GEORGE
edited by L. GILBERT
Denison University Granville. Ohio 43023
Buffer Capacity ol Various Acetic Acid-Sodium Acetate Systems: A Lecture Experiment S U B M ~ EBY D Craig.J. Donahue and Maw Universlly of Mlchigan-Dearborn Dearborn, MI 48128
has been added. As was the case for flask 3 no color change is observed. For flasks 3 and 6, which have a tenfold greater buffer capacity than flasks 2 and 5, the demonstrator should be prepared to add over 100 ml of titrant in the event a student should ask about this. For moderately sized classes, a pH meter with larger digital readout may he used to follow the course of the titration or to allow students to plot their own titration curves.
G. Panek
CHECKEDBY
David A. Franz Lycomlng College Willlamsport, PA 17701
Previouslv ~ublisheddemonstrations dealing with buffers compare the p~ change in water or an unbuffekd solution to that in a buffer solution upon the addition of an acidor a base (1-5). None of these demonstrations, however, explore the response of two buffers with the same initial pH value but different buffer capacities to added acid or base. One demonstration we know of does compare two such buffers and their response t o added acid using a pH meter; this demon~ - system (6). stration utilizes a H Z P O ~ - / H P O ~buffer We have devised a demonstration using indicators to put across the concept of differing buffer capacities by titrating some acetic acidlsodium acetate buffers with 1.0 M HCI and 1.0 M NaOH. Prior to the demonstration copies of the table and the figure are distributed to each student. Alternativelv. the table and the figure could he projected on a screen in tie form of a trans~arencv.The titrations corres~ondto the m a ~ h ~ or ~ O ' M in the figure in thaceach milliliter of 1 . 0 HC1 NaOH added to a flask containing 100 ml of the buffer solution corresponds to 0.01 moles of reagent per liter volume. The indicators chosen, bromophenol blue (pH range: 3.0-4.6; colors: yellow to purple) and bromothymol blue (pH range: 6.0-7.6; colors: yellow to blue) produce very visible color changes that correspond well to the calculated pH values. In the actual demonstrations acid is added continuously until the initial color of hromophenol blue is converted to yellow. For flasks 1 and 2 this occurs at approximately 2 ml and 10 ml, res~ectivelv.For flask 3 the titration is s t o ~ o e dafter 10 ml even though the initial purple color persists. For the second srt of titrations the same procedure is followed. For flasks 4 and 5 approximately 1.5 ml and 10 ml of 1.0 M NaOH is rewired to ronvrn the initial vellow color of hromothvmol blue to blue. For flask 6 the titration is stopped after 10 k l of base
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Procedure Prepare the following buffersand place 100.0ml of each in apprapriately labeled 250-ml Erlenmeyer flasks: Solution 1: 0.18 M HOAc10.018 M OAc- (To 90.0 rnl1.OM HOAe add 1.22 g NaOAc4HzO and dilute to 500.0 ml with distilled HzO.) Solutions2 and 5: 0.1 M HOAc1O.l M OAe- (To 50.0ml of 1.0M HOAc add 6.80 g NaOAc4HzO and dilute to 500.0 ml with distilled HzO.)
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Tibation curves for acetic acidlsodiumacetate buffer solutions.
Data Used to Plot Tltratlon Curve (