Lab Manual
CBB20303 BIOCHEMISTRY
EXPERIMENT 2
ACID-BASE TITRATION
Lab Objectives:

To determine the concentration of acetic acid by titration with a standard sodium
hydroxide solution.
Introduction:
Titration is used to calculate the amount of unknown acid by measuring the amount of base,
or titrant, it takes to neutralize the acid. There are two major ways to know when the solution
has been neutralized. The first uses a pH meter in the receiving flask adding base slowly until
the pH reads exactly 7. The second method uses an indicator. An indicator is an acid or base
whose conjugate acid or conjugate base has a color different from that of the original
compound. The color changes when the solution contains a 1:1 mixture of the differently
colored forms of the indicator. From the Henderson-Hasselbalch equation, the pH equals the
pKa of the indicator at the endpoint of the indicator. Since we know the pH of the solution and
the volume of titrant added, we can then deduce how much base was needed to neutralize the
unknown sample.
Titration Curves
A titration curve is drawn by plotting data attained during a titration, titrant volume on the xaxis and pH on the y-axis. The titration curve serves to profile the unknown solution. In the
shape of the curve lies much chemistry and an interesting summary of what we have learned
so far about acids and bases.
If a strong base is used to titrate a weak acid, the pH at the equivalence point will not be 7.
There is a lag in reaching the equivalence point, as some of the weak acid is converted to its
conjugate base. You should recognize the pair of a weak acid and its conjugate base as a
buffer. We see the resultant lag that precedes the equivalence point, called the buffering
region. In the buffering region, it takes a large amount of NaOH to produce a small change in
the pH of the receiving solution.
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Lab Manual
CBB20303 BIOCHEMISTRY
Figure 1: Titration curve of a strong base titrating a weak acid
Because the conjugate base is basic, the pH will be greater than 7 at the equivalence point.
You will need to calculate the pH using the Henderson-Hasselbalch equation, and inputting
the pKb and concentration of the conjugate base of the weak acid.
Materials
Sodium hydroxide
White vinegar
Distilled water
Mass cylinder
Biuret
Support and clamp
Beaker
pH meter
Method
1.
2.
3.
4.
5.
Prepare 50 ml of 10 times dilution of vinegar solution (acetic acid, CH3CO2H).
Set the apparatus as in Figure 2.
Fill a buret with a 0.1 M solution of sodium hydroxide (50 ml), and allow the liquid in
the buret to come down to the zero mark and close the tap.
Slowly drip 0.5 – 1 ml of sodium hydroxide into the solution to be titrated and mix
slowly with stirrer. After the pH is stable, record the pH and volume of sodium
hydroxide added.
Record until pH of solution reach 11. Plot the graph of pH solution against total
volume of sodium hydroxide added.
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Lab Manual
CBB20303 BIOCHEMISTRY
Figure 2: Titration setting
Result and Discussion
1. Write the chemical equation for this particular acid-base reaction. Determine type of
acid and base used (strong or weak).
2. Determine equivalence point of this particular reaction.
3. Find out the molarity of acetic acid in the titrated solution and percentage of acetic
acid used in vinegar.
4. Using Henderson-Hasselbach equation, determine the acid dissociation constant (Ka
and pKa) of acetic acid.
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