. When excited by the LED light, they respond with a red fluorescence. Oxygen molecules attach themselves to these markers and attenuate, or quench, the light. The photodiode will then detect the light’s intensity.

The number of oxygen molecules quenching the light dictates the light’s intensity. Therefore, the transmitter can translate this intensity into the amount of DO in the medium.

Over time the LEDs in optical probes can also lose accuracy. Thus, some sensors come equipped with reference LEDs to reduce maintenance, just as the extra silver anode does for the amperometric sensors.

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