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Dissolved Oxygen - GLOBE

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  • pg 1
									    Hydrology
Dissolved Oxygen




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    Why Measure Dissolved
         Oxygen?




1
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 Scientists want data for…

• Determine the mixing of air and
  water at the water’s surface
• Determine what animals can live in
  the water




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          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.


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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)



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        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

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    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.



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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.
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             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 + I2     blue (to improve endpoint determination)




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              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.



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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.



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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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