April, 2010

High Range (1-20 mg/L) Dissolved Oxygen Test Instructions

1.	Fill the Dissolved Oxygen bottle (round bottle with glass stopper) with the water to be used by allowing the water to overflow the bottle for two or three volumes.  Do not aerate this water. 

2.	Use the clippers (or knife) to open one Dissolved Oxygen 1 Reagent Powder Pillow and one Dissolved Oxygen 2 Reagent Powder Pillow.  Add the contents of each of the pillows to the bottle.  Stopper the bottle carefully with a quick thrust to exclude air bubbles.  Glass stoppers have pointed tips to assist this action.  If air becomes trapped in the bottle in Steps 2 or 4 the sample should be discarded and the test repeated. 

3.	Grip the bottle and stopper firmly; shake vigorously to mix.  A flocculant (floc) precipitate will be formed.  If oxygen is present in the sample the precipitate will be brownish orange in colour.  A small amount of powdered reagent may stick to the bottom.  This will not affect the test results.

4.	Allow the sample to stand until the floc has settled to the bottom of 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, such as sea water.  No interference with the test results will occur as long as the sample is allowed to stand for four or five minutes.

5.	Use the clippers (or knife) to open one Dissolved Oxygen 3 Reagent Powder Pillow.  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 colour will develop if oxygen is present.  The sample may now be stored for up to several hours if need be.

6.	Fill the plastic measuring tube level full of the sample prepared in Steps 1 through 5.  Pour the sample into the square mixing bottle.

7.	Add Sodium Thiosulfate 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 colourless.

8.	Each drop used to bring about the change in Step 7 is equal to 1 mg/L of dissolved oxygen (DO).  Record this result (eg; 8 drops is 8 mg/L DO).

Recommended: Before step 7 add one or two drops of Starch Solution Indicator (until the sample turns dark blue).  The Starch simply provides a more sensitive colour contrast when titrating the sample to a clear state.  

If storing the oxygen bottles with glass lids in place, use paper between the bottle and the stopper to prevent the lid from becoming seized.

Low Range (0.4 - 4 mg/L) Dissolved Oxygen Test Instructions

Perform Steps 1 to 5 as for the High Range Test.  

In those cases where low oxygen concentrations are suspected (very light yellow colour), pour off the contents of the Dissolved Oxygen bottle until the level reaches the 30 mL mark on the bottle.  Note that not all Dissolved Oxygen bottles have this 30 mL mark.  Save marked bottles for bottom or deep water samples.

Add Sodium Thiosulfate Solution drop by drop to the Dissolved Oxygen 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 colourless.

Multiply by 0.2 the number of drops of Sodium Thiosulfate Solution used.  This is the total mg/L of Dissolved Oxygen (ie. 10 drops is 2 mg/L). 

Note: Again, Starch Solution may be added to the Dissolved Oxygen bottle to attain a dark blue colour before the titration.  More starch will be required here.

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