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Project Watershed Central New York

Dedicated to facilitating water resource education in Central New York

1. Dissolved Oxygen

Dissolved oxygen (DO) is the most important chemical parameter to test when monitoring a stream. DO is essential for the survival of aquatic organisms. The level of DO will determine what types of organisms can inhabit an aquatic system. High concentrations of DO are usually indicative of healthy aquatic ecosystems that can support a diversity of aquatic organisms (Mitchell and Stapp, 1997). Reductions in the level of DO can have a dramatic effect on this diversity. If DO levels drop significantly, undesirable organisms such as "nuisance" algae and various anaerobic organisms can dominate a stream system. Excessive inputs of nutrients to a stream system can lead to DO depletions.


DO levels are determined utilizing traditional and mechanical water testing instruments. Project Watershed CNY/SOS utilizes a DREL Portable Water Quality Laboratory, designed by Hach Company, that includes a Digital Titrator for DO testing. The following procedures were adapted from the Hach Digital Titrator Manual.

  1. Student or adult volunteers collect a water sample in a 60-ml BOD bottle by submersing the bottle for several seconds to allow all air bubbles to escape. Replace the stopper underneath the surface of the water to avoid air bubbles. The water sample should be taken in the middle section of the stream within the main current. The temperature of the water (°C) is also recorded on the form under the DO listing.
  2. Add one Dissolved Oxygen 1 Reagent Powder Pillow and one Dissolved Oxygen 2 Reagent Powder Pillow to the sample.
  3. Insert the stopper and invert the BOD bottle several times to mix the solution. An orange-brown flocculent precipitate will form in the presence of oxygen; a white precipitate will form if no oxygen is present in the water. Avoid the development of air bubbles in the bottle.
  4. Wait until the flocculent had settled and the top half of the solution is clear. Again invert the bottle several times to mix the solution and wait for the flocculent to settle.
  5. Remove the stopper and add one Dissolved Oxygen 3 Powder Pillow. Replace the stopper and invert the bottle several times to mix the solution. The flocculent will dissolve and the solution will take on a yellow color.
  6. Accurately measure 20 ml of the sample solution and transfer it to a titration flask (Erlenmeyer flask). Place the flask on white paper for best viewing.
  7. Attach a clean delivery tube to a 0.2000 N Sodium Thiosulfate Titration Cartridge. Connect the cartridge to the Digital Titrator.
  8. Flush the delivery tube by turning the delivery knob to release a few drops of titrant. Reset the counter to zero (0000) and wipe the tip. remains. Titrate more sodium thiosulfate until the blue color disappears.
  9. Titrate the sample solution with 0.2000 N Sodium Thiosulfate until the solution changes from yellow to colorless. The sample solution should be swirled continuously during the titration. The starch test is also used by creating a starch solution of  about 100 mL of cold water and a pinch of starch. After boiling the mixture, the starch will dissolve. As you reach the "clear" end point of the titration, add about 5 drops of this starch solution to the DO test solution in the flask. Swirl the contents of the flask; a blue color indicates that iodine in the reaction This is the end point of the titration.
  10. Record the number of digits from the counter window. In order to determine the concentration of dissolved oxygen in mg/L, multiply this number by 0.1. Using the current adjustment determined by standard testing, add +0.6 to results determined by DREL.
  11. Dispose of all sample solutions in the waste carboy.
  12. Refer to the level of oxygen saturation chart in the Field Manual for Water Quality Monitoring (Mitchell and Stapp) or Appendix 3 to determine the percent saturation of dissolved oxygen. Determine the Q-value for this result (percent saturation) from the Weighting Curve Chart. Multiply this value by the weighting factor of 0.17 to get the WQI for DO. Record all results on the Chemical Water Quality Measurements form.

#10 Protocol

Using the current adjustment determined by standard testing (calibration of the equipment with with standard solutions is done each year) adjust the dissolved oxygen reading as necessary.

#9 Protocol DO

If the additon of the starch solution creates a "Blue" color Sodium Thiosulfate should be added drop by drop (swirl continuously) until the sample is clear.

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