LAB – PROPERTIES OF BUFFER SOLUTIONS
NAME______________________
Introduction
Salts that react with water play an important role in buffer systems. Buffers usually consist of solutions of
a weak acid and its salt, or of a weak base and its salt. In other words, a buffer can be created by the
partial neutralization of a weak acid by a strong base, or by partial neutralization of a weak base by a strong
acid. For example, a solution containing acetic acid, (HC2H3O2), and its salt, sodium acetate, (NaC2H3O2),
is a buffer. Sodium acetate is formed in the reaction between acetic acid and the strong base, sodium
hydroxide. Buffered solutions maintain a relatively constant pH when limited amounts of acid or base are
added to them. An important buffering system in the blood is the carbonic acid/bicarbonate system. This
buffering system helps to maintain the blood at a pH between 7.35 and 7.45. The chemical reactions in this
system that buffer against hydrogen ions or hydroxide ions produced in the body are as follows.
HCO3-1 + H+  H2CO3
H2CO3 + OH-1  HCO3-1 + H2O
In this experiment, you will examine the effectiveness of different buffer systems in resisting changes in
pH.
Equipment
2 50-mL beakers, 7 medium test tubes, test tube rack, 2 Beral pipets 10-mL graduated cylinder, pH meter
Materials
0.1 M sodium dihydrogen phosphate (NaH2PO4), boiled distilled water, paper towels, 1M hydrochloric
acid, (HCl), 1 M sodium hydroxide, (NaOH), 0.1 M sodium carbonate, (Na 2CO3), 0.1 M sodium
bicarbonate, (NaHCO3), 0.1 M sodium monohydrogen phosphate, (Na 2HPO4), universal indicator
Safety
Hydrochloric acid and aqueous sodium hydroxide are irritants at the concentrations used here. If you spill
either of these solutions on yourself, immediately flush the affected area with water for 2-3 minutes and
notify the teacher. If either of these solutions should get into your eyes, begin flushing your eyes with
running water immediately and continue doing so for at least 20 minutes. If there is an eye wash fountain
with continuously running water in the laboratory, use it.
Procedure
1. Label seven medium test tubes with the numbers 1-7.
2. Mix 5 mL of 0.1M Na2CO3 and 5 mL of 0.1M NaHCO3 in a 50-mL beaker. This is a carbonate/
bicarbonate (CO3-2 / HCO3-1) buffer. Divide this buffer solution equally between test tubes 1 and 2.
3. Mix 5 mL of 0.1 M Na2HPO4 and 5 mL of 0.1M NaH2PO4 . This is a monohydrogen phosphate/
dihydrogen phosphate (HPO4-2 / H2PO4-1) buffer. Divide this buffer equally between test tubes 3 and 4.
4. Put 5 mL of 0.1M NaHCO3 in test tube 5, 5 mL of 0.1M NaH2PO4 in test tube 6, and 5 mL of cool,
previously boiled distilled water in test tube 7. Put a drop of universal indicator in all 7 tubes.
5. Measure the pH of each solution by using the pH meter. Record the results in the table provided.
6. Use a dropper pipet to add 1 drop of 1M HCl to tubes 1, 3, 5, 6, and 7. CAUTION: 1M HCl is an irritant.
Shake the solutions, measure the pH, and record the results.
7. Use a dropper pipet to add 2 drops of 1M NaOH to the tubes (1,3, 5, 6, and 7) that received the HCl
solution. Caution: 1M NaOH is an irritant. Flick the solutions and measure and record the pH.
8. Add hydrochloric acid to tubes 2 and 4, dropwise and with flicking, until the pH drops to about 2.
Record the number of drops required for this to occur.
9. Rinse all the test tubes with running tap water.
LAB – PROPERTIES OF BUFFER SOLUTIONS
NAME______________________
Data
Tube
Contents of Tube
1
CO3-2 / HCO3-1
2
CO3-2 / HCO3-1
3
HPO4-2 / H2PO4-1
4
HPO4-2 / H2PO4-1
5
0.1 M NaHCO3
6
0.1 M NaH2PO4
7
boiled distilled water
Initial pH
pH after adding
1 drop HCl
pH after adding
2 drops NaOH
Number of drops
of HCl added
Conclusions
1. Based on your data, do the carbonate (tubes 1 & 2) and phosphate (tubes 3 & 4) solutions appear to be
buffer systems? Explain your answer.
2. Do 0.1M NaHCO3 (Tube #5) and 0.1 M NaH2PO4 (Tube #6) solutions buffer as effectively as the
carbonate and phosphate systems? Explain your answer.
3. Write equations for the reaction of the HPO4-2 / H2PO4-1 buffer system with an acid and a base.