D2896_12
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Calculate the quantity of sample
required from its expected base
number as follows--
approximate weight
of sample, g, grams,
equals 10 divided by the
expected base number.
For the back
titration procedure,
see 17.2 of the
Written Standard.
It may be necessary to use
a smaller sample weight.
Weigh the sample into
the titration beaker,
applying the limits
shown as follows.
A maximum of 10 grams should
be taken for analysis.
It is especially
important for procedure B
that great care be exercised
in obtaining accurate weights,
particularly for the high
base number samples which
require small sample weights.
Add 60 milliliters of titration
solvent to the sample.
Place the sample on
the titration stand
and stir the solution until
the sample is dissolved.
If the solvent of the
sample proves difficult,
dissolve it in 40 milliliters of
chlorobenzene in the titration
beaker.
Then add 20 milliliters
of glacial acetic acid.
Prepare the
electrodes as directed
in 10.1, 10.2, and 10.3
of the Written Standard.
Position the electrodes
in the solution
so that they are immersed
as far as possible.
Continue stirring
throughout the determination
at a rate sufficient to
produce vigorous agitation
without spattering and without
stirring air into the solution.
Adjust the meter so that
it reads in the upper part
of the millivolt scale--
for example, 700 millivolts.
For simple meters
without this adjustment,
it may be necessary
to incorporate
a source of potential in
series with the electrode.
A 1.5-volt dry cell and
potential divider is suitable.
Fill the burette with 0.1
normal perchloric acid solution
and place the
burette in position
in the titration
assembly, taking care
that the tip is immersed
below the level of the surface
of the liquid in the beaker.
Record the initial burette and
meter cell potential readings.
Add suitable small
portions of titrant.
And after waiting until
a constant potential
has been established--
note 12 of the
Written Standard--
record the burette
and meter readings.
At the start of the titration
and in any subsequent regions,
inflections, where 0.1
milliliters of titrant
consistently produces a total
change of more than 0.03 volts,
corresponding to 0.5 pH scale
unit in the cell potential,
add 0.05 milliliters portions
in the intermediate regions,
plateaus, where 0.1 millimeter
increments change the potential
by less than 0.03 volts.
Add large portions
sufficient to produce
a total potential change
approximately equal to but not
greater than 0.03 volts.
Titrate in this manner
until the potential
changes less than 0.005 volts,
corresponding to 0.1 pH scale
unit per 0.1 millimeters.
Consider the cell
potential constant
when a change is less than
0.005 volts per minute.
Adjust the instrument
in accordance
with the manufacturer's
instructions
and set the titration
speed at 1.0 milliliters
per minute maximum.
On completion of the
titration, remove the beaker
and proceed with cleaning
of the electrodes.
See 10.3 of the
Written Standard.
Store the electrodes in
water when not in use.
See 10.1 of the
Written Standard.
For each set of samples, make
a blank on 60 milliliters
of titration solvent.
For a manual titration, add 0.1
normal perchloric acid solution
in 0.05 milliliter increments,
waiting between each addition
until a constant cell
potential is established.
Record meter and burette
readings after each increment.
Follow the procedure in
12.6.2 of the Written Standard
for an automatic titration.