University of Puget Sound
Department of Chemistry
Chem 111
Spring, 2010
BUFFER CAPACITY AND POLYPROTIC ACIDS
LEARNING GOALS AND ASSESSMENTS
1. Be familiar with qualitative differences in titration curves of various kinds of analyte.
a. Describe qualitative differences between the titration of water vs a weak acid.
b. Describe qualitative differences between the titration of a weak vs strong acid.
c. Sketch the titration curve of a given polyprotic acid, making it quantitative at the pKas.
2. Be familiar with the preparation of buffers.
a. Demonstrate how to make a buffer of desired pH from given stock solutions.
INTRODUCTION
Buffer capacity is defined as the amount of strong base (or acid) required to change the pH of a
buffer by 1 pH unit. In Part I of this experiment, you will explore what this means in terms of
actual buffers. In part II, you will prepare a buffer of a desired pH. Lastly, you will investigate pH
properties of a polyprotic acid.
EXPERIMENTAL PROCEDURE:
Solutions:
NaOH (0.010 M and 0.10 M)
CH3COOH and NaCH3COO (0.010 and 0.10 M)
H3PO4 (0.010 M)
EXPERIMENTS:
I.
Buffer capacity.
Buffer #1 will be pure water. Put 100 mL of DI water into an Erlenmeyer flask and label it.
For Buffer #2, combine 50 mL of 0.010 M CH3COOH with 50 mL of 0.010 M
NaCH3COO, and label appropriately.
For Buffer #3, combine 50 mL of 0.10 M CH3COOH with 50 mL of 0.10 M NaCH3COO,
and label appropriately.
Record the pH of all three buffers, taking care to wash the pH probe well with DI between
readings. Then, carry out an experiment to determine the buffer capacity the three solutions.
Record your result as mL of NaOH per mL of buffer. For proper comparison, be sure to use
the same stock solution of NaOH for all three.
Q.1. State which buffer has the highest capacity, and explain why.
II.
Preparation of a buffer. Using only the provided solutions of acetic acid and sodium
acetate, prepare 100-200 mL of a buffer with pH=4.0. It will help to carry out a calculation
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using the Henderson-Hasselbalch equation first. Show your solution to your TA or
instructor before proceeding to part III.
III. Titration of a polyprotic acid. Using as analyte 15.0 mL of 0.010 M H3PO4, titrate with
0.010 M NaOH. Keep going until you reach pH > 11, and save a copy of the titration graph.
You may wish to confer with your instructor or TA as to the interpretation of this curve.
WHAT TO DO
Before Lab: Read this handout carefully. In your Lab Notebook, write your title page,
objectives and an outline of the procedures. This will be your pre-lab.
During the Lab: Do all of the investigations assigned to you as described above.
To Be Turned in at the Beginning of the Next Lab:
1. Buffer capacities and your answer to the question in part I.
2. A description of how you prepared the buffer in part II.
3. A plot of the titration curve obtained in part III, with buffer regions circled. Above each
buffer region, write the relevant titration reaction equation.
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