GEORGE L. GILBER; Denison University
tested demonstrcrtions Visual Demonstration of Buffer Action James C. Chang
University of Northern Iowa Cedar Falls, IA50614 In the past 15 years or so several papers were published on the demonstrations of the actions of buffer solutions, using a pH-meter (1-21, showing with a n overhead projector (3). or using color changes of indicators (45). In the papers using indicators, one paper required a mixed indicator while the other required careful preparation of solutions. In my classes I have been using a fast demonstration without elaborate preparation of solutions. Only a 1Macetic acid and a 3 M HCI solutions are needed. To three test tubes each containing about 10 mL of the 1M acetic acid. two or three drops oflbenzopurpurine 4B or congo red indi: cator are added. The color is red-brown. Up to 20 drops of benzopurpurine 4B may be needed to produce an incense color. To one test tube five drops of 3 M HC1 are added, and the color of the red-brown solution changes to violet-blue. " indicating a change to a low pH. If benzopurpurine 4B is
-
Granville, OH 43023
used as the indicator, u p to 20 drops of HCI may he needed to produce the color change. To another test tube a scoop of solid sodium acetate is added. and the color of the redbrown solution changes to orange, indicating a change to a hieher DH. Now 3 M HC1 is added d r o ~ w i s eto this oranee sorution, but the color does not change even after adding 40 to 50 d r o ~ of s HCI, indicatine the buffer action of the mixture ofaEetic acid and sodium acetate. Alternatively, methyl orange indicator may be used for a dilute acetic acid solution. The dilute acetic acid solution is DreDared by mixing 10 d r o ~ sof 1 M acetic acid with 200 m ~ ~ water. o f The color of the indicator i n the dilute acetic acid solution is orange but changes to red upon addition of only one drop of 3 M HC1. After adding a scoop of solid sodium acetate, the color of the orange solution changes to yellow, and this yellow color persists upon adding five to 10 drops of 3 M HCI. Literature Cited 1. Chang.d. C J C1ii.m Eduu 1916.53.228. 2. Felty, W.L.J.Chern. Educ 1976.5.3. 229. 3. Hobe. P.G.J Cimm Educ 1979.56.47. 4. Go1d.M.J Clicni.Eilur 1982.5'3. 156. 5. Ophardf. C. E.J. Chrm. Ediii..1985. 62. 608.
Volume 72
Number 4 Auri11995
345
