A natural impurity in water. The test will measure the amount of free oxygen "gas" dissolved in your water sample in ppm (mg/l).

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Jasmin Bruce
5 years ago
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1 What is? A natural impurity in water. The test will measure the amount of free oxygen "gas" dissolved in your water sample in ppm (mg/l). Aquatic animals such as fish and zooplankton, can not use the oxygen in the water molecules to breathe. They rely on "free" molecules of oxygen gas dissolved in the water, without sufficient levels of which they would suffocate. 1

2 Oxygen accounts for one of every five molecules in the air; on the other hand, in water, roughly five of every million molecules are dissolved oxygen. The amount of dissolved oxygen in water is affected by several factors, such as temperature, altitude, total dissolved solids, respiration, and photosynthesis. Significant Values Dissolved oxygen levels of at least 5-6 ppm (mg/l) are usually required for growth. Dissolved oxygen levels of below 3 ppm are stressful to most aquatic organisms. 2

3 Instrumentation Test Kit that meets the specifications found in the Toolkit of the GLOBE Program Teacher's Guide (please note that this kit contains hazardous chemicals) Distilled water 250 ml polyethylene bottle with lid Thermometer Hydrology Investigation Data Work Sheet Latex gloves/safety goggles 3

4 Sample Preservation and Sample Testing test kits involve two overall parts: sample preservation and sample testing. The preservation part involves addition of a chemical that precipitates in the presence of dissolved oxygen. This is followed by addition of a chemical that causes the solids to dissolve and produce a colored solution. Most DO test kits are based on the Winkler titration method. 4

5 Instrument Placement Take water sample near the same location as the water temperature measurement. Collect water in the sampling bottle. Conduct the dissolved oxygen test immediately after you have taken the temperature of the water sample. Oxygen should be measured within 30 minutes of collecting the sample. 5

6 Calibration Rinse the 250 ml bottle twice with distilled water. Measure 100 ml of distilled water with a graduated cylinder and pour this water into the 250 ml bottle. Put the lid on tightly and shake it vigorously for 5 minutes. Uncap the bottle and take the temperature of the water. Be sure the tip of the thermometer does not touch the bottom or sides of the bottle. Wait 30 seconds before reading the temperature. Record the temperature on the Hydrology Investigation Data Work Sheet. 6

7 Calibration (continued) To determine the dissolved oxygen value use a dissolved oxygen test kit that meets the specifications in the Toolkit of the GLOBE Program Teacher's Guide. Follow the instructions carefully. On the Hydrology Investigation Data Work Sheet, record the value as mg/l DO for the shaken distilled water. 7

8 Calibration (continued) The DO of the shaken distilled water must be within 1.0 mg/l of the expected value for a distilled water sample saturated with oxygen. To find the expected DO value for a saturated distilled water sample: Step 1: Using Table HYD-P-1 find the solubility of oxygen (mg/l) that corresponds to the temperature of your sample. Example: A temperature of 22 C has a corresponding DO solubility of 8.7 mg/l. Step 2: Using Table HYD-P-2 find the value that corresponds to your elevation. Example: An altitude of 1,544 meters has a corresponding saturation calibration value of

9 Calibration (continued) 9 Step 3: Multiply the solubility of oxygen found in Step 1 by the calibration value found in Step 2. Example: At an altitude of 1,544 meters and a temperature of 22 C, 8.7 mg/l X 0.83 = 7.25 mg/l. Step 4: Compare this value to the DO value of the shaken distilled water. If the the shaken distilled water DO value is not within 0.4 mg/l (LaMotte kit) or 1.0 mg/l (Hach kit) of the "expected" value, try the measurement again on the shaken distilled water. If it is within the above values, record the DO value obtained for your shaken distilled water on the Hydrology Investigation Data Work Sheet. Calibrate every six months to check technique.

10 Taking a sample Rinse sampling bottle with distilled water before taking the sample. Submerge the bottle in sample water and remove the cap. Allow the bottle to fill. Tap the bottle to release air bubbles. While the bottle is submerged, replace the cap. Remove the capped bottle from the water. Check to ensure that no bubbles are present in the bottle. If bubbles are found, repeat the sampling procedure. 10

11 Element Reporting Make measurements weekly. Complete the Hydrology Investigation Data Work Sheet entry for. Take an average of the dissolved oxygen values measured by the student groups. If all values are within 1 mg/l of the average, submit the average DO value to the GLOBE Student Data Server or repeat the measurement. 11

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