ANALYSIS OF VINEGAR BY TITRATION
Lab 4
PURPOSE
Students will use a pH probe to perform potentiometric
titrations to determine the mass percent of acetic acid in
vinegar and the pKa of acetic acid.
POTENTIOMETRIC TITRATIONS
 Potentiometric titrations can be used to determine:
 proticity (how many acidic hydrogens are donated to
solution)
 the amount or concentration of acid or base present
(using MaVa= MbVb)
 pKa (the “– log” of the acid dissociation constant)
PROTICITY
Titration curve of a
monoprotic acid
such as HCl
(hydrochloric acid)
Titration curve of a
diprotic acid such as
H2SO3 (sulfurous acid)
Titration curve of a
triprotic acid such as
H3PO4 (phosphoric acid)
PROTICITY CONTINUED
Acetic acid is a monoprotic weak acid that reacts with NaOH or
KOH in a 1:1 ratio and produces a single sigmoidal curve.
12.00
11.00
10.00
9.00
8.00
pH 7.00
6.00
5.00
4.00
3.00
2.00
0.00
5.00
10.00
15.00
Volume, mL
20.00
25.00
EQUIVALENCE
20
18
16
14
12
10
8
6
4
2
0
0.00
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
3.00
5.00
10.00
15.00
Volume, mL
20.00
25.00
pH
Derivative
The equivalence point volume is determined by plotting a derivative
curve of the titration curve. The steepest point on the derivative
curve corresponds to the equivalence point volume. (Find the
exact value on your spreadsheet!)
SPREADSHEET
Time, s
0.01
35.95
60.24
79.47
103.54
142.46
151.31
187.66
222.13
268.71
pH
Vol, mL
3.02
0.02
4.15
1.97
4.65
5.21
4.74
10.44
5.16
13.45
5.34
16.11
8.50
20.89
10.58
21.52
11.35
22.95
11.73
25.36
Deriv
0.001
0.000
0.003
0.000
0.002
0.010
18.750
1.960
0.001
0.000
pKa
Half-equivalence
Point Volume
Highest
derivative
Equivalence
Point Volume
EQUIVALENCE POINT VOLUME
 The concentration of acetic acid can be determined from:
 the equivalence point volume of the base
 the concentration of the base
 the volume of the acid used in the titration
 The equation to use is:
 MaVa = MbVb
PKA
The pKa of acetic acid is the pH at the half-equivalence
point volume of the titration, because:
For a weak acid:
HA
H+ + A-
H   A  
and Ka =
HA 
At the half-equivalence point, half the acid has been converted to
its salt, so:
[HA] = [A-]
Ka = [H+]
pKa = pH
VINEGAR TITRATION
 Make the required dilution of vinegar.
 Calibrate your pH probe.
 Titrate the specified aliquots to obtain titration curves.
 Determine the volume of base delivered at each time
point, using the base delivery rate.
 Graph the derivative of your titration curves following the
instructions in the manual.
 Make up a spreadsheet that will allow you to calculate the
indicated values.
 Perform statistical analysis on your data and complete
your report form.
EQUIPMENT SETUP
SAFETY CONCERNS
 Reagents:
Acetic Acid (1 N)
 Sodium Hydroxide (0.1 N) / Potassium Hydroxide (0.1 N)

 Eye Contact:
 Irritation, tearing, redness, pain, impaired vision, severe burns and irreversible
eye injury.
 Skin Contact:
 Severe skin irritation, soreness, redness, destruction of skin (penetrating
ulcers) . May cause sensitization and / or allergic reaction.
 Inhalation:
 May cause coughing, serious burns, pneumonitis, pulmonary edema, and
coma.
 Ingestion:
 Toxic. Corrosive to mucous membranes. May cause perforation of the
esophagus and stomach, abdominal pain, nausea, vomiting, diarrhea, general
gastro-intestinal upset.
WASTE
 Dispose of waste in the appropriate waste receptacles.
 Acidic and basic solutions / waste need to be disposed in
the acid/base waste container in the fume hood.
 Solutions with a pH between 6 and 8 can be disposed
down the drain.
LAB 5 REMINDER
Read the required reading sections in your
textbook and lab manual as you prepare for the
next experiment.
Complete and submit the pre-lab questions.
Study for your quiz.
Submit your Lab 4 Report.