Pour 50 mL of 0.1 M HC2H3O2 and 50 mL of 0.1 M NaC2H;i02 into a 250-mL beaker (pH about 4.7). Into a second 250-mL beaker pour 100 mL of a very dilute acetic acid solution of the same pH. (This solution is best prepared beforehand by mixing about 6 drops of 0.1 M HC2H3O2 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 HC1 dropwise to the dilute acetic acid 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, at least at first, rather than in one large aliquot.
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