Lab 6: Buffer Solutions
Objective: Learn to make buffer solutions and study buffer characteristics.
Background:
Buffers are used to maintain a constant pH of a solution, even when other acids
and bases are added. Buffers contain one substance that can act like an acid (neutralizing
added base) and one substance that can act like a base (neutralizing added acid.)
Usually, buffers are made of conjugate acid-base pairs, so that the buffer does not
neutralize itself. In this lab, we be testing the buffering capacity of water and two
different solutions: acetic acid/acetate and ammonia/ammonium:
Acetic acid: CH3COOH
Ammonium: NH4+
&
&
Acetate: CH3COOAmmonia: NH3
Note: sodium (in the acetate buffer) and chloride (in the ammonium buffer)
are spectator ions, so we don’t need to include them in chemical reactions.
When excess acid (in the form of HCl in this lab) is added, the base in the buffer reacts to
neutralize it. When excess base (in the form of NaOH in this lab) is added, the acid in the
buffer reacts to neutralize it. After a certain point, the buffer capacity will be exceeded
and it can no longer neutralize any additional acid or base.
Pre-Lab Questions (To be submitted at the beginning of lab on a separate sheet
of paper)
1.
2.
3.
4.
What substance in the acetic acid/acetate buffer reacts with HCl?
What substance in the acetic acid/acetate buffer reacts with NaOH?
What substance in the ammonia/ammonium buffer reacts with HCl?
What substance in the ammonia/ammonium buffer reacts with NaOH?
Procedure:
Part 1: Making the Acetic Acid / Acetate Buffer
1. Put approximately 10-15 mL of 0.100 M acetic acid in one small beaker and
approximately 10-15 mL of 0.100 M sodium acetate in another small beaker.
2. Obtain a 5 mL volumetric pipette and rinse it out with DI water.
3. Label two large test tubes as acetic acid / acetate. Using the rinsed 5 mL
volumetric pipette, put 5 mL of the acetic acid (obtained in step 1) and 5 mL of
sodium acetate (obtained in step 1) to each of the two test tubes. These two test
tubes now contain acetic acid/acetate buffer. You will use one of these in Part 3
and one of these in Part 4.
Part 2: Making the Ammonia / Ammonium Buffer
1. Put approximately 10-15 mL of 0.100 M ammonium hydroxide in one small
beaker and approximately 10-15 mL of 0.100 M ammonium chloride in another
small beaker.
2. Rinse out your 5 mL volumetric pipette with DI water.
3. Label two large test tubes as ammonia / ammonium. Using the rinsed 5 mL
volumetric pipette, put 5 mL of the ammonium hydroxide (obtained in step 1)
and 5 mL of ammonium chloride (obtained in step 1) to each of the two test
tubes. These two test tubes now contain acetic acid/acetate buffer. You will use
one of these in Part 3 and one of these in Part 4.
4. Rinse the pipettes with DI water and return to the cart.
Part 3: Buffer capacity when adding a strong acid
1. Put 10 mL of water in an empty large test tube. Label it as water.
2. Using a pH meter and LabQuest, measure and record the pH of the water.
3. Remove the pH meter, then add 2 drops of 0.500 M HCl to the water and swirl
the test tube gently (while the pH meter is removed from the test tube).
4. Submerge the end of the pH meter in the solution to measure and record the pH.
5. Add an additional 2 drops of 0.500 M HCl to the test tube (swirl gently after
removing the pH meter), then measure and record pH.
6. Repeat step 8 until a total of 12 drops are added.
7. Rinse the pH probe with DI water and dab gently with a KimWipe.
8. Obtain one of the test tubes from Part 1 (acetic acid / acetate buffer). Measure
and record the pH.
9. Repeat steps 3-7 with the acetic acid / acetate buffer.
10. Obtain one of the test tubes from Part 2 (ammonia / ammonium buffer). Measure
and record the pH.
11. Repeat steps 3-7 with the ammonia / ammonium buffer.
Part 4: Buffer capacity when adding a strong base
1. Put 10 mL of water in an empty large test tube. Label it as water.
2. Measure and record the pH of the water.
3. Remove the pH meter, then add 2 drops of 0.500 M NaOH to the water and swirl
the test tube gently (while the pH meter is removed from the test tube).
4. Submerge the end of the pH meter in the solution to measure and record the pH.
5. Add an additional 2 drops of 0.500 M NaOH to the test tube (swirl gently after
removing the pH meter), then measure and record pH.
6. Repeat step 8 until a total of 12 drops are added.
7. Rinse the pH probe with DI water and dab gently with a KimWipe.
8. Obtain one of the test tubes from Part 1 (acetic acid / acetate buffer). Measure
and record the pH.
9. Repeat steps 3-7 with the acetic acid / acetate buffer.
10. Obtain one of the test tubes from Part 2 (ammonia / ammonium buffer). Measure
and record the pH.
11. Repeat steps 3-7 with the ammonia / ammonium buffer.
Part 5: Clean up
1. Rinse the pH probe with DI water, dab gently with a KimWipe and put it back
into the solution it came in.
2. Put all waste into the beaker in the fume hood.
3. Power off the LabQuest and return it to the cabinet in the back of the room.
Data Tables:
Drops of HCl
added
pH Beaker 1
Acetic Acid
pH Beaker 2
Ammonia
pH Beaker 3
Water
pH Beaker 1
Acetic Acid
pH Beaker 2
Ammonia
pH Beaker 3
Water
0 drops
Drops of
NaOH added
0 drops
Post-Lab:

Your data tables neatly filled out with data

Two graphs: (Don’t forget to label your axes and put a title on your graph!)
o Plot the pH (y-axis) versus drops of NaOH added (x-axis), for each beaker 1-3.
o Plot the pH (y-axis) versus drops of HCl added (x-axis), for each beaker 1-3.

Brief discussion comparing the three buffer solutions. Describe possible sources of error.
Please be specific.

Typed answers to the following questions:
1. According to your graphed data is one buffer (beaker 1-3) better or worse
than the other(s)? Explain.
2. Write out the chemical reaction that occurs in the Acetic Acid / Acetate
buffer when:
a. NaOH is added
b. HCl is added
3. Write out the chemical reaction that occurs in the Ammonia/Ammonium
buffer when:
a. NaOH is added
b. HCl is added
4. Does distilled water make a good buffer? Why or why not?
5. What happens when you go beyond the buffer capacity? How is buffer
capacity indicated on your graph(s)?