Abstract
To survive, we need oxygen in the air we breathe. Oxygen is also essential for most aquatic organisms, but there is much less oxygen available in water than in air. How much oxygen can dissolve in water? Does the temperature of the water matter? Learn how to measure dissolved oxygen and then see how oxygen concentration changes with water temperature.Summary
Andrew Olson, Ph.D., Science Buddies
Sources
- Maryland Department of Natural Resources. (2005). "Eyes on Dissolved Oxygen," Maryland Department of Natural Resources, Eyes on the Bay Lesson Plan Series
Objective
Dissolved oxygen is an important measure of water quality for aquatic life. In this project you will use a test kit to measure the level of dissolved oxygen in water samples. This project has two goals:
- Measure dissolved oxygen in water samples at different temperatures, and
- Determine the saturating oxygen concentration for water samples at different temperatures.
Introduction
Dissolved oxygen (the amount of oxygen dissolved and freely available in water) is one of many measures of water quality, but an important one for aquatic life. Like land animals, fish and shellfish require oxygen to survive. When oxygen levels fall below 5 milligrams per liter (mg/L), fish are stressed. At oxygen levels of 1-2 mg/L, fish die.
The amount of oxygen that can dissolve in water (i.e., the saturating concentration of oxygen) depends on water temperature. Colder water can hold more oxygen than warmer water. You'll see for yourself just how much more in this project.
Where does dissolved oxygen come from?
There are two main sources of dissolved oxygen: air and photosynthesis. Consider photosynthesis first. You probably know that photosynthesis is the fundamental biological process that uses light energy to produce sugar from carbon dioxide and water. Oxygen is a by-product of photosynthesis. Both algae (phytoplankton, seaweeds) and plants can be found in natural bodies of water. These organisms are net producers of oxygen in the daytime, but at night become net consumers of oxygen.
Now consider oxygen from the air. At the surface of the water, oxygen from the air equilibrates with oxygen dissolved in the water. This is a dynamic equilibrium: the oxygen molecules are in constant motion. At any given moment, some are leaving the water for the air, and some are leaving the air to dissolve in water. At equilibrium, there is a balance. On average, an equal number of oxygen molecules are leaving and entering the water. If the water temperature increases, the water can't hold as much oxygen as before—the water is oversaturated with oxygen. For a time, there will be more oxygen molecules leaving the water than entering it from the air. Then a new equilibrium will be reached, with less oxygen in the water than before.
Moving water has a rougher surface than still water. With more surface area in contact with air, moving water will equilibrate with air more quickly. (You'll make use of this in your experiment.) In natural situations, water can also become stratified into different layers (see the Science Buddies project Can Water Float on Water? for a hands on demonstration). For example, cold water is denser than warm water, and salt water is denser than fresh water. Can you think of ways that different layers of water might form in a lake or ocean? What do you think happens to the oxygen in a colder layer of water trapped under a warmer layer of water? (Remember that the warmer layer cannot hold as much dissolved oxygen as the colder layer. See the Make It Your Own tab for a Variation for a project idea on this topic.)
What causes dissolved oxygen levels to vary?
So far we've seen that dissolved oxygen can come from the air or from photosynthesis, and that when water warms up, there is a net loss of dissolved oxygen. Besides warming, how else can dissolved oxygen become depleted? The answer is another fundamental biological process: respiration. Respiration uses oxygen to break down molecules, in order to produce energy for cells. So the amount of dissolved oxygen will be determined by:
- how much oxygen the water can hold (temperature-dependent),
- how much surface area is available for diffusion from the air,
- how much oxygen is produced by photosynthesis, and
- how much oxygen is consumed by respiration.
Here is a real-world example of variations in dissolved oxygen levels from a continuous monitoring site in the Chesapeake Bay (Maryland DNR, 2006). All of the data were collected at the same location over the same time period. The first graph shows dissolved oxygen, the second graph shows temperature and the third graph shows chlorophyll concentration (a measure of how much algae is present in the water). Notice the daily fluctuations in oxygen level and water temperature. Notice also how the oxygen level and chlorophyll level both declined toward the end of the time period.
Example graph showing the concentration of dissolved oxygen in the waters of Chesapeake Bay over the course of a week. The oxygen levels fluctuate over the course of a day, and remain relatively stable (except for a slight overall decrease near the end of the graph).
Sometimes imbalances occur that lead to skyrocketing concentrations of algae. For a project that investigates water quality measures and algal blooms, see the Science Buddies project Harmful Algal Blooms. You can also check out the references in the Bibliography section.
How is dissolved oxygen measured?
