LaMotte SMART Spectro Spectrophotometer - Manual (Page 221)

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: 0.04 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 SMART Spectro Test Procedures 2.11
Test P
rocedures