6
TEST METHODS SPECIFICATIONS
INTRODUCTION
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 an increase in algae and weed
nuisances. When algae cells die, oxygen is used in the decomposition and fi sh
kills often result. Rapid decomposition of dense algae scums with associated
organisms give rise to foul odors and hydrogen sulfi de gas.
APPLICATION
Drinking, surface and saline waters; domestic and industrial wastes (Method
based on reactions that are specifi c for orthophosphate).
RANGE
0 to 3.0 ppm Orthophosphate (Range can be extended by dilution.)
METHOD
Ammonium molybdate and antimony potassium tartrate react in a
fi ltered acid medium with dilute solution of PO
4
3-
to form an antimony-
phosphomolybdate complex. This complex is reduced to an intense blue
colored complex by ascorbic acid. The color is proportional to the amount
of phosphate present. (Only orthophosphate forms a blue color in this
test.) Polyphosphates (and some organic phosphorus compounds) may be
converted to the orthophosphate form by sulfuric acid digestion. Organic
phosphorus compounds may be converted to the orthophosphate form by
persulfate digestion.
HANDLING & PRESERVATION
If benthic deposits are present in the area being sampled, great care should
be taken not to include these deposits. If the analysis cannot be performed
the same day of collection, the sample should be preserved by the addition
of 2 mL of concentrated sulfuric acid or 40 mg mercuric chloride per liter and
refrigerated at 4°C.
INTERFERENCES
1. No interference from copper, iron, or silicate at concentrations many
times the concentration of sea water. However, high iron concentrations can
cause precipitation and subsequent loss of phosphorus.
2. Salt error for samples ranging from 5% to 20% salt content was found to
be less than 1%.
3. Mercuric chloride, HgCl
2
, when used as the preservative, interferes
when the chloride levels are low (less than 50mg/L). This interference
is overcome by spiking samples with a minimum of 50 mg/L of sodium
chloride.
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