Dissolved oxygen can be measured with an electronic metering device or with a chemical test. Dissolved oxygen meters cost hundreds of dollars, so this project will use the chemical testing method. Commercial test kits are based on the "modified Winkler method." You can research more details yourself (the references in the Bibliography will help), but here is a basic outline of how the test works:
- A water sample is collected and the sampling container is quickly sealed (sealing under water is ideal if possible). This prevents exposure of the sample to the atmosphere.
- A chemical is added to the water sample to react with all of the dissolved oxygen in the sample. An insoluble precipitate is formed.
- Additional chemicals are added to drive the first reaction to completion, and to prevent an unwanted reaction from occurring in the final step.
- A third addition causes the precipitate to change color.
- The oxygen is now "fixed" and can no longer react with the atmosphere.
- In the final step, a titration is performed. In this step, a chemical is added in liquid form, one drop at a time. The added compound reacts with the colored precipitate, causing it to lose color. The water sample is gently mixed after the addition of each drop. When the color change is complete (sample achieves the end color stated in the kit directions), it means that the added compound has reacted with all of the fixed oxygen in the sample. By counting the number of drops that were added, the amount of oxygen in the sample can be calculated.
The amount of oxygen can be reported using different units. You have already seen one above, mg/L meaning miligrams of dissolved oxygen in a liter of water. Another common unit is parts per million (ppm). Ppm is also a weight ratio, meaning 1 part by weight of oxygen per every million parts per weight by oxygen. The "parts" can be any weight unit. For example, 1 pound of oxygen per million pounds of water. Since a liter of water contains one million mg of water (1000 mg per gram X 1000 g per liter = 1,000,000 mg per liter) mg/L and ppm are interchangeable.
Terms and Concepts
To do this project, you should do research that enables you to understand the following terms and concepts:
- Dissolved oxygen
- Photosynthesis
- Saturation and oversaturation,
- Respiration
- Milligrams per liter (mg/L)
- Parts per million (ppm)
- Saturation
More advanced students will want to study the chemistry used in the test kits (modified Winkler method).
Questions
- What concentration of dissolved oxygen is required to support aquatic life?
- What are some of the processes that increase dissolved oxygen concentration in natural bodies of water?
- What are some of the processes that decrease dissolved oxygen concentration in natural bodies of water?
Bibliography
For more background information on water quality measures, including dissolved oxygen, see:
- Munson, B.H. et al. (2005). Water on the Web: Understanding: Water Quality: Parameters: Dissolved Oxygen. Retrieved September 25, 2012.
- Department of Ecology Staff, State of Washington. (2012). How to Measure Dissolved Oxygen. Retrieved September 25, 2012.
Archived data (including dissolved oxygen) from Maryland DNR continuous monitoring stations in coastal bays can be found at:
- Maryland Department of Natural Resources. (n.d.). Dissolved Oxygen. Retrieved October 3, 2013.
This website has background information on harmful algal blooms, which can deplete water of dissolved oxygen:
- Anderson, Don. (2012, July 31). Harmful Algae. Retrieved September 25, 2012.
These sites have background information on the Gulf of Mexico "Dead Zone," a massive area of hypoxic water that appears every summer near the mouth of the Mississippi River:
- Science Museum of Minnesota Staff. (n.d.) Dead Zone Home. Retrieved September 25, 2012.
- Roach, J. (2005, May 25). "Gulf of Mexico 'Dead Zone' Is Size of New Jersey". National Geographic News. Retrieved September 25, 2012.
Materials and Equipment
These items can be purchased from Carolina Biological Supply Company, a Science Buddies Approved Supplier:- Thermometer (range 0°C-50°C)
- Safety glasses/goggles
- Spray bottle; for rinsing test container in the field
- Aerator pump with plastic tubing and a 2.5 cm or larger fine-mist air stone that all fits together. Alternatively, these supplies may be purchased from a local aquarium store.
- Disposable gloves. Alternatively, these can be purchased at a local drug store or pharmacy. If you are allergic to latex, use vinyl or polyethylene gloves.
- Dissolved oxygen test kit; can be purchased from aquarium stores and online vendors like
Amazon.com.
- Tip: Look for a dissolved oxygen test kit that can measure a wide range (at least 0-12 ppm) of dissolved oxygen.
- Containers for water (3); 1 L volume or more
- Sealable container; at least 500 ml volume, for test waste in the field
- Lab notebook
Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools, Amazon.com, Carolina Biological, and Jameco Electronics. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.
Experimental Procedure
- Read and follow all of the instructions in your dissolved oxygen test kit, including all safety precautions.
- Avoid skin contact with dissolved oxygen test kit reagents.
In this science project, you will measure how dissolved oxygen changes in water samples at different temperatures. You will test both aerated and non-aerated water samples at each temperature.
- Do your background research and make sure that you are knowledgeable about the terms, concepts, and questions, above.
- Read the instructions that came with your dissolved oxygen test kit so that you know how to perform the test. Different test kits will have slightly different directions. All the kits will have a color read out though. Figure 2 is one example of a color read out for a dissolved oxygen test kit.
