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Hydrology Dissolved Oxygen

Learn why measuring dissolved oxygen is crucial in hydrology, affecting aquatic life and ecosystem health. Explore methods and factors influencing levels, such as temperature and elevation. Discover sampling techniques and chemical reactions for accurate testing. Ensure quality control for precise results in environmental monitoring.

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Hydrology Dissolved Oxygen

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  1. Hydrology Dissolved Oxygen

  2. Why Measure Dissolved Oxygen? 1

  3. Scientists want data for… • Determine the mixing of air and water at the water’s surface • Determine what animals can live in the water

  4. Dissolved Oxygen • 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, mg/L (ppm by mass) • Test will measure the amount of free oxygen “gas” dissolved in your water sample in mg/L (ppm) • 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.

  5. The amount of oxygen that water can hold decreases with: • Temperature increases • Elevation increases (due to decreasing atmospheric pressure) • Increasing amounts of dissolved substances (e.g., salts)

  6. Biological Influences and Dissolved Oxygen • As photosynthesis increases, oxygen levels increase: CO2 + H2O Biomass + O2 • As respiration increases due to decay or organic materials, oxygen levels decrease: Biomass + O2 CO2 + H2O

  7. Taking a Sample and testing • Rinse sampling bottle 3 times with sample water • Submerge bottle in water and allow to fill. • Tap bottle to release air bubbles • While bottle is submerged, replace cap • If there are air bubbles in the bottle, empty and repeat • Preserve sample immediately. Test within 2 hours. • Repeat 3 times. Take the average to see if all values are within the precision of the kit. Discard outliers.

  8. Sample Preservation and Sample Testing Dissolved Oxygen test kits involve two overall parts: sample preservation and sample testing. • Preservation: • 1st - addition of a chemical that precipitates in the presence of dissolved oxygen • 2nd - addition of a chemical that causes the solids to dissolve and produce a colored solution. This should be done in the field. • Sample Testing: • Titration of preserved sample. This can be done in the lab. • Most DO test kits are based on the Winkler titration method.

  9. Chemical Reactions To Preserve DO: Done in the field O2 + 2 Mn2+ + 2H2O 2Mn(IV)O2 + 4H+ (pH >10) Allow precipitate to settle (reaction goes to completion) 2Mn(IV)O2 + 4H+ + 2I- Mn2+ + I2 (yellow) + 2H2O (low pH) DO is preserved -------------------------------------------------------------------------- To Test Sample: This step can be done in the lab. Na2S2O3 + 4I2 + 5H2O 8I- + 2SO42- + 10H+ + Na+(the titration) Starch + I2blue (to improve endpoint determination)

  10. Quality Control Check technique and quality of kit chemicals every 6 months. • 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 vigorously for 5 minutes. The water will be saturated with dissolved oxygen. • 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. Record the temperature on the Hydrology Investigation Quality Control Procedure Data Sheet.

  11. Quality Control (continued) • To determine the dissolved oxygen value use a dissolved oxygen test kit that meets the specifications in the Toolkit of the GLOBE Teacher's Guide. Follow the instructions carefully. • On the Hydrology Investigation Quality Control Procedure Data Sheet, record the value as mg/L DO for the saturated distilled water. • 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.

  12. Quality Control (continued) 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 0.83. • 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.

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