LaMotte SMART3 Colorimeter User Manual

Page 258

APPLICATION:

Drinking, surface and saline waters; domestic and

industrial wastes (Method based on reactions that are

specifi c for orthophosphate).

RANGE:

0–3000 ppm Orthophosphate

MDL:

50 ppb

METHOD:

Ammonium molybdate and antimony potassium tartrate

react in a fi ltered acid medium with dilute solution of PO

–

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, ppb

SMART3 Test Procedures 11.10

Test P

rocedures