Monitoring Methods: Dissolved Oxygen

Collecting the Sample

Water can gain or lose oxygen while sitting in the sample bottle, therefore, you should begin D.O. testing immediately upon reaching the shore after you have collected the sample.  You should measure water temperature at the same time and location that you collect the D.O. sample.

1. Use bottle with the stopper included in the Hach kit.
2. Collect your sample in roughly one-foot deep, normally moving water.
3. Facing upstream (against the current), slowly lower the bottle so the opening of the bottle faces away from you and water current is entering the bottle.
4. Allow the bottle to fill with water gradually, turning it to allow air bubbles to float out.
5. Cap bottle while still submerged and leave extra water in the neck of the bottle.
6. When lifting the bottle out of water, look for bubbles.  If you see any, take another sample using the same procedure.
7. Then test using the Hach Dissolved Oxygen Test Kit.

Testing for Dissolved Oxygen Using Hach Kit

Note:  If you see any air bubbles trapped in the sample bottle during steps 2-4 below, discard the sample and start over.

1. Put on protective gloves and safety goggles. If your skin comes in contact with any powder or titrant, rinse the area liberally with water.
2. Remove the stopper and add the contents of the silver packets D.O. reagent #1 (manganous sulfate) and D.O. reagent #2 (alkaline iodide azide) to the sample.
3. Insert the stopper, being careful not to trap an air bubble.  With a finger over the stopper, shake vigorously.  A brownish-orange flocculant (suspended particles) will form if oxygen is present, as is seen in image A on the right.
4. Allow the sample to stand until the flocculent (floc) settles to the bottom half of the bottle.  Shake the bottle a second time and allow the floc to settle halfway again, like in image B on the right.
5. Remove the stopper and add the contents of D.O. powder pillow #3 (sulfamic acid).
6. Stopper and shake vigorously until all the floc is dissolved.  This is called the prepared sample.   It can be stored in the dark for a short time if it is more convenient to return to your home/school to complete the analysis.
7. Transfer two plastic measuring tubes full of prepared sample to the square glass mixing bottle.  Hach kit instructions say one tube, however, using two measuring tubes of sample allows us to determine D.O. to the nearest 0.5 mg/L instead of 1 mg/L.
8. Complete the following:
   1. Holding the dropper vertically, add one drop at a time of sodium thiosulfate titrant to the square mixing bottle, and count each drop.  
   2. Swirl the solution after each drop. 
   3. Continue adding and counting sodium thiosulfate drops until the sample is a very light yellow.
   4. Add 3 to 4 drops of starch solution. The prepared sample will turn blue from the starch. 
   5. Continue adding drops of sodium thiosulfate, mixing, and counting until the prepared sample turns from blue to colorless (the endpoint).  Often this is just one or two more drops, so be careful.
9. The dissolved oxygen content of the water in mg/L is the total number of drops of titrant used to get to the endpoint divided by two (because you used two tubes of sample).  Example: 13 drops divided by two tubes = 6.5 mg/L). 
10. To determine percent saturation, use a straight edge to draw a line from your water temperature (top bar) to your oxygen mg/L (bottom bar). The measured percent saturation is the point where that line crosses the % oxygen saturation line. For example, 5˚C with 10 mg/L of oxygen aligns with 75% saturation. (See diagram below.)

     

    

     

11. On your data recording form, report number of drops, number of tubes, D.O. mg/L and % saturation.

 Further information can be found in the factsheet "Dissolved Oxygen: Aquatic Life Depends on It".

Video Demonstration

Watch this video to learn how WAV measures dissolved oxygen. 

 Back to "The Methods"