Theory and Measurement
Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in a system. Measurements are usually taken in water using a DO probe and meter. Measurements taken follow Henry’s Law, which states that the concentration of gas in a solution is directly proportional to the partial pressure of that gas above the solution. Henry’s Law constant is a factor of proportionality, and so is specific to the gas in the solvent being measured.
The partial pressure of oxygen is in fact a measurement of the thermodynamic activity of its molecules. The rate at which oxygen dissolves, diffuses, and reacts is not determined by its concentration, but by its partial pressure. The Earth’s atmosphere is composed of 20.9% oxygen, and at sea level the atmosphere is 100% saturated with oxygen.
Percent saturation is the amount of DO present per amount of DO possible at a given temperature and pressure. Percent saturation is a common unit for DO measurement since it is based upon the partial pressure of a gas; thus it is correct for determination in any solvent.
Concentration measurements of DO can also use the units of parts per million (ppm) or milligrams per liter (mg/L). In meters that report DO concentration in ppm or mg/L, the solvent is always assumed to be water. In other solvents such as oils or acids, the Henry’s Law constant would be different. In those cases, percent saturation should be used as it is incorrect to use ppm or mg/L.
What affects dissolved oxygen?
Temperature
Temperature is one of the biggest factors that directly affects dissolved oxygen. Temperature itself is a measurement of the energy of motion within a system. As temperature increases, particle motion increases. When temperature climbs, the particles, molecules, atoms, etc. get more and more energy. This energy makes them bounce around more rapidly. As these particles bounce around more, they collide with each other and can break the bonds that hold them together. DO particles can then have the bonds that hold them in the liquid break, and they can bounce out of the solution. Therefore, the higher the temperature, the lower the DO concentration. Inversely, as temperature decreases, particle motion decreases, and therefore DO concentrations go up.
Pressure
Pressure when talking about dissolved oxygen, refers to atmospheric pressure. Have you ever been at sea level and then traveled to a place of higher elevation such as Denver, CO? You may have noticed that you feel a bit lighter, but also that the air feels “thinner”. There is less atmospheric pressure pressing down at that altitude. It probably took a day or two for you to adjust to the oxygen and pressure differences. When atmospheric pressure decreases, the partial pressure of oxygen decreases too. As altitude increases, the concentration of DO decreases since there is not as much pressure keeping the oxygen diffused into the liquid. As atmospheric pressure increases (i.e. going back towards sea level), the partial pressure of oxygen increases as well. So, as altitude decreases, the concentration of DO increases.
Salinity
Salinity can also affect the amount of DO in a solution. This goes back to chemistry; how certain molecules can carry different types of charges. The charge that a salt molecule carries is very attracted to the water molecules, and is more likely to be dissolved into a solution. Oxygen isn’t as attracted to the water molecules in a solution if salt is present. This is due to the fact that salt will bump oxygen out of a solution since there just isn’t enough room. As salinity increases, DO will decrease.
Humidity
Humidity or water vapor can affect DO concentration, and the calibration of certain DO measurement technologies. When you have an increased humidity level, you have an increased partial pressure of oxygen, and therefore you have an increased level of dissolved oxygen.
Applications
Water quality measurements are vital to environmental monitoring. In quiescent lakes and rivers, the decay of organic matter can cause bacteria levels to increase. The aerobic bacteria consume oxygen, triggering a deficiency that can cause a water body "to die," killing aquatic plants and animals.
- DO plays a role in the quality of water for all life, not just aquatic.
- High DO levels can cause corrosion of the pipes that transport the water
- Higher DO levels improve the taste of drinking water
Aquaculture is the breeding, rearing, and harvesting of plants and animals in all types of water environments. Dissolved oxygen is needed by fish, zooplankton, and plants to survive and reproduce. DO measurements are used to monitor and control the environment required for success.
- DO monitoring is important in fish farming.
- Sufficient DO levels are required for respiration and continued growth.
- Can change in a matter of minutes greatly effecting fish populations.
- Need accurate fast way to measure levels in order to activate aeration devices if needed.
Wastewater treatment plants rely on bacteria to break down the organic compounds found in water. If the amount of dissolved oxygen in the wastewater is too low, these bacteria will die and septic conditions will occur. The amount of DO must be consistently monitored to ensure proper waste treatment.
- Microorganisms are introduced to the sludge during primary treatment
- These organisms turn the organic waste into inorganic byproducts
- These byproducts are more stable and do not harm receiving waterways
- Most plants want a 4-5 mg/L DO level in effluent waters.
Wine and beer are both affected by oxygen at various stages during production and storage. DO is an important parameter to monitor for those who wish to produce consistent, high quality products.
WINE
- Oxygen is needed to ensure active yeast during fermentation
- The way a bottle is sealed affects the oxygen content
- Oxygen levels affect the quality and shelf life of the wine
- It should be noted that post fermentation, winemakers are usually looking at DO in the ppb range
- At this time, HANNA does not make a sensor that is suitable for this low range
BEER
- Any oxidative flavor changes generally happen up to 3 months after packaging
- Low oxygen levels mean the batch of beer will have a longer shelf stability
- Problems can be caused by the pump, centrifuge or filter
- Higher oxygen levels can lead to a fruitier flavor in the beer as there is a higher ester production
Oxygen is required by yeast for fermentation. Insufficient oxygen can lead to stuck fermentations. Air can be introduced to get that moving but a balance is always struck between how much air to introduce to help the wine vs the consequential DO that comes with it. DO is introduced through the movement of wine and beer. SO2 takes care of some of this but it’s the DO hanging around at bottling that is of concern since there is still opportunity of oxidation. Winemakers are pretty good at mitigating all of this… (i.e. moving wine slowly, measuring DO, etc…)
Hanna Instruments offers a variety of methods to measure dissolved oxygen.Products include portable and benchtop meters that use either a Clark-Type Polarographic, Galvanic, or Optical probes.
Dissolved Oxygen can also be measured photometrically with reagents. Photometric analysis is based on the Beer-Lambert principle of absorbance. Photometric analysis products include portable and benchtop photometers, and spectrophotometers. Photometric methods include reagent chemistries based on an adaptation of Standard Methods for Examination of Water and Wastewater (23rd edition) Azide modified Winkler method in which there is a reaction that causes a yellow tint in sample.
Chemical Test Kits (CKT) are also available and are simple titrations using a modified Winkler method.
