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water using vacuum filtration. Rinse the crucible and residue with a few milliliters of acetone to speed the
drying time. Dry the crucible in a 100 5C oven for 15 minutes, and cool it to room temperature. Weigh
the crucible plus third residue.
(Ε Φ)
Ξ 100
Περχεντ α ονιυ šερχηλ ρατε
ο
Α
where:
E = Weight of crucible and rod plus second residue of 4.6.2.2 in grams
F = Weight of crucible and rod plus third residue in grams
A = Weight of sample in grams
(Φ Γ )
Περχεντ αλ ινυ
υ
Ξ 100
Α
where:
F = Weight of crucible and rod plus third residue in grams
G = Weight of crucible plus rod in grams
A = Weight of sample in grams
4.6.2.4 Resorcinol. Determine the resorcinol content by using a UV/VIS spectrophotometer and the
following procedure:
a.
Prepare a standard curve (see step e) as follows: weigh a 25.0 2.0 mg sample of pure
resorcinol, and dissolve it in 100 mL of 5 percent sodium carbonate (25 grams of sodium
carbonate dissolved in 475 grams of distilled water). Dilute the dissolved resorcinol with
distilled water to 250 mL, and use this as the stock solution to make the following dilutions
for the standard curve.
b.
Pipette 1, 2, 4, 6, and 8 mL of the stock solution to 100 mL volumetric flasks, and dilute
them with distilled water to the mark. These standards contain 0.1, 0.2, 0.4, 0.6, and 0.8 mg
of resorcinol.
c.
Prepare a reference solution by diluting 100 mL of 5 percent sodium carbonate solution
from step a to 250 mL with distilled water and then further diluting 10 mL of this solution
to 100 mL with distilled water.
d.
Set the instrument to zero at 288 with the reference solution in both the reference and
sample cells. With the reference solution in the reference cell, read the absorbance of the
standard solutions (from step b) in a matched, 1-cm silica cell.
e.
Plot the absorbance versus the concentration (mg of resorcinol per 100 mL of solution).
The best straight line through the points is the standard curve.
f.
Transfer the methylene chloride extract (from 4.6.2.1) to a 500-mL separatory funnel, and
wash it with two 50-mL portions of 5 percent sodium carbonate solution. Collect the
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