APPENDIX B: METHOD DOCUMENTATION FIELD METHOD FOR
DETERMINATION OF AMMONIUM PICRATE AND PICRIC ACID IN WATER
4.0 INTERFERENCES
1.0 SCOPE AND APPLICATION
Identification and quantification is based on the
This method is suitable for the determination
absorbance of picrate ions at 400 nm. Other sub-
of ammonium picrate and picric acid in well wa-
stances that can produce yellow extracts include
ter or surface water. A battery-operated spectro-
humic materials and certain nitroaromatic second-
photometer and hand vacuum pump can be used;
ary explosives and their environmental transfor-
however, a powered vacuum source is recom-
mation products. Extraction with ion-exchange
mended.
resins followed by a rinse with methanol removes
the nitroaromtic interferences. Background cor-
rection using the initial absorbance of the acid
2.0 SUMMARY OF METHOD
methanol eluent accounts for humic materials.
Rinsing the spectrophotometer cuvette with
A 2-L subsample of water is drawn through an
methanol between samples is necessary. Carry-
anion extraction disk under vacuum. Picrate ions
over of any water from the previous wateracid
are retained by the disk. Interferences are removed
methanol mixture will convert colorless, proto-
by washing the disk with methanol. Picrate ions
nated picric acid in the acidmethanol mixture to
are converted to protonated picric acid and eluted
yellow picrate ions. This will invalidate the "Initial
from the disk using a mixture of 4.5 mL of metha-
Absorbance" measurement that is required for
nol plus 0.5 mL of concentrated sulfuric acid. The
background correction.
initial absorbance of the eluent at 400 nm is re-
corded and provides a background correction. The
eluted colorless, protonated picric acid is converted
to a yellow picrate ion solution by dilution with
5.0 SAFETY
water. The absorbance is measured again at 400
The normal safety precautions associated with
nm. The background-corrected absorbance is con-
verted to g/L of picric acid based on the response
the use of flammable organic solvents, strong ac-
ids and potentially toxic chemicals should be em-
from calibration standards.
ployed. Eye, hand and clothing protection is rec-
ommended when handling the concentrated sul-
furic acid and acidified eluents.
3.0 DEFINITIONS
Ammonium picrate (ammonium 2,4,6-trinitro-
phenoxide)
6.0 EQUIPMENT AND SUPPLIES
MW: 246.14
Solubility in water at 25C: >10 g/L
A. Instrumentation:
1. Field-portable, battery-operated spectro-
Octanolwater partition coefficient: 0.2
photometer with a 1-cm path-length cell
CAS# 131-74-8
(Hach DR2000 or equivalent).
2. Analytical balance for preparation of stan-
Picric acid (2,4,6-trinitrophenol)
dards.
MW: 229.11
Solubility in water at 25C: >10 g/L
3. Vacuum source.
B. Labware and Equipment
Octanolwater partition coefficient: 4.4
1. 1-L (Qorpak or equivalent) amber glass
CAS# 88-89-1
sample bottles (two per sample).
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