LaMotte DC1600 Colorimeter - Manual (Page 73)

PHOSPHATES ~ LOW-RANGE
ASCORBIC ACID REDUCTION METHOD CODE 3653
QUANTITY CONTENTS CODE
60 mL *Phosphate Acid Reagent *V-6282-H
5 g *Phosphate Reducing Reagent *V-6283-C
1 Pipet, 1 mL, plastic 0354
1 Spoon, 0.1 g, plastic 0699
*WARNING: Reagents marked with a * are considered hazardous substances. Material Safety Data Sheets (MSDS) are supplied for
these reagents. For your safety, read label and accompanying MSDS before using.
Phosphorus is an important nutrient for aquatic plants. The amount found in water is generally not more than 0.1 ppm
unless the water has become polluted from waste water sources or excessive drainage from agricultural areas. When
phosphorus is present in excess of the concentrations required for normal aquatic plant growth, a process called
eutrophication takes place. This creates a favorable environment for the increase in algae and weed nuisances. When algae
cells die, oxygen is used in the decomposition and fish kills often result. Rapid decomposition of dense algae scums with
associated organisms give rise to foul odors and hydrogen sulfide gas.
APPLICATION: Drinking, surface and saline waters; domestic and industrial wastes (Method based on reactions
that are specific for orthophosphate).
RANGE: 0 – 3.0 ppm Orthophosphate (Range can be extended by dilution.)
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.
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