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.)
