Preparation and Properties of Buffer Solutions
Experiment Overview
In this experiment the pH of water and a number of solutions will be measured, and then acids and bases
will be added to see how the pH is affected. Several buffer solutions of different pH values will be
prepared and tested for their ability to resist change in pH.
Materials
Balance, 0.01 g precision
Beakers, 100 mL – 4
Pipet, dropper – 3
Buret, ring stand, buret clamp
Erlenmeyer flasks, 250 mL – 2
Graduated cylinders, 25 mL & 100 mL
pH meter
Stirring rod
Wash bottle & distilled water
Acetic acid, 0.10 M, 20mL
Ammonia, 0.10 M, 20mL
Ammonium chloride, 0.10 M, 20mL
Hydrochloric acid, 0.10 M, 5 mL
Phenolphthalien, 1.0%, 1mL
Sodium acetate, 0.10 M, 20 mL
Sodium Chloride 0.10 M, 40 mL
Sodium hydroxide 0.10 M, 5 mL
Unknown solid acid sample, 2.0 g
Safety Precautions
Sodium hydroxide solution and ammonia solution are txoic by inhalation, ingestion, and are corrosive to
all body tissues. Hydrochloric acid solution is toxic by ingestion or inhalation and is corrosive to skin and
eyes. Phenolphthalein is an alcohol solution that is flammable and is moderately toxic. Wear chemical
splash goggles, chemical resistant gloves, and a chemical resistant apron. Wash hands thoroughly with
soap and water before leaving the laboratory.
Procedure
Part 1. ph Changes in Non-buffered Solutions
1. Place 20 mL of distilled or de-ionized water in a 100 mL beaker. Test the pH using a pH meter.
Record the pH.
2. Using a pipet, add one drop of 0.10 M HCl, stir the solution and again test the pH and record.
3. Repeat step 2 with a second and third drop of 0.10 M HCl
4. Repeat steps 1-3 with 20 mL of 0.10 M NaCl solution.
5. Place 20 mL of distilled or de-ionized water in a 100 mL beaker. Test the pH using a pH meter.
Record the pH.
6. Using a pipet, add one drop of 0.10 M NaOH, stir the solution and again test the pH and record.
7. Repeat step 6 with a second and third drop of 0.10 M NaOH
8. Repeat steps 5-7 with 20 mL of 0.10 M NaCl solution.
Part 2. pH Changes in Buffered Solutions
9. To prepare a buffer, use 25 mL graduated cylinder to add 10 mL of 0.10 M HC2H3O2 and 10.0 mL of
0.10 M NaC2H3O2 to a 100 mL beaker.
10. Measure the pH of the buffer solution and record.
11. Add 1 drop of 0.10 M HCl, stir, measure and record the pH.
12. Repeat step 11 for two additional drops of 0.10 M HCl.
13. Prepare a fresh sample of the HC2H3O2 - C2H3O2- buffer as in step 9.
14. Repeat steps 11 and 12 by adding 0.10 M NaOH.
15. Prepare a basic buffer by combining 10.0 mL of 0.10 M NH3 and 10.0 mL of 0.10 M NH4Cl in a 100
mL beaker.
16. Repeat steps 11 and 12 for this basic buffer solution.
17. Repeat step 14 by preparing a fresh sample of the basic buffer and testing it with a dropwise addition
of three drops of 0.10 M NaOH.
Part 3. Preparation of Buffer of a Given pH
18. Weigh out 1.8 to 2.0 g of a solid acid assigned to you.
19. Dissolve the acid in 150 mL of distilled or de-ionized water in a 250 mL Erlenmeyer flask (flask 1).
20. Pour 75 mL of this solution into a second 250 mL Erlenmeyer flask (flask 2), add 2 drops of
phenolphthalein solution, and titrate to the phenolphthalein color change with 0.20 M NaOH. Record
the volume of titrant.
You now have a solution of a weak acid in flask 1, and a solution of the sodium salt of the weak acid
(its conjugate base) in the second flask 2.
Make the concentrations of the two solutions the same by adding the same volume of distilled water
to the acid flask 1 as the volume of NaOH you used to titrate the solution in flask 2.
21. Combine 10.0 mL of your weak acid solution in flask 1 with 10.0 mL of the conjugate base solution in
flask 2, mix, and measure the pH and record. This is the pKa of the acid.
22. Your teacher will assign you the pH of a buffer to prepare. Calculate the volume of weak acid and
conjugate base that you will need to prepare 50 mL of a buffer of the assigned pH. Record these
values. Because the concentration of the two solutions are the same , the volume rations will give the
needed concentration ratios:
[base]
volume conjugate base
(50 – x)
=
=
[acid]
volume conjugate acid
x
23. Measure out the volumes of acid and conjugate base, mix then measure and record the pH.
24. Test your buffer solution to see if it resists changes in pH by measuring 20.0 mL of the and adding 3
drops of 0.10 M HCl, and repeating with 20.0 mL of buffer and 3 drops of NaOH. Record the pH
values after each addition.
Calculations and Analysis
1. In step 1 how did the strong acid affect the pH of the distilled water?
2. Compare the pH values of steps 1 and 2. Did the addition of NaCl, a salt of a strong acid and a
strong base, buffer the pH changes? Explain.
3. Compare the pH values of steps 1 and 3. How did the strong base affect the pH of distilled water?
4. Compare the pH values of steps 1 and 4. Did NaCl, the salt of a strong acid-strong base, buffer the
pH changes? Explain.
5. Compare the pH values of steps 1 and 5. How did the acetic acid-acetate ion buffer affect the pH
changes with HCl? Explain. Write a chemical equation to show the reaction of HCl with the buffer.
6. Compare the pH values of steps 1 and 6. How did the acetic acid-acetate ion buffer affect the pH
changes with NaOH? Explain. Write a chemical equation to show the reaction of NaOH with the
buffer.
7. Compare the pH values of steps 1 and 7. How did the ammonia-ammonium ion buffer affect the pH
changes with HCl? Explain. Write a chemical equation to show the reaction of HCl with the buffer.
8. Compare the pH values of steps 1 and 8. How did the ammonia-ammonium ion buffer affect the pH
changes with NaOH? Explain. Write a chemical equation to show the reaction of NaOH with the
buffer.
9. Did your prepared buffer match your assigned pH value? Explain.
10. Comment on the ability of your prepared vuffer to resist the change of pH when HCl and NaOH were
added in step 24. Explain.