LaMotte SMART Spectro Spectrophotometer - Manual (Page 199)

APPLICATION: Drinking, surface, and saline waters; domestic and industrial
wastes (Method based on reactions that are specific for
orthophosphate).
RANGE: 0.00–3.00 ppm Orthophosphate
METHOD: Ammonium molybdate and antimony potassium tartrate
react in a filtered 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–LR 2/3 SMART SPECTRO TEST PROCEDURES 05.04