Dissolved Oxygen

What is dissolved oxygen?

Oxygen gas dissolves freely in fresh water. Thus, oxygen from the atmosphere as well as that produced as a by-product of photosynthesis may increase the dissolved oxygen concentration in water. Oxygen is removed from the water through the processes of respiration (by both plants and animals) as well as other chemical reactions, including the decomposition of organic wastes entering the water.

The distribution of dissolved oxygen (DO) within an aquatic environment may vary horizontally or vertically and with time. Its distribution is dependent upon atmospheric contact, biological activity, wave and current actions, thermal phenomena, waste inputs and other characteristics of a lake or stream. High levels of oxygen are likely in surface water on windy days. Dissolved oxygen levels are also temperature and pressure dependent. Cold water holds more oxygen than warm water.

There are biological processes in water that consume oxygen such as respiration by organisms and decomposition of organic matter by microorganisms. The oxygen consumed by these processes is called the Biological Oxygen Demand or BOD. When demand for oxygen is high and oxygen production from photosynthesis is not occurring such as before sunrise, dissolved oxygen readings can be low. Photosynthesis contributes to an increase in dissolved oxygen levels during the day. Deep areas of a lake would be expected to yield low dissolved oxygen readings.

How is dissolved oxygen measured?

Since an adequate supply of oxygen is necessary to support life in a body of water, a determination of the amount of oxygen provides a means of assessing the quality of the water with respect to sustaining life. A standard chemical method to determine the amount of oxygen dissolved in a water sample is a type of titration, the Azide Modification of the Winkler Method. Precisely measured amounts of chemicals (reagents) are added to a water sample until a color change is achieved. A color change (or electrical measurement for other types of titration) marks the endpoint of the test. Another way to measure dissolved oxygen is to use a dissolved oxygen (DO) meter and probe.

Since an adequate supply of oxygen is necessary to support life in a body of water, a determination of the amount of oxygen provides a means of assessing the quality of the water with respect to sustaining life. A standard chemical method to determine the amount of oxygen dissolved in a water sample is a type of titration, the Azide Modification of the Winkler Method. Precisely measured amounts of chemicals (reagents) are added to a water sample until a color change is achieved. A color change (or electrical measurement for other types of titration) marks the endpoint of the test. Another way to measure dissolved oxygen is to use a dissolved oxygen (DO) meter and probe. Units for measuring dissolved oxygen are parts per million (ppm) or milligrams per liter (mgL).

Because the solubility of oxygen in water is dependent upon temperature, pressure, and ionic concentrations, it is also important to calculate percentage saturation. The accompanying nomogram will permit you to quickly approximate oxygen saturation values (Figure 11). The saturation point indicates the level at which water will not generally hold any more oxygen at a given temperature. Supersaturation occurs when the water holds more oxygen molecules than usual for a given temperature. Sunny days with lots of photosynthesis or turbulent water conditions can lead to supersaturation. A water sample is "saturated" at 100% and "supersaturated" above 100%.

What is the significance of dissolved oxygen?

Dissolved oxygen levels provide information about the biological, biochemical, and inorganic chemical reactions occurring in aquatic environments. Most aquatic organisms are highly dependent upon dissolved oxygen and will experience stress, or perhaps even be eliminated from a system, when dissolved oxygen levels fall below about 3.0 ppm (parts per million). Trout species normally require an oxygen concentration greater than 10 ppm (10 mgL) whereas carp can live in water containing as little as 1-2 ppm (1-2 mgL) oxygen.

Poor water quality is also indicated by low percent saturation readings. Levels below 60% may happen with rapid biological processes such as decomposition or high temperatures. Supersaturation can be a problem for organisms in that blood oxygen levels can increase resulting in gas bubbles in the blood.

Adapted from Testing the Waters, S. Behar, River Water Network, 1996

Instructions for Dissolved Oxygen determination:

Note: These instructions are primarily for reference. The aquatic science instructors will lead participants through the dissolved oxygen measurements on the vessel. Two methods may be used: the Hach method with a digital titrator or a LaMotte kit. See Appendix C for pictorial directions.

Chemicals are used in the determination of the amount of dissolved oxygen in a water sample. Normal safety rules should be followed when performing this analysis. This includes wearing the safety glasses provided at the dissolved oxygen (DO) station in the main cabin.

