How Does Dissolved Oxygen Probe Work?

Working principle of dissolved oxygen probe

Dissolved oxygen probe works by measuring the amount of oxygen that diffuses across a permeable (or semi-permeable) membrane into a dissolved oxygen sensor (probe). Once oxygen is inside the sensor, a chemical reduction reaction occurs, producing an electrical signal. This signal is read by the DO probe and is displayed on a meter.

Dissolved oxygen (DO) comes from two main sources: the atmosphere (molecular oxygen) and photosynthesis, where it is a byproduct. Most aquatic organisms require DO to survive and it is important for chemical processes in aquatic environments.

Depending on the industry you work in, will depend on why you are testing DO and what Dissolved oxygen probes are best for you. For example, if you are testing DO when brewing beer, the reason you test DO will be different from regularly testing water samples as a lab technician.

Dissolved Oxygen Probe

While there are many ways to test DO, if you are testing in the lab or the field, the easiest way to test DO in water is to use an electroanalytical or electrochemical dissolved oxygen sensor/probe, also known as amperometric or Clark-type sensors.

How Does Dissolved Oxygen Probe Work?

Most people are familiar with the titrimetric method to measure DO in water. ​​Titrations use one liquid where the concentration is already determined (titrant), to identify the concentration of another (your sample), however, with advances in technology, precision equipment such as a DO probe offers precision and accuracy from measuring DO during basic laboratory work to industrial water quality monitoring.

A dissolved oxygen probe works off redox (oxidation-reduction) reactions, providing continuous and live measurements. As they have an applied voltage, most DO probes require a “warm-up time” before use which polarizes the electrodes before measuring the DO in water.

Once the DO probe is connected to the meter, the electrode sensor (DO probe) is dipped into the solution being tested. When the DO probe is connected to the meter, the voltage can be applied to the electrodes inside. The oxygen (O2) molecules reach the membrane, reaching the electrodes through the permeable/semi-permeable membrane via diffusion. A small electrical current flows between the electrodes (cathode and anode). The amount of current flowing between the electrodes is proportional to the O2 concentration of the solution. The amount of DO in the solution is measured by measuring the current flow using the calibrated meter.

Dissolved oxygen sensors do, however, have a disadvantage, while taking the DO measurement. It consumes an amount of O2 from the solution equal to the diffusion in the sensor. So, the amount of O2 near the DO probe is reduced, therefore, to obtain an accurate measurement if using a galvanic probe, the DO probe must be stirred in the solution.

Dissolved oxygen sensor
Galvanic DO Probes/Sensors

Galvanic DO probes produce a voltage as O2 diffuses across the probe’s membrane. The thin semi-permeable membrane allows O2 to pass through, blocking anything else from passing.

As O2 passes through the membrane and electrolytic solution to the electrodes, it dissolves inside the probe cap that contains the buffered electrolyte which allows O2 to react with the cathode (platinum electrode), thus gaining an electron. It is this electron that is given to the O2 molecule that creates a voltage between the anode (silver chloride electrode) and cathode inside the DO probe.

Once the DO probe has detected a current, the meter attached can then convert the reading into a DO concentration.

Galvanic Dissolved Oxygen Probe

Polarographic DO Sensors/Probes

Polarographic DO probes also contain a thin semi-permeable membrane, however, the voltage is applied between the anode and cathode in the DO probe. As O2 reaches the cathode, an electron is added to the O2 molecule, which creates a current. It is this voltage that drives the O2 reaction and determines the DO concentration.

How To Use A Lab Grade DO Probe?

The Lab Grade DO probe contains a PTFE membrane, a cathode, and an anode soaked in an electrolyte. During use, the O2 molecules diffuse through the DO probe membrane at a continuous rate. Once the O2 molecules cross the membrane, they are reduced at the cathode, where a small voltage is produced. When no O2 molecules are present, the DO meter will read the voltage as 0 mV (millivolts). As the O2 molecules increase, so does the reading from the DO probe.

Once your probe has recorded the voltage, it can be easily read by a multimeter or an analogue digital converter.

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