Phosphates ~ low-range – LaMotte DC1600 Colorimeter User 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

Pipet, 1 mL, plastic

0354

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

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

, 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.