dard with deionized water as described in Table
to the vacuum flask, with its drain tube inserted
B1. Volumetric pipets are used to measure the
into the test tube.
working standard.
6. A 4.5-mL aliquot of methanol is added to one
10.2 INITIAL CALIBRATION: Calibration so-
10-mL graduated cylinder. A 0.5-mL aliquot of con-
lutions are prepared in the 2-L graduated cylin-
centrated sulfuric acid is added slowly with an
der and processed according to the PROCEDURE
eyedropper. The dropper should be held close to
starting at Step 1. Absorbance readings of Solu-
the surface of the methanol to prevent splashing.
tions AE should range from 0.0 to 0.7 absor-
7. The acidmethanol mixture is poured into
bance units. The slope of the relationship be-
the extraction funnel and allowed to soak the disk
tween the concentration of picric acid and ab-
for 2 minutes. This converts the picrate ions to
sorbance should be linear, with zero intercept
protonated picric acid.
and a response factor of about 280 g/L per ab-
8. Vacuum is applied and the acidmethanol
sorbance unit.
eluent is collected in the test tube. The vacuum is
10.3 DAILY CALIBRATION: A 150-g/L picric
vented and the test tube withdrawn and capped.
acid solution is made by diluting 30 mL of work-
9. The Anion disk is removed from the holder
ing standard to 2 L with deionized water and pro-
and placed in a waste container. The holder and
ceeding with the method. The daily response fac-
funnel are assembled and returned to the vacuum
tor is then determined:
flask and rinsed with 10 mL of deionized water,
which is collected in the vacuum flask and then
Daily response factor = (150 g/L)/
transferred to a waste container.
(Absorbance @ 400 nm) .
10. A portion of the acidmethanol eluent is
transferred from the test tube to a spectrophotom-
eter cuvette using a 9-in. pipet.
11. The absorbance at 400 nm is measured and
11. PROCEDURE
recorded as Initial ABS. The spectrophotometer is
1. A 47-mm Empore Anion Extraction Disk is
zeroed with water.
placed on a vacuum filter holder. A glass fiber
12. The eluent is returned to the test tube and
filter may be placed on top of that if the sample
the cuvette rinsed with water.
appears cloudy. The funnel is clamped in place.
13. Any color in the eluent is qualitatively de-
2. The water sample is measured in a 2-L gradu-
scribed and recorded. A 5.0-mL aliquot of deion-
ated cylinder and a portion is added to the extrac-
ized water is measured in a second 10-mL gradu-
tion funnel. The vacuum is applied and the funnel
ated cylinder and added to the 5.0 mL of acid
refilled as needed. After the first 1-L is extracted,
methanol eluent in the test tube.
the vacuum is shut off and vented and the vacuum
14. Any change in color should be qualitatively
flask emptied into a waste container. The vacuum
described and recorded. An increase in yellow color
is reapplied, the second liter extracted and the
confirms the presence of picrate in the sample.
vacuum flask emptied again. Picrate ions in the
15. A portion of the diluted eluent is transferred
sample are retained on the anion disk.
to the cuvette and the absorbance at 400 nm mea-
3. The glass fiber filter, if used, is removed and
sured and recorded as Final ABS.
placed in a waste container. The funnel is re-
16. If the Final ABS is greater than 0.7, the di-
clamped in place. Air is drawn through the Anion
luted eluent may be diluted further with water
disk for 5 minutes using vacuum.
and remeasured.
4. The vacuum is turned off and vented, then 10
17. After measurement, the eluent is returned
mL of methanol is added to the funnel. This is
to the test tube.
allowed to soak into the disk for 2 minutes, then
18. The cuvette is then rinsed, first with water
pulled through with vacuum. This rinse removes
from a squirt bottle, then with unacidified metha-
interfering compounds.
nol from a squirt bottle. It is critical that no re-
5. The vacuum is vented and the extraction
sidual water be present in the cuvette when the
holder and funnel are removed so that the metha-
next sample is added to the cuvette for the Initial
nol rinse may be transferred to a waste container.
ABS measurement. Water will convert any picric
A 25- 200-mm test tube is placed into the vacuum
acid to picrate ion and invalidate the background
flask. The extraction holder assembly is returned
correction.
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