GEORGEL. G I L B E R ~ Denison University Granville. Ohio 43023
Visualization of Buffer Action' and the Acidifying Effect of C 4 Niels D. Schonbeck Melropolltan Slate College Denver. CO 80204
David Blackman Unlrerrlty ol the Dlstrlct of Colvmbla Washington. OC 20005 Many students finish one ye& of general chemistry with embarrassingly little understanding of acid-base buffers. Formal classroom explanations are considerably aided by visual demonstrations using pH indicators, for which a (1-5). number of protocols have appeared in THISJOURNAL The following buffer demonstration features visualization of the effects of COz on pH. Discussion of the physiological huffering of COz in the human body serves to revive student curiosity and stimulate active interest in the subject of acidhase huffers. Materials 3 magnetic stirring bars 2 magnetic stirrers 1hot plate magnetic stirrer 3 500-ml Erlenmeyer flasks 15-ml maduated o i ~ e t 2 disoo&le oio& kith rubber holhs
100 ml of 1M NaOH &I0 small pieces (0.5-1.5 g each) dry ice, or compressed COz gas, or COz gas generator (NaHCOs, HCI). Procedure Place 250 ml of distilled water in each of two 500-ml Erlenmeyer flasks along with magnetic stirring bars. Add 5 ml of 0.04% hromthvmol hlue solution to each flask. and turn on hoth magnetic s&rers. Depending upon the p ~ ' o the f water used. the solutions will have a ~ossihleranee of colors from hlue'(hasic) through green ( P H I ) to yellow(acidic). T o the t f i s t flask add 1M HC1 dronwise with a disposable o i ~ euntil the indicator turns yel1ow;and note the number of drops of acid (1-3) that this transition reauires. Next add 1M NaOH dropwise: observe the solution a s i t turns green and finally a deep hlue, again noting the volume of hase required. T o the first flask add approximately 0.2 g of KHzP04 (estimation with a scoopula suffices), and allow to dissolve with stirring. T o the same flask add approximately 0.2 g of K2HPOa, and allow t o dissolve. Note the expected color changes that accompany these additions. Repeat the addition of 1M HC1 and 1M NaOH to hoth flasks and compare the dramatically different volumes required to cause the expected color changes in the first, now huffered, and second, unhuffered, solutions. Set u p a third flask as above hut with 2 g of hoth phosphate
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Journal of Chemical Education
salts. Observe the volumes of acid and hase required to generate the color changes and compare with the fir& 2 solut~ons. (Note that after about 10 drops of 1M HCl or 1M NaOH, which has no visible effect on
