D7318_11
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Obtain an ethanol
sample in accordance
with practice D4057
or practice D4177.
The sample should be well
mixed to ensure homogeneity.
A representative portion
shall be taken for analysis.
Samples should be
collected in containers
with closures that seal well to
prevent evaporation of ethanol.
Sample containers should not
contain any residual sulfate.
If containers have been
cleaned and rinsed with water,
they must be rinsed
with type III water
and dried prior to use.
Determine the blank value by
analysis of a denatured ethanol
sample used for making
standards, 100 grams,
using the procedure described
in 11.3 of the written standard.
An aliquot of aqueous
sulfate blank solution, 8.5
of the written standard,
is added to all samples
as part of the blank.
This is necessary to provide
a stable blank value.
The average of three
blank measurements
is recorded as v sub b
for use in the calculation
of sulfate content of samples.
Accurately weigh an
ethanol sample or blank,
approximately 100 grams,
into a 150-milliliter beaker
and record the weight as g.
Carefully pipette 0.200
milliliters of aqueous sulfate
blank solution, 8.5 of
the written standard,
into the beaker containing
the ethanol sample.
Add 1 milliliter of 0.1
molar perchloric acid,
8.6 of the written standard.
The sample should be
slightly acidic to pH paper
before titrating. pH should
be in the range of 3 to 5.
The pH can be checked by
dipping a glass stirring rod
into the sample and testing
a droplet on the rod
with a moistened pH test strip.
If 1 milliliter of 0.1
molar perchloric acid
is insufficient to achieve
a pH in the range of 3 to 5,
add an additional milliliter
of acid and recheck the pH.
To prepare the
auto-titrator for operation,
immerse the electrodes in
the titration solution.
Initiate stirring and titrate
to the potentiometric endpoint
with lead titrant.
Record the volume of
titrant used as v.
Duplicate determinations
should agree to
within the repeatability
limits in section 15
and table 2 of the
written standard.
If not, additional titrations
should be performed
until agreement is achieved.
If satisfactory titration
curves are not obtained,
the lead electrode
should be polished,
9.2 of the written standard.
Typical titration curves
for sulfate standards
in denatured ethanol are shown
in figure 1 of the written
standard, and examples from
commercial denatured ethanol
samples are shown in figure
2 of the written standard.