Experiment: Buffer Solutions

Purpose:

Determine the effect on pH when a strong acid and a strong base are added to buffer solutions made from acetic
acid and sodium acetate.

To compare the behaviour of pure water and acetic acid when similar amounts of the same strong acid and strong
base are added.

Theory:

Acidic solutions are those that contain hydrogen ions, and the greater the concentration of the hydrogen ions, the
more acidic the solution and the lower the pH of the solution. Basic solutions are those that contain an excess of
hydroxide ions, and the greater the concentration of hydroxide ions, the more basic the solution and the higher the
pH of the solution.

Strong acids are acids that dissociate completely in water producing hydronium ions, H

3

O

+

, as the sample reaction

below illustrates.

HCl (aq) → H

+

(aq) + Cl

–

(aq)

Thus when a strong acid, such as HCl, is added to a solution, the concentration of hydrogen rises and the pH goes
down.

Strong bases are generally alkali metal hydroxides which dissociate completely in water producing hydroxide ions, as
the sample below illustrates:

NaOH (aq) → Na

+

(aq) + OH

–

( aq)

Thus, when a strong base, such as NaOH, is added to a solution, the concentration of hydroxide ions rises and the
pH goes up.

If a solution contains the conjugate base of a weak acid, such as acetate ions, CH

3

COO

–

, the added acid will react

with the conjugate base to create some of the weak acid, acetic acid CH

3

COOH in this example.

HCl (aq) + CH

3

COO

–

(aq) → CH

3

COOH (aq) + Cl

–

(aq)

Because the added hydrogen ions are bonded to the acetate ions, the added hydrogen ions do not decrease the pH
as much as when the same amount of acid was added to pure water. If hydroxide ions were added to the solution
with acetate ions, the hydroxide ions would not react and just become part of the solution thereby raising the pH. To
create a solution that resists change when acid or base is added, you need some weak acid and some of the
conjugate base of the weak acid in the same solution.

A solution that is able to resist change in pH when a strong acid or when a strong base is added is called a buffer.
Buffered solutions are almost always solutions that have both a weak acid and its conjugate base in the same
solution. The more of each component, the better the solution is at resisting pH changes.

Materials:

Measurement

PASPORT Xplorer GLX

PASPORT Chemistry Sensor

pH electrode

graduated cylinder, 100-mL

Lab Equipment eye dropper

stirring rod

beakers (3), 50-mL

protective gear

Chemicals

water, 10.0 mL

0.1 M acetic acid (CH

3

COOH), 10.0 mL

1.0 M hydrochloric acid (HCl), 2.0 mL

0.1 M sodium acetate (CH

3

COONa), 20.0 mL

1.0 M sodium hydroxide (NaOH), 2.0 mL

Safety:

• Follow your teacher’s directions for using the equipment.

• Wear safety glasses and follow standard laboratory safety procedures.

Pre-Lab Questions
1. Acetic acid (CH

3

COOH) is a weak acid and it does not completely dissociate in solution. As it dissociates, equilibrium will be

established. Write the complete ionic equation for this equilibrium.

2. Sodium acetate is a strong electrolyte and completely dissociates when added to water. Write the complete ionic equation for this

reaction.

3. The pH of a 0.1 M sodium acetate solution is about 8.5. What will happen to the pH of 1.0 M acetic acid (pH 3.0) if you add some

of the sodium acetate solution? (Hint: refer to the equations you wrote above.)

Procedure

Equipment Setup

Part A — Water


Measure 20.0 mL of water into a 50-mL beaker.

Part B — Acetic Acid/Sodium Acetate


Measure 10.0 mL of acetic acid into a 50-mL beaker.



Add 10.0 mL of 0.1M sodium acetate

Xplorer GLX Setup



Connect the Chemistry Sensor to Port #1 on the GLX.



Connect the pH electrode to the Chemistry Sensor.



Press

,

to open the graph display.



If pH is not shown press

to activate the active fields. Press

again and select pH from the list of

measurements.

Record Data

Part A — Water


Place the pH electrode in the beaker of water.



Press

to begin recording the pH.



Watch the Graph display for the pH to stabilize. Record the pH in the appropriate



Using an eye dropper, add a drop of HCl and stir the solution.



Continue adding drops of HCl and stirring until you have added a total of 20 drops.



Press

again to end data collection.



Rinse the pH Sensor thoroughly with distilled water.

Part B – Acetic Acid


Place the pH electrode in the beaker of acetic acid/sodium acetate solution.



Press

to begin recording the pH.



Watch the Graph display for the pH to stabilize.



Using an eye dropper, add a drop of HCl and stir the solution.



Continue adding drops of HCl and stirring until you have added a total of 20 drops.



Press

again to end data collection.



Rinse the pH Sensor thoroughly with distilled water.



Repeat the entire procedure, parts A and B, adding NaOH instead of HCl.

Analysis



Record calculations in your data tables as you complete your analysis.

Analysis and Synthesis Questions

1. How did the pH change with the addition of hydrochloric acid to each solution?

2. How did the pH change with each addition of sodium hydroxide?

3. Examining your graph displays, describe any differences between how pH changes occurred in the water beakers compared to the

buffer solutions?)

4. What is the purpose of a buffer solution and where might you find one in the "real world"?

5. Buffer capacity is a term used to describe how much acid or base a buffer can react with while still exhibiting only a small change

in pH. In essence, buffer capacity can be seen in the length of the portion of the graph before the pH changed. What determines
the buffer capacity and how could it be increased?

6. Is the purpose of an antacid to buffer or neutralize stomach acid?