Select Page
Dissolved oxygen measurement – easier than ever

Dissolved oxygen measurement – easier than ever

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:

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 is 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:

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 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:

What affects the dissolved oxygen concentration? 

Temperature

The temperature has a large influence on DO concentration. The higher the temperature, the less oxygen is dissolved in the liquid phase. Why? I will explain it to you a bit more visually:

When the temperature of a solution increases, the ions and molecules therein move and vibrate due to the increased energy. This leads to more and more collisions between particles and thus, some of the bonds that hold them together break. As more particles vibrate, more collisions occur, and even more bonds are broken. That also means that the bonds which hold oxygen molecules in the liquid will break, and oxygen will be released from the solution. This results in a decrease in the DO content. The opposite happens if the temperature decreases: particle motion decreases, and therefore the DO concentration increases.

Pressure

For our purpose, here «pressure» refers to the atmospheric pressure. Perhaps you’ve been on the top of a mountain, or inside of an airplane flying at altitude, and had a drink from your water bottle up there. When you were back on the ground, or at the base of your hike, and checked the bottle again, maybe you noticed that it was compressed slightly, or had a suction noise as you opened it again. This is due to the difference in atmospheric pressure, which is inversely proportional to altitude.

As atmospheric pressure decreases, the partial pressure of oxygen also decreases. Therefore at higher altitudes, less oxygen is dissolved in the liquid since the pressure does not hold it there. Oxygen diffuses out of the liquid, the higher we get. When we go to lower altitudes, the DO concentration increases as the atmospheric pressure increases.

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.

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 how easy it is to use!

The O2-Lumitrode, the optical sensor for DO measurement from Metrohm, is the fastest of its kind on the market. It measures the DO content in liquids in less than 30 seconds! The working principle is based on luminescence quenching.

Let me explain how this works: the sensor cap contains a membrane with an embedded luminophore that is excited by red light. When there is no oxygen present, the luminophore returns to its ground state via emission of luminescence.

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.

Our 913 pH/DO Meter or 914 pH/DO/Conductometer can be equipped with the O2-Lumitrode. Both of these are combined instruments, meaning you can additionally measure pH and/or conductivity alongside dissolved oxygen.

As stated earlier, temperature, pressure, and salinity impact the dissolved oxygen content in liquids. Therefore, the O2-Lumitrode is equipped with a temperature sensor and a pressure sensor so automatic temperature and pressure compensation can be applied for the most reliable results. If you are measuring DO in a saline solution or in seawater, you can measure the conductivity in parallel to DO and switch the automatic salinity compensation on.

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.

Want to learn more?

Download our free White Paper:

Determining dissolved oxygen in water: Titration or direct measurement?

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.