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G-12
water, gently dry it, and immerse the probe into the solution to be titrated. You will titrate the acetic acid with
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 using this plastic rod.
Tabulate your data under the following headings on your Observations Sheet:
Buret Reading (mL) Measured pH
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 value shoot up from about 6 to about 9. This is all right because the pH must increase
when all of the acid has been neutralized. 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.
