LaMotte Dissolved Oxygen Water Quality - Manual (Page 13)

WHERE DOES THE OXYGEN GO?
Once in the water, oxygen is used by the aquatic life. Fish and other aquatic
animals need oxygen to breathe or respire. Oxygen is also consumed by
bacteria to decay, or decompose, dead plants and animals.
Respiration
All animals, whether on land or underwater, need oxygen to respire, grow and
survive. Plants and animals respire throughout the night and day, consuming
oxygen and producing carbon dioxide, which is then used by plants during
photosynthesis.
Decomposition
All plant and animal waste eventually decomposes, whether it is from living
animals or dead plants and animals. In the decomposition process, bacteria use
oxygen to oxidize, or chemically alter, the material to break it down to its
component parts. Some aquatic systems may undergo extreme amounts of
oxidation, leaving no oxygen for the living organisms, which eventually leave
or suffocate.
OTHER FACTORS
The oxygen level of a water system is not only dependent on production and
consumption. Many other factors work together to determine the potential
oxygen level, including:
•
Salt vs. fresh water - Fresh water can hold more oxygen than salt
water.
• Temperature - Cold water can hold more oxygen than warm water.
• Atmospheric pressure (Altitude) - The greater the atmospheric
pressure the more oxygen the water will hold.
TESTING DISSOLVED OXYGEN
Dissolved oxygen is often tested using the Azide modification of the Winkler
method. When testing dissolved oxygen it is critical not to introduce
additional oxygen into the sample. Many people avoid this problem by filling
the sample bottle all the way and allowing the water to overflow for one
minute before capping.
The first step in a DO titration is the addition of Manganous Sulfate Solution
(4167) and Alkaline Potassium Iodide Azide Solution (7166). These reagents
react to form a white precipitate, or floc, of manganous hydroxide, Mn(OH)
2
.
Chemically, this reaction can be written as:
MnSO
4
+ 2KOH
⎯ → ⎯
Mn(OH)
2
+ K2SO
4
Manganous
Sulfate
+ Potassium
Hydroxide
⎯ → ⎯
Manganous
Hydroxide
+ Potassium
Sulfate
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