ANALYSIS OF VINEGAR BY TITRATION Lab 4 PURPOSE Students will use a pH probe to perform potentiometric titrations to determine the mass percent.
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Transcript ANALYSIS OF VINEGAR BY TITRATION Lab 4 PURPOSE Students will use a pH probe to perform potentiometric titrations to determine the mass percent.
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.
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.