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G-10
standardized NaOH solution.
Clean your plastic stirring rod and place it in the acid solution.
During the
titration, you should stir the solution
rapidly but carefully after each addition using this plastic rod.
Record the initial pH of the solution
to 2 decimals before any NaOH is added. Begin
the pH titration by
adding the NaOH solution in approximately
2 mL increments. After each addition read the buret
(to 2 decimals: DO
NOT try to set these exactly to whole
mL additions, for example 2.00 mL) and read the pH
meter; record your readings.
Plot this point on your graph.
If the solution changes color, record
on your observation sheet the pH values between which this change
occurs. As you approach the
end point of the titration (within 5 mL of the volume used in Part A),
gradually
reduce the volume of NaOH added
in each increment until the base is being added dropwise. After
each drop,
stop and stir the solution.
Check the pH. In this region surrounding the
equivalence point, you should
record your buret reading and the
pH value only if the volume of titrant added has changed by 10
drops, OR if the pH value has changed
by 0.2 units.
Near the equivalence point, the
pH value will increase rapidly on addition of a drop or two of NaOH.
You
might see the pH
dramaticaly increase from about 6 to about 9. This is expected
because the pH must
increase when all of the acid has
been neutralized. Note: If you think you have overshot
the equivalence point
(pH jumps up past 10), immediately
consult with your laboratory instructor before restarting. Continue
the
titration drop by drop, recording
your buret reading and the pH value each time that the volume has changed
by
10 drops or the pH value has changed
by at least 0.2 units.
When you are about 5 mL past the
equivalence point, resume adding the NaOH solution in
2 mL increments.
Continue until you have added about 15 mL
of NaOH past the equivalence point. NOTE:
In this basic region,
record data whenever the volume of titrant added has changed by
2 mL OR the pH
value has changed by 0.2 units.
A summary of the above information
for the addition of NaOH through the pH titration is given in the following
table. Add the NaOH in increments
so that your titration curve will look like the one given in Figure G-3.
The following information summarizes
the format that should be used for the NaOH additions:
i)
start to pH ~ 5.2
add NaOH in about 2
mL increments.
ii)
pH 5.2 to pH 5.8
add NaOH in about 0.5
mL increments.
iii)
pH 5.8 to pH 11.5
add NaOH in about 2
to 3 DROPS at a time.
(NOTE: In this region only,
record data only if the
pH is > 0.2 pH units
or after the addition of 10 drops
of NaOH.)
iv)
pH 11.5 to pH 12.5
add NaOH in about 0.5
mL increments.
v)
pH 12.5 to end
add NaOH in about 2
mL increments.
