LaMotte SMART3 Soil Operator's Manual page 102

RANGE:
METHOD:
INTERFERENCES:
PHOSPHATE
1.800.544.2843
0.00–99.00 lb/acre Phosphorus
Ammonium molybdate and antimony potassium tartrate
react in a fi ltered acid medium with dilute solution of
–3
PO to form an antimony-phosphomolybdate complex.
4
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.
a. 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.
b. Salt error for samples ranging from 5% to 20% salt
content was found to be less than 1%.
c. Mercuric chloride, HgCl
interferes when the chloride levels are low (less than 50
mg/L). This interference is overcome by spiking samples
with a minimum of 50 mg/L of sodium chloride.
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15
10 20
5 25
0 30
, when used as the preservative,
2
Smart3 Soil Test Procedures 9.18
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