1. Analysis of Vinegar by Titration
Lab 4

2. Outline Purpose Potentiometric Titrations Vinegar Titration
Proticity
Equivalence
pKa
Vinegar Titration
Safety Concerns
Waste
Next Lab Reminder

3. 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.

4. 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)

5. Proticity
Acetic acid is a monoprotic weak acid that reacts with NaOH or KOH in a 1:1 ratio and produces a single sigmoidal curve.

6. Equivalence
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!)

7. 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

8. 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- and Ka =
At the half-equivalence point, half the acid has been converted to its salt, so:
[HA] = [A-]
Ka = [H+]
pKa = pH

9. 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.

10. Spreadsheet Time, s pH Vol, mL Deriv 0.01 3.02 0.02 0.001 35.95 4.15
1.97
0.000
60.24
4.65
5.21
0.003
79.47
4.74
10.44
103.54
5.16
13.45
0.002
142.46
5.34
16.11
0.010
151.31
8.50
20.89
18.750
187.66
10.58
21.52
1.960
222.13
11.35
22.95
268.71
11.73
25.36
pKa
Half-equivalence Point Volume
Highest derivative
Equivalence Point Volume

11. Safety Concerns Reagents: Eye Contact:
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.

12. 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.

13. 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.

14. After you are finished, did you…
Wash, dry and put away your glassware?
Put your buret clamp back in the drawer!?
Turn off your hotplate?
Turn off your computer (if your class is the last class for the day)?
Turn off your MicroLAB™ interface?
Turn off your monitor (if your class is the last class for the day)?
Wipe down your counter?