Environmental Engineering Reference
In-Depth Information
contaminate the labware. All labware should be acid-washed. The only form
of phosphorus this text recommends for field analysis is total orthophos-
phate, which requires use of the ascorbic acid method on an untreated sam-
ple. Analysis of any of the other forms requires adding potentially hazardous
reagents, heating the sample to boiling, and using too much time and too
much equipment to be practical. In addition, analysis for other forms of phos-
phorus is prone to errors and inaccuracies in field situations. Pretreatment
and analysis for these other forms should be handled in a laboratory.
Ascorbic Acid Method
In the ascorbic acid method, a combined liquid or prepackaged powder
reagent consisting of sulfuric acid, potassium antimonyl tartrate, ammonium
molybdate, and ascorbic acid (or comparable compounds) is added to either
50 or 25 mL of the water sample. This colors the sample blue in direct propor-
tion to the amount of orthophosphate in the sample. Absorbance or trans-
mittance is then measured after 10 minutes, but before 30 minutes, using
a color comparator with a scale in milligrams per liter that increases with
the increase in color hue, or an electronic meter that measures the amount
of light absorbed or transmitted at a wavelength of 700 to 880 nm (again,
depending on manufacturer's directions).
A color comparator may be useful for identifying heavily polluted sites
with high concentrations (greater than 0.1 mg/L); however, matching the
color of a treated sample to a comparator can be very subjective, especially at
low concentrations, and can lead to variable results. A field spectrophotom-
eter or colorimeter with a 2.5-cm light path and an infrared photocell (set for
a wavelength of 700 to 880 nm) is recommended for accurate determination
of low concentrations (between 0.2 and 0.02 mg/L). Use of a meter requires
that a prepared known standard concentration be analyzed ahead of time to
convert the absorbance readings of a stream sample to milligrams per liter or
that the meter reads directly in milligrams per liter.
For information on how to prepare standard concentrations and on how
to collect and analyze samples, consult the latest edition of
Standard Method
s
and Method 365.2 of
Methods for Chemical Analysis of Water and Wastes
( U S E PA ,
1991).
Nitrates
Nitrates are a form of nitrogen found in several different forms in terrestrial
and aquatic ecosystems. These forms of nitrogen include ammonia (NH
3
),
nitrates (NO
3
), and nitrites (NO
2
). Nitrates are essential plant nutrients, but
excess amounts can cause significant water quality problems. Together with
