LaMotte AM-21 Water Pollution Detection Outfit - Manual (Page 38)

Phosphorus (Phosphate) Test
Phosphorus is an important nutrient for aquatic plants. The amount found
in water is generally not more than 0.1 ppm unless the water has become
polluted from waste water sources or excessive drainage from agricultural
areas. When phosphorus is present in excess of the concentrations required
for normal aquatic plant growth, a process called eutrophication takes place.
This creates a favorable environment for the increase in algae and weed
nuisances that produce scums and odors. When algae cells die, oxygen is
used in the decomposition and fish kills often result. Rapid decomposition of
dense algae scums with associated organisms give rise to foul odors and
hydrogen sulfide gas. Inorganic phosphate, which is largely the form of
phosphorus required for plant growth, is determined by its reaction with a
molybdate solution to form a phosphomolybdate which, when reduced,
forms a blue color which is the basis for a very sensitive test for phosphorus.
The production of more than a faint blue color in this test is cause for
suspicion of phosphate pollution, and when the other factors such as
available nitrogen, iron, trace metals, etc. are present, will cause the
conditions described above.
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