- Collect your water sample. You will need at least 300 mL. The sample can be from a natural body of water, such as an estuary, ocean, lake, pond, or stream. You can also use plain old tap water.
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Take a baseline dissolved oxygen measurement.
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When you collect your water sample, bring along your dissolved oxygen test kit, thermometer, spray bottle, sealable waste container, lab notebook, and a pen.
- For tap water you will not need the spray bottle or waste container as you will not be testing the samples away from a sink.
- Measure the temperature of the water at the collection site. Write this, and all other data, down in your lab notebook.
- Test the dissolved oxygen content of the water at the collection site. This is your baseline measurement of dissolved oxygen.
- When your measurement is complete, discard the test sample down a drain; do not throw it back in the body of water you sampled. Do the same with the rinse water when you clean the sampling container. If need be, bring the test waste back home in a sealable container and flush it down the drain at home.
- To be sure that your results are consistent, you should repeat the test at least three times, using a fresh sample each time. Use the spray bottle to rinse your test container. Discard rinse water down a drain or into your waste container for disposal at home.
- Be sure to record the temperature of the water.
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When you collect your water sample, bring along your dissolved oxygen test kit, thermometer, spray bottle, sealable waste container, lab notebook, and a pen.
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At home, divide your water sample equally into three separate containers:
- Container 1 will be cooled with ice,
- Container 2 will be allowed to equilibrate to room temperature, and
- Container 3 will be heated slightly.
- Add enough ice to container 1 to bring the water to about 4-8°C. When the water has cooled, record the temperature and measure the dissolved oxygen concentration. As before, you should run the test at least three times, to be sure that your results are consistent.
- Next, aerate the sample and re-test. The point of aeration is to saturate the water with oxygen (i.e., dissolve as much oxygen as the water can hold). Aerate the water with an aquarium aeration pump and airstone. Lots of small bubbles work best. Allow 5-10 minutes for equilibration. Check the temperature periodically and add more ice if needed to maintain the temperature.
- When the water has been aerated, repeat the dissolved oxygen test. Make sure to record the temperature. As before, you should run the test at least three times to be sure that your results are consistent.
- Run similar tests (aerated and non-aerated) for container 2, the water sample at room temperature (it may take a few hours to equilibrate, depending on how cold the sample was to start).
- Run similar tests (aerated and non-aerated) for container 3, the water sample that you heat. You can warm it on the stove, or in the microwave. Mix the sample gently and check the temperature frequently. Aim for a temperature from 35-40°C. You don't want to scald yourself when testing the dissolved oxygen concentration.
- Summarize your results in a table. For example:
| Sample | Aerated? | Temp #1 | DO #1 | Temp #2 | DO #2 | Temp #3 | DO #3 |
| Baseline | N | ||||||
| Chilled | N | ||||||
| Chilled | Y | ||||||
| Room temp | N | ||||||
| Room temp | Y | ||||||
| Warmed | N | ||||||
| Warmed | Y |
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Make a graph of your results. You can plot dissolved oxygen vs. temperature. Use separate symbols for:
- Your baseline sample.
- Your non-aerated samples.
- Your aerated samples.
- From your graph, do you think your original baseline sample was saturated with oxygen? Why or why not?
Ask an Expert
Variations
- Does seawater hold as much dissolved oxygen as freshwater at the same temperature? Compare aerated fresh- and salt-water samples at different temperatures. If the ocean is too far away, make your own saltwater by adding between 30 and 35 g of table salt for each liter of water. Use a double bath for cooling the saltwater sample down without diluting it (saltwater sample in the inner container, surrounded by ice water in the outer container).
- How could you modify the experiment to test whether the initial baseline sample is saturated with oxygen?
- Measure dissolved oxygen in water with and without aquatic plants. If you have access to a planted aquarium, it would be interesting to monitor oxygen levels both in daytime and at night. As in the experiment described above, this experiment could be done both with and without aeration. If fish are present, monitor the nighttime oxygen level frequently. Also, monitor the fish for signs of oxygen stress (e.g., increased gill beat rate, gulping at the surface).
- If you live near an estuary or other natural body of water, you could monitor dissolved oxygen from one or more sites over time. For example, you could sample multiple times during a 24-hour period to track the daily fluctuation of oxygen. Alternatively, you could sample over a longer time period, and look for changes correlated with weather systems. What effect would you predict for cloudy weather?
- Compare dissolved oxygen in a still portion of a stream vs. a rapidly flowing portion. Or compare oxygen levels in water sampled at different depths.
- For a more advanced project that uses archived water quality data from monitoring stations in the Chesapeake Bay, see the Science Buddies project Harmful Algal Blooms in the Chesapeake Bay.
Careers
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