Lab Activity
12 Buffer Solutions
Background
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,
as the sample reaction below illustrates.
HCl(aq)  Cl
-1
(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:
+1
-1
NaOH (s)  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, the
added acid will react with the conjugate base to create some of the weak acid, acetic
acid in this example.
HCl(aq) + CH3CO2-  HCH3CO2(aq) + ClBecause 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 it's conjugate base in the same solution. The more of each
component, the better the solution is at resisting pH changes.
In this lab, you will experiment with a buffer solution made of acetic acid and acetate
ions and will the measure the pH of the solution as you add strong acid and strong
base. You will compare these results that those obtained with pure water as a starting
solution.
Purpose
Determine the effect of a strong acid and strong base on the pH of water and on the
pH of a buffer solution.
Materials
PASCO & Other Equipment
•
PASPORT Xplorer GLX
•
eye dropper
•
PASPORT Chemistry Sensor
•
graduated cylinder, 100-mL
•
pH electrode
•
stirring rod
•
beakers (3), 50-mL
•
protective gear
•
0.1 M acetic acid (CH3CO2H),
10.0 mL
Consumables
•
water, 10.0 mL
•
1.0 M hydrochloric acid (HCl),
2.0 mL
•
0.1 M sodium acetate
(CH3CO2Na), 20.0 mL
•
1.0 M sodium
hydroxide (NaOH), 2.0 mL
Safety Precautions
•
Follow your teacher’s directions for using the equipment.
•
Wear safety glasses and follow standard laboratory safety procedures.
Pre-Lab Questions
1) Acetic acid (CH3COOH) is a weak acid and it does not completely dissociate in
solution. As it dissociates, an 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
1) Measure 20.0 mL of water into a 50-mL beaker.
Note: For more accurate results, first calibrate the pH Sensor. Refer to the Sensor Information Sheet
for more information.
Part B — Acetic Acid/Sodium Acetate
1) Measure 10.0 mL of acetic acid into a 50-mL beaker.
2) Add 10.0 mL of 0.1M sodium acetate
Xplorer GLX Setup
1) Connect the Chemistry Sensor to Port #1 on the GLX.
2) Connect the pH electrode to the Chemistry Sensor.
3) Press
,
to open the graph display.
4) If pH is not shown press
to activate the active fields. Press
select pH from the list of measurements.
again and
Record Data
Part A — Water
1) Place the pH electrode in the beaker of water.
1) Press
to begin recording the pH.
2) Watch the Graph display for the pH to stabilize. Record the pH in the appropriate
3) Using an eye dropper, add a drop of HCl and stir the solution.
4) Continue adding drops of HCl and stirring until you have added a total of 20 drops.
5) Press
again to end data collection.
6) Rinse the pH Sensor thoroughly with distilled water.
B — Acetic Acid
1) Place the pH electrode in the beaker of acetic acid/sodium acetate solution.
2) Press
to begin recording the pH.
3) Watch the Graph display for the pH to stabilize.
4) Using an eye dropper, add a drop of HCl and stir the solution.
5) Continue adding drops of HCl and stirring until you have added a total of 20 drops.
6) Press
again to end data collection.
7) Rinse the pH Sensor thoroughly with distilled water.
8) Repeat the entire procedure, parts A and B, adding NaOH instead of HCl.
Analyze
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?