LaMotte COD3 Plus, Operator'S Manual

Page: 88 / 119

Phosphate
, Low Range COD3 Plus Colorimeter 10.16
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.
Testing