How Do I Analyze Water for Dissolved Oxygen?

Oxygen is vital to the life cycle common to water. It is essential to keep organisms living, to sustain species reproduction, and for the development of populations. Oxygen is soluble in water in direct proportion to the partial pressure in the gas phase and solubility decreases as temperature increases. Oxygen enters the water by absorption directly from the atmosphere or by plant photosynthesis. it is removed by respiration of organism and by organic decomposition. During respiration and decomposition, animals and plants consume dissolved oxygen and liberate carbon dioxide. Organic waste from municipal, agricultural and industrial sources may overload the natural system causing a serious depletion of the oxygen supply in the water. Waters rich in nutrients produce algae in quantity which upon decomposition deplete the oxygen supply. Fish kills are often associated with the process of eutrophication.

Dissolved oxygen (DO) determination must be done in the field. The DO is in equilibrium, based on gas pressures and temperature. As these conditions change, so will the DO. Electronic DO probes are a quick and accurate method for dissolved oxygen determination. Test kits also provide accurate information. The following are instructions for DO determination using the Hach DO Test Kit.

High Range Test Instructions

1. Fill the DO bottle (round bottle with glass stopper) with the water to be tested by allowing the water to over flow the bottle for two or three minutes. To avoid trapping air bubbles in the bottle, incline the bottle slightly and insert the stopper with a quick thrust. If air bubbles are trapped, the sample should be discarded.
   
2. Use the clippers to open one DO 1 reagent powder pillow and one DO 2 reagent powder pill. Add the contents of each of the pillows to the bottle. Stopper the bottle carefully to exclude air bubbles. Shake vigorously to mix. A flocculent precipitate will be formed. If oxygen is present in the sample the precipitate will be brownish orange in color. A small amount of powdered reagent may remain stuck to the bottom of the bottle.
   
3. Allow the sample to stand until the floc has settled halfway in the bottle, leaving the upper half of the sample clear. Shake the bottle again. Again let it stand until the upper half of the sample is clear. Note the floc will not settle in samples with high concentrations of chloride.
   
4. Use the clippers to open DO 3 reagent . Remove the stopper from the bottle and add the contents of the pillow. Carefully restopper the bottle and shake to mix. The floc will dissolve and a yellow color will develop if oxygen is present.
   
5. Fill the plastic measuring tube level full of the sample prepared in steps 1-4. Pour the sample into the square mixing bottle.
   
6. Add sodium thiosulfate standard solution drop by drop to the mixing bottle, swirling to mix after each drop. hold the dropper vertically above the bottle and count each drop as it is added. Continue to add drops until the sample changes from yellow to colorless.
   
7. Each drop used to bring about the color change in step 6 is equal to 1 mg/l of dissolved oxygen.

Low Range Test Instructions

1. If the result of step 7 is very low, 3 mg/l or less, it is advisable to obtain a more sensitive test. To do so, use the prepared sample left from step 4 in the high range test. Pour off the contents of the DO bottle until the level just reaches the mark (30 ml) on the bottle.
   
2. Add sodium thiosulfate standard soln. drop by drop, directly to the DO bottle. Count each drop as it is added and swirl the bottle constantly to mix while adding the titrant. Continue to add drops until the sample changes from yellow to colorless.
   
3. Each drop of standard soln. used to bring about the color change in step 2 is equal to 0.2 mg/l DO.