Dissolved Oxygen: Lakes Method

Learn more about DO (leaving MassWWP pages)

**For the Mass. Department of Environmental Protection-approved SOP, get the pdf file here **

Field Equipment

photo of Wisconsin sampler use

  • Modified Wisconsin Sampler with 60 ml Dissolved Oxygen bottle inside
  • Calibrated line
  • Thermometer (depth-electronic or manual)
  • D.O. chemicals - Powder pillows #1, #2, and #3
  • Scissors
  • Field data sheet
  • Cooler
  • Frozen Koolits and ice
  • Extra 60 ml DO bottle for replicate
  • Marble
  • Zip-loc bags (1 gallon bags)

Sampling Protocol

Determine the depth of water at the sampling site. A convenient way to do this is to lower the Secchi Disk until the line goes slack. Retrieve the line until all slack is removed. Record this bottom depth on your data sheet. Subtract 0.4 meters (two marks on the sampler's lowering line) from this depth. This will be the depth to collect the dissolved oxygen sample. Lower Wisconsin sampling bottle to desired depth. Check that no bubbles are coming from the sampler. Give a short, sharp yank on the lowering line to pull intake and outlet plugs. Wait until all bubbles disappear at the surface. (Some volunteers report that it takes about 90 seconds for the bottle to fill). Retrieve bottle.

Carefully unscrew and remove sampler top to avoid spilling any water. Extract D.O. bottle from sampler. Place the thermometer in the Wisconsin sampling bottle (not in the D.O. bottle). Pour D.O reagent #1 (manganous sulfate) into D.O. bottle; pour reagent #2 (alkaline iodide-azide) into the D.O. bottle. Do not be concerned if a small amount of water overflows the bottle, but do be careful not to introduce any bubbles. Carefully cap bottle to avoid trapping air bubbles inside (the cap is shaped like a cone to minimize this; holding the cap 1" above the lip, let go of the cap so it drops neatly inside the bottle). Holding the D.O. bottle and with your index finger on the cap to prevent its dislodging, invert bottle 50 times with a motion like turning a doorknob clockwise and counterclockwise in succession. Allow bottle to sit for 5 minutes. A brownish flocculent material should accumulate on the bottom. [You will notice that more of this will form with higher D.O. samples than low.]

Carefully remove cap of D.O. Bottle by twisting slightly and lifting. Pour reagent #3 (sulfamic acid) into bottle. You can now put a marble in the bottle to ensure that there is enough water and that no air bubble will form when you cap the bottle. Carefully drop the cap in the bottle again and repeat bottle inversion process. The liquid will turn a yellow color: the stronger the color, the more dissolved oxygen. Recap the bottle and store in the cooler for delivery to the lab. The process you have just completed has "fixed" the dissolved oxygen as a reasonably stable compound. The sample must be analyzed within 8 hours.

As the sample temperature will rise in your boat, it is advisable to use a depth electronic thermometer. If you don't have one, however, read the manual thermometer in the WI sampler and record in °C on the field data sheet.
Repeat this procedure at this site and depth, filling and fixing a second(replicate) D.O. Bottle

image of Wisconsin sampler
drawing by Tom Burke

Dissolved Oxygen Table*

Temperature (°C) Dissolved oxygen (mg/l)
at 100% saturation
0
14.6
1
14.2
2
13.8
3
13.5
4
13.1
5
12.8
6
12.5
7
12.2
8
11.9
9
11.6
10
11.3
11
11.1
12
10.8
13
10.6
14
10.4
15
10.2
16
10.0
17
9.7
18
9.5
19
9.4
20
9.2
21
9.0
22
8.8
23
8.7
24
8.5
25
8.4

Figuring Percent Saturation of Dissolved Oxygen

Water can hold a limited amount of dissolved oxygen. When it holds the maximum amount it can, a water body is said to be at saturation, or 100% saturated. The dissolved oxygen (in mg/l) of water at saturation changes with temperature: the higher the temperature, the less oxygen water holds. Massachusetts Surface Water Quality Standards express minimum criteria for dissolved oxygen in both mg/l and % saturation.

To calculate % saturation of the sample, you divide the measured dissolved oxygen content of your sample by the maximum oxygen content at the temperature of your sample. The maximum oxygen content of water at various temperatures is given in the table at left

%DO saturation = your DO Measurement / Max. DO Concentration at tour measured temperature

For example, if you measured a DO concentration of 5 mg/l at 20°C (lake temperature) you would divide 5 mg/l by 9.2, the maximum concentration at 20°C. The percent saturation would be 54%.
*Note in high altitude areas (none in Massachusetts) or water with high chloride concentrations, an additional correction may be necessary.These values are correct for a barometric pressure of 760 mm or sea level altitude.

Lab Equipment

  • 50 ml graduated cylinder
  • 100ml beaker
  • digital titrator
  • starch solution
  • thiosulfate cartridge
  • magnetic stirrer and stir bar (optional)

Lake Sample Analysis

  1. Uncap sample bottle and rinse calibrated cylinder with a few mls of sample. Measure exactly 50 mls of sample. Careful measurement is crucial to accurate measurement of dissolved oxygen. Simply reading the meniscus incorrectly can introduce an error of 2-5%. No special care needs to be taken to avoid introducing bubbles into the sample.
  2. Pour sample from cylinder into clean beaker, insert magnetic stirring rod and place on bright white paper on the magnetic stirrer. Set stirrer to mix sample without splashing.
  3. Set digital titrator at zero.
  4. Begin titrating the standard solution with sodium thiosulfate. (You can go pretty quick at first). Continue until the solution turns from a strong yellow to a pale yellow.
  5. Add a dropperful of the starch indicator solution (enough to turn the standard solution from pale yellow to dark blue).
  6. Continue titrating, adding about 5 to 10 digits every few seconds. As the color turns to light blue, slow down, adding about 2 digits every few seconds until the blue just disappears.
  7. Write the number of digits down, but then continue titrating by adding one more digit as you look carefully for a blue swirl. Keep doing this every few seconds, one digit at a time until your last digit causes no visible change. Record the previous digit as the correct amount. For instance, if you saw a change at 194 digits, but none at 195, write down 194 as the units of titrant used.
  8. Calculate the dissolved oxygen of the sample by this formula: Dissolved oxygen (milligrams per liter) = (digits of titrant) * (.04). In the example, 194 * .04 = 7.76 mg/l DO, or 7.76 parts per million
  9. Rinse sample bottle, volumetric flask and beaker once with distilled water.