4) Dilute 10 ml of 1.0 F acetic acid to 100 mi and measure the p H of this dilute solution. Repeat these four steps using the acetic acid-sodium acetate buffer solution. Remarks Tabulation of the p H values allows comparison of the effect of addition of acid or base to a huffered or unbuffered solution. A class exercise of calculating the predicted values using the K. for acetic acid further reinforces this principle. A similar system employing NH3(aq)-NH&1 can also be used to demonstrate this effect. Typical values are tahulated below: acetic acidsodium acetate 1.0 F acetic acid meas- calculameasured calculated ured ted initial solution 2.40 2.37 4.68 4.74 1ml HCI to 50 ml 1.82 1.71 4.62 4.73 1ml NaOH to 50 3.07 3.09 4.68 4.77 ml ten-fold dilution 2.90 2.87 4.67 4.74
, Weak and Strong Acids and Buffers Submitted by:, Checked by:
Charles E. Ophardt Elmhurst Collexe Elmhurst, Illinois 60126 Paul T. Ruda Cleveland Hill Schools Cheektowaga, New York
Preparation Put 5 ml of 0.01 M HCI into 2 small test tubes, 5 ml of 0.01 M HCzH302 (HAc) into 2 small test tubes, 0.1 g of NaCl into 2 test tubes, and likewise 0.1 g of NaC2H302 (NaAc).' Dropping bottles of metacresol purple (MCP), hromcresol green (BCG), and 0.3 M HC1 are also needed. The indicators have the following color changes: MCPred to yellow a t p H 1.2-2.8 and BCG-yellow to blue a t p H 3.8-4.4. Demonstration 1) Add 5 drops of MCP indicator to one tuhe of HCI and HAc. Note the difference in colors. Add 5 drops of BCG indicator to the remaining two acids. Note the difference in colors.
2) Pour one-half of the MCP-HC1 solution into a tuhe with NaCl and one-half of the MCP-HAc solution into a tuhe with NaAc. No color changes should he apparent. In a similar manner combine one-half of the BCG-acid solutions with their respective salts. Note the color changes this time. 3) Finally add 5 drops of 0.3 M HCI to all of the tuhes and watch for color changes. 10-15 drops may he added to the HAc-NaAc solution with no color changes.
Buffer Action with Precise pH Monitoring Submitted by:
Checked by:
Wayne L. Felty, The Pennsylvania State Uniuersity Wilkes-Barre Campus Wilkes-Barre, 18708 Wayne L. Smith Colby College Waterville, Maine
Preparation Provide a digital p H meter readable to a t least *0.01 p H units by the class, p H and reference electrodes, magnetic stirrer, four 600-ml beakers each containing a magnetic stirrer bar and 400 ml of (1) distilled water, prehoiled, (2) 0.025 M NaHzPO4, 0.025 M Na2HP04, (3) 0.50 M NaHzP04, 0.50 M Na2HP04, and (4) 0.50 M acetic acid, 0.50 M sodium acetate. Demonstration Measure the p H of distilled water (solution I), with stirring. Then add 10 drops concentrated HCI and again note the pH. The p H changes from about 6-7 to 2.5. Repeat the procedure for solutions 2,3, and 4, rinsing the electrodes between solutions. Respective p H changes are typically 6.87 to 6.46,6.54 to 6.51, and 4.83 to 4.81. Remarks Solution 1 is unhuffered-the initial p H drifts with stirring and a dramatic p H change is observed upon acidification. Solution 2 has an initial p H comparable to that of solution 1, hut is huffered (addition of HCl converts HPO& to HzP04-). Solution 3, of greater buffer concentration, exhibits a greater "huffer capacity." The p H differs somewhat due to the difference in ionic strength. Solution 4 has the same buffer capacity (SpH) as solution 3, but the p H (= pK,) is quite different.
Volume 53,Number 4, April 1976 / 229
