Analysis of Vinegar by Titration

Lab 10

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 acidsuch 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

Acetic acid is a monoprotic weak acid that reacts with NaOH or KOH in a 1:1 ratio and

produces a single sigmoidal curve.

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

11.00

12.00

0.00 5.00 10.00 15.00 20.00 25.00

Volume, mL

pH

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!)

0

2

4

6

8

10

12

14

16

18

20

0.00 5.00 10.00 15.00 20.00 25.00

Volume, mL

Der

ivat

ive

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

11.00

12.00

pH

Spreadsheet

Time, s

pH Vol, mL

Deriv

0.01 3.02 0.02 0.00135.95 4.15 1.97 0.00060.24 4.65 5.21 0.00379.47 4.74 10.44 0.000

103.54 5.16 13.45 0.002142.46 5.34 16.11 0.010151.31 8.50 20.89 18.750187.66 10.58 21.52 1.960222.13 11.35 22.95 0.001268.71 11.73 25.36 0.000

Equivalence Point Volume

Half-equivalence Point Volume

pKa

Highest derivative

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- and Ka =

At the half-equivalence point, half the acid has been converted to its salt, so:

[HA] = [A-]Ka = [H+]pKa = pH

H A

HA

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.

Calculations• At the equivalence point:

moles of base = moles of acid(because they react in a 1:1 ratio)

• Moles of base = (Eq Pt Vol base, L) x ([base], M)

• Mass of acetic acid = (molar mass of acetic acid, g/mol) x (moles of acid, moles)

• Mass of vinegar = (density, g/mL) x (Vol vinegar, mL)

• Mass % = (mass of acetic acid / mass of vinegar) x 100%

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

• Only solutions with a pH between 6 and 8 can go down the drain.

• All other solutions need to go in the acid/waste container in the fume hood.

Lab 11 Reminder

• Lab 11 is next.

pH Probe Calibration

• Calibrate your pH probe according to the instructions in your lab manual.

• Be careful not to break the bulb.• Be careful not to contaminate the

buffer solutions.• Ask your instructor if you need

help.