LaMotte SMART Spectro Spectrophotometer User 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

–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.

PHOSPHATE, Low Range

SMART Spectro Test Procedures 2.11

Test P

rocedures