LaMotte SMART3 Colorimeter - Manual (Page 252)

APPLICATION: Drinking, surface and saline waters; domestic and
industrial wastes (Method based on reactions that are
specifi c for orthophosphate).
RANGE: 0.00–3.00 ppm Orthophosphate
MDL: 0.05 ppm
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.
SAMPLE 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: 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
2
, when used as the
preservative, 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.
PHOSPHATE, Low Range SMART3 Test Procedures 11.10
Test P
rocedures