A Simple Demonstrationof Buffer Action Understanding buffer solutions, particularly the reasons why buffers are capable of resisting pH change4 upon addition of strong acid or strong base, is often a difficult milestone for the beginning chemistry student. The following demonstration is a simple, yet impressive visual illustration of such buffer action. Buffer Action upon Addition of Acid Pow 50 mL of 0.1 M HCzHzOz and 50 mL of 0.1 M NaC2HzOz intoa 250-mLbeaker (pH about 4.1). Intoa second 250-mL beaker pour 100 mL of a very dilute acetic acid solution of the same pH. (Thissolution is best prepared beforehand by mixing about 6 drops of 0.1 M HC2H302 into 250 mL of distilled water, and the pH tested with a pH meter.) Bromphenol blue indicator is added to each beaker; both solutions are blue. Add 0.1 M HCI dropwise to the dilute acetic aeid solution; only 5-6 drops of the acid are enough to cause the pH to drop to about 3, where the color of the solution is yellow. Approximately 40 mL of the same acid solution are required to achieve the same color change in the buffer solution. The effectiveness of the demonstration is enhanced by adding the acid to the buffer in mL increments, a t lesst a t first, rather than in one large aliquot. Buffer Action upon Addition of Base Two solutions are prepared as above. Methyl red mdirator is added to each beaker; both the buffer and the weak aeid solution turn a peach rulor at pH 4.7.0.1 M NaOH is added dropMse to the arid solution; only about 5-6 drops are required for the solution 11, change to yellow ( p H about 6 ) .In stark contra-t, at least 30 mL of the same base is required to achieve the same pH rise in the buffer Marllyn D. Duerst University of Minnesota Minneapolis. MN 55455
Volume 62
Number 5
May 1985
435
