Buffer Solutions
It is often difficult for the beginning science student to appreciate the ability of buffer solutions to resist changes in pH. Using Bogen universal indicator solution, a microplate, and small quantities of only a few solutions, the property of buffer capacity can be studied on microscale.
Mix 2 parts 0.1 M acetic acid and one part 0.1 M sodium hydroxide to make a buffer solution whose pH is approximately 4.7. Add a drop of Bogen universal indicator solution and note the color. From the Bogen universal indicator color chart, find the color of the solution if the pH were to increase one unit. Add 1 M sodium hydroxide one drop at a time until the pH of the solution increases one unit. Note the number of drops required.
Now make a buffer solution by mixing 2 parts 1 M acetic acid and one part 1 M sodium hydroxide. Add 1 M sodium hydroxide one drop at a time until the pH of the solution increases one unit, and note the number of drops. From these observations, students learn that only a certain amount of base may be added to a buffer solution before the pH changes and that this amount depends on the concentration of the buffer components.
The color changes of acid-base indicators can be determined using buffer solutions of different pH values. Fill each well of a microplate with a different buffer solution and add a drop of an indicator. Find the pH values over which the indicator changes colors. This range of pH values is the effective pH range for the indicator. Repeat this method for several indicators. Topics for further discussion include how to choose the right indicator for a titration and the relationship between the acid strength of an indicator and its effective pH range.
In many ways microscale experiments are superior to traditional laboratory activities. They can be safer, faster, and less expensive. If you haven't tried microscale in your laboratory yet, now might be the time!
Figure 3 Different acid-base indicators and their colors from pH 2 to 12, left to right. The indicators are Bogen universal indicator (top row), methyl red (second row), bromothymol blue (third row), and phenolphthalein (bottom row).
Further Resources
Beran, J. A. 1994. Laboratory Manual for Principles of General Chemistry. John Wiley & Sons, Inc., New York.
Elder, J.W. 1995. Thin-layer chromatography of analgesics – an update. Journal of Chemical Education. 72:1049.
Hall, J. F. 1993. Experimental Chemistry. D.C. Heath and Company, Lexington, MA.
O'Neill, M. D. (ed.). 1990. Chemistry Laboratory Manual. Addison-Wesley Publishing Company, New York.
Zumdahl, S. S. 1993. Chemistry. D.C. Heath and Company, Lexington, MA.
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