Fill the BOD bottle to the brim with 300 ml of water from the system, being careful to avoid any air bubbles in the bottle. Carefully stopper the BOD bottle and check for air bubbles, discarding and starting over if you see any.
Add the 2 ml of manganese sulfate to the sample with the pipette by inserting the pipette below the water's surface. Repeat with the 2 ml alkali-iodide-azide.
Adding the reagents above the water's surface could cause air bubbles in the sample.
Close the BOD bottle and invert several times to mix, being careful to avoid introducing air into the sample. If there are air bubbles, discard the sample and start over.
A brownish precipitate will appear if there is oxygen present in the sample. Ensure you have mixed the sample thoroughly and allow the precipitate to settle.
Add the 2 ml of sulfuric acid using the pipette just above the sample's surface. The sample is now "fixed" and you can store the sample for up to eight hours in a cool, dark place.
Titrate the sample by dripping sodium thiosulfate slowly from a calibrated pipette into a glass flask containing 201 ml of the sample, continually stirring or swirling until the sample is a pale straw color.
Add 2 ml of the starch solution to form a blue color and continue slowly titrating until the sample turns clear. Ensure that each drop of the sodium thiosulfate is thoroughly mixed and the sample has not turned clear before adding another. You can use a white sheet of paper to check the color of the sample as it changes.
Measure the amount of sodium thiosulfate used in the sample to determine the amount of dissolved oxygen in the sample. Each milliliter of sodium thiosulfate used in steps 5 and 6 equals one milligram per liter (mg/l) of dissolved oxygen.