Dissolved oxygen measurement – easier than ever

Apr 14, 2020

Article

Do you know why your drinking water becomes flat after you leave it untouched for a few hours? Or why your orange juice changes its color and darkens a bit when the bottle is left open for a longer time?

One of the key driving factors behind these changes is the amount of oxygen in your beverage.

I would like to share some information with you about the effects (both positive and negative) oxygen has when dissolved in liquids, which parameters affect the dissolved oxygen (DO) content, as well as how to accurately assess the DO concentration.

Why is DO concentration important?

Next to pH and conductivity, dissolved oxygen is one of the most important water quality indicators. Oxygen dissolves in surface water according to its partial pressure (Henry’s law), but also due to aeration processes (e.g., wind, rapids). Additionally, oxygen is introduced into water as byproduct of photosynthesis by plants and phytoplankton. Dissolved oxygen is essential for the survival of fish and any other aquatic organism that breathes oxygen.

The DO content may be reduced when too many bacteria or algae contaminate the water. Bacteria feed on dead algae and other organic material, consuming oxygen and producing carbon dioxide. If all DO is consumed by bacteria, it is called eutrophication. When the DO content in water drops below 5 mg/L, aquatic life is put under stress, and if the concentration is even lower, a large amount of aquatic life can die. Dissolved oxygen can be directly assessed, in-situ in surface water, by the direct measurement technique.

Learn more about dissolved oxygen measurement in surface water by downloading our free application note:

Application Note: DO in surface water based on ISO 17289

Getting back to the example of your drinking water or orange juice:

Water only tastes good to us when there is a certain amount of oxygen dissolved into it. When your glass or water bottle is standing around, DO is released as it equilibrates with the atmosphere and additionally it will warm up to the ambient temperature, releasing even more oxygen. This is why the taste of your water turns flat over time.

If you would like an overview of how dissolved oxygen in your water supply can be determined, download our free application note:

Application Note: DO in tap water according to ISO 17289

Orange juice exhibits the contrary situation. Orange juice (and other fruit and vegetable juices) are kept almost DO free. The reasoning is because oxygen, as an oxidizing agent, has a negative influence on the overall quality, taste, nutritional value, and color of a beverage. The longer you keep your orange juice open to the atmosphere, the more oxygen will dissolve into your juice, until a certain point. Furthermore, this DO will start to react with other ingredients of your juice. For example, DO will oxidize any present Vitamin C (ascorbic acid, an antioxidant) to dehydroascorbic acid. To prevent quick browning, as well as the flavor and quality of your juice, keep it in a closed bottle.

Do you want to know more about the determination of dissolved oxygen in fruit juices? Download our free application note:

Application Note: DO measurement in fruit juices

What affects the dissolved oxygen concentration?

Pressure

Salinity

The salinity also plays a part in the amount of dissolved oxygen which is available in a liquid.

Again consider the ions and molecules present in the solution. When we have a dissolved salt present in the water, these charged ions are very much attracted to the water molecules. Dissolved oxygen has no charge, and is therefore not attracted to anything. The higher the salinity content, the more ions are present. This increased density of particles coerces oxygen to leave the solution as its interaction with water is not so strong.

How can we assess the DO concentration?

There are two possibilities to determine the dissolved oxygen content in liquids, either by direct measurement or by titration. We have summarized the pros and cons for each of the methods in a free white paper which you can download below.

Determining dissolved oxygen in water – Titration or direct measurement?

However, I will only cover direct measurement using an optical sensor here. Why? Because you can measure the DO content online or in-situ without tedious sampling and sample preparation and your equipment is almost maintenance-free – you will be surprised at how easy it is to use!

If oxygen is present and these molecules collide with the excited luminophore, the luminophore returns to its ground state emission-free because the energy is transferred to the oxygen molecule. By evaluating the lifetime of the excited state of the luminophore (by using the phase shift), it is possible to determine the DO content.

The O₂-Lumitrode does not need much maintenance—a regular one-point calibration with 100% air saturation is enough. From time to time, we recommend performing a two-point calibration with 100% and 0% air saturation.

The O₂-cap must be replaced from time to time, as the luminophore becomes less reactive. This effect is called photo bleaching. However, the sensor will tell you when this is necessary due to its active performance monitoring. Never worry again about inaccurate DO measurements due to poor quality instrumentation.

To summarize, depending on the application and matrix, a wide range of dissolved oxygen can be found. The determination of the DO content fast and accurately is extremely important. Using an optical sensor with a mobile device makes it very easy to assess the DO content in-situ. For the most reliable data, additionally measure the temperature and pressure (and eventually the salinity) in parallel to minimize the effect of these physical parameters on your results.