APPENDIX A: METHOD DOCUMENTATION: FIELD METHOD FOR
DETERMINATION OF AMMONIUM PICRATE AND PICRIC ACID IN SOIL
1.0 SCOPE AND APPLICATION
4.0 INTERFERENCES
This method is suitable for the determination
Identification and quantification is based on the
of ammonium picrate and picric acid in field-moist
absorbance of picrate ions at 400 nm. Other sub-
or dried soil, using a battery-operated spectropho-
stances that can produce yellow extracts include
tometer.
humic materials and certain nitroaromatic second-
ary explosives and their environmental transfor-
mation products. Extraction with ion-exchange
resins followed by a rinse with methanol removes
2.0 SUMMARY OF METHOD
the nitroaromatic interferences. Background cor-
A 20-g subsample of soil is extracted with 100
rection using the initial absorbance of the acid
mL of acetone by manually shaking for three min-
acetone eluent accounts for humic materials. Rins-
utes. A 30-mL aliquot is filtered into a graduated
ing the spectrophotometer cuvette with acetone
cylinder and diluted with 30 mL of deionized wa-
between samples is necessary. Carry-over of any
ter. This converts picric acid or ammonium picrate
water from the previous wateracidacetone mix-
in the extract to picrate ions. The diluted extract is
ture will convert colorless, protonated picric acid
then passed through an ion-exchange solid-phase
in the acidacetone mixture to yellow picrate ions.
extraction (SPE) tube. Picrate ions are retained on
This will invalidate the "Initial Absorbance"
the resin. Interferences are removed by washing
measurement that is required for background
the resin with methanol. Picrate ions are converted
correction.
to protonated picric acid and eluted from the resin
by washing with 10 mL of acetone containing four
drops of concentrated sulfuric acid. The initial ab-
5.0 SAFETY
sorbance of the eluent at 400 nm is recorded and
provides a background correction. The colorless,
The normal safety precautions associated with
protonated picric acid solution is converted to a
the use of flammable organic solvents, strong ac-
yellow picrate ion solution by diluting the acidi-
ids and potentially toxic chemicals should be em-
fied acetone extract with water. The absorbance is
ployed. Eye protection is recommended when
measured again at 400 nm. The corrected absor-
shaking bottles to protect against splash from
bance is converted to g/g of picric acid based on
poorly sealed containers. Eye, hand and clothing
the response from calibration standards.
protection is recommended when handling the
concentrated sulfuric acid and acidified eluents.
3.0 DEFINITIONS
6.0 EQUIPMENT AND SUPPLIES
Ammonium picrate (ammonium 2,4,6-trinitro-
phenoxide)
A. Instrumentation:
MW: 246.14
1. Field-portable, battery-operated spec-
Solubility in water at 25C: >10 g/L
trophotometer with a 1-cm path-length
Octanolwater partition coefficient: 0.2
cell (Hach DR2000 or equivalent).
CAS# 131-74-8
2. Mechanical or battery-operated balance
to measure soils.
Picric acid (2,4,6-trinitrophenol)
3. Analytical balance for preparation of
MW: 229.11
standards.
Solubility in water at 25C: >10 g/L
B. Labware and equipment
Octanolwater partition coefficient: 4.4
1. 250-mL (Nalgene or equivalent) plastic
CAS# 88-89-1
bottles with caps containing three 1/4-
inch steel ball bearings, one per sample.
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