1. Check to see that the temperature of the water samples in the Van Dorn bottles on the aft (rear) deck have been recorded. Obtain a 300 mL, glass-stoppered dissolved oxygen (D.O.) bottle from the main cabin and follow the directions to fill it with water from the appropriate Van Dorn sampling bottle. Be sure to match the symbol on the D.O. bottle with the same symbol on the Van Dorn bottle (the symbol "T" for top and "B" for bottom). A Van Dorn bottle has rubber tubing on the spigot that will reach the bottom of the D.O. bottle. Always fill the D.O. bottle from the bottom, slowly, until the water overflows from the top of the D.O. bottle. Make sure there are no air bubbles trapped inside the D.O. bottles.
2. Take the sample to the dissolved oxygen station in the main cabin. PUT ON YOUR SAFETY GOGGLES.
3. Clip off the end of Powder Pillow #1 (manganous sulfate) and add the contents to the water sample in the D.O. bottle. Follow the same procedure to add the contents of Powder Pillow #2 (alkaline iodide azide). Carefully stopper the bottle with the glass stopper and invert the stoppered bottle slowly, count to 10, and return the bottle to the upright position. Invert the bottle in the same manner 14 more times.
4. A cloudy, brownish-orange to white flocculent mass (floc) will form. This may take 15 to 20 minutes. Allow the D.O. bottle to stand until the cloudy floc has settled leaving the top quarter of the solution clear. Invert the bottle several more times to mix the solution then let stand for one minute.
5. Remove the stopper and carefully add the contents of the big (#3) Powder Pillow (sulfamic acid). Replace the stopper and invert the bottle several times and the cloudy floc will change to a clear orange to yellow (amber) solution.
6. Slide (do not drop) a magnetic stir bar down the side of a clean 125 mL Erlenmeyer flask. Measure 25 mL of solution from the D.O. bottle into a graduated cylinder and pour this measured amount into the Erlenmeyer flask with the magnetic stir bar.
7. Add one dropper full (approximately 1 mL) of Starch Indicator Solution to the flask. The contents of the flask should turn blue-black.
8. Place the flask with stir bar and solution on the magnetic stir plate. Turn the stir plate on slowly to insure that the stir bar will rotate (not jump around) to mix the solution.
9. While the solution is mixing, prepare a digital titrator. Hold the digital titrator in your left hand while turning the delivery knob clockwise until a few drops of solution (titrant) are expelled at the "J" shaped end of the delivery tube. Using the larger knob at the opposite end of the titrator, reset the digital counter to zero and wipe the tip with paper toweling.
10. While the solution continues to mix, place the "J" shaped end of the digital titrator into the blue-black solution in the flask and begin to turn the delivery knob clockwise to release the titrant. Continue turning the delivery knob clockwise until the solution in the flask changes from dark blue to clear - STOP! The clear solution is the endpoint of the titration.
11. Remove the tip of the titrator from the flask and read the number on the digital counter. Divide that number by 100 to determine the concentration of dissolved oxygen in ppm or mgL. If you put a decimal point two digits in from the right end of the number shown on the digital counter it will be the same as dividing by 100. Round off dissolved oxygen readings to one decimal place (>/= .05 would be .1 and <.05 would be .0).
12. Knowing the ppm of dissolved oxygen and the temperature of the water, determine the % dissolved oxygen saturation using the dissolved oxygen chart (nomogram) and ruler. This is done by aligning the oxygen ppm and water temperature with the ruler then reading where the ruler crosses the % saturation line. Record both the ppm (mgL) and % saturation for the top and bottom samples in the appropriate places on the data board. GLOBE trips will require taking dissolved oxygen measurements in triplicate and averaging the readings.

NOTES:

1. Sampling and sample handling are important considerations in obtaining meaningful results. The dissolved oxygen content of the water being tested can be expected to change with depth, turbulence, temperature, sludge deposits, light, microbial action, mixing, travel time, and other factors. A single dissolved oxygen test rarely reflects the accurate overall condition of a body of water. Several samples taken at different times, locations, and depths are recommended for most reliable results. Samples must be fixed immediately upon collection. This means that if conditions on Lake Michigan make it uncomfortable to be in the main cabin, it is essential to complete STEP 3 as quickly as possible. It is then possible to delay the titration until the vessel has returned to calmer waters. If you are collecting stream samples the titration can be delayed at most 6 hours and the samples should be kept in the dark and on ice until the titration is completed.
2. Allowing the floc to settle twice insures reaction of the chemicals with all of the dissolved oxygen present. The floc will settle very slowly in salt water and usually will require an additional five minutes before proceeding with Step 6. Results will not be affected if the floc does not settle.
3. The nitrite ion frequently interferes with the dissolved oxygen determination. Nitrites occur primarily in effluents from sewage treatment plants, which use biological processes, in river waters, and in incubated biochemical oxygen demand (BOD) samples. Nitrite interference may be easily overcome by the use of sodium azide. It is most convenient to incorporate the azide in an alkali-KI reagent. When sulfuric acid is added, the nitrite is destroyed. In this way, nitrite interference is eliminated, and this modification retains the simplicity of the original Winkler method.

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