Environmental Engineering Reference
In-Depth Information
is then measured by the ascorbic acid method. Because the sample is
not filtered, the procedure measures both dissolved and suspended
orthophosphate.
3. The dissolved phosphorus test measures that fraction of the total phos-
phorus that is in solution in the water (as opposed to being attached
to suspended particles). It is determined by first filtering the sample,
then analyzing the filtered sample for total phosphorus.
4. Insoluble phosphorus is calculated by subtracting the dissolved phos-
phorus result from the total phosphorus result.
All of these tests have one thing in common—they all depend on measur-
ing orthophosphate. The total orthophosphate test measures the orthophos-
phate that is already present in the sample. The others measure that which is
already present and that which is formed when the other forms of phospho-
rus are converted to orthophosphate by digestion.
Sampling and equipment Considerations
Monitoring phosphorus involves two basic steps:
1. Collect a water sample.
2. Analyze it in the field or lab for one of the types of phosphorus
described above.
Refer to Standard Methods for the specific laboratory methods.
Sample Containers
Sample containers made of either some form of plastic or Pyrex ® glass are
acceptable to the USEPA. Because phosphorus molecules have a tendency
to adsorb (attach) to the inside surface of sample containers, containers that
are to be reused must be acid-washed to remove adsorbed phosphorus. The
container must be able to withstand repeated contact with hydrochloric acid.
Plastic containers, either high-density polyethylene or polypropylene, might
be preferable to glass from a practical standpoint, because they are better
able to withstand breakage. Some programs use disposable, sterile, plastic
Whirl-Pak ® bags. The size of the container depends on the sample amount
needed for the phosphorus analysis method chosen and the amount needed
for other analyses to be performed.
Dedicated Labware
All containers that will hold water samples or come into contact with reagents
used in this test must be dedicated. They should not be used for other
tests, to eliminate the possibility that reagents containing phosphorus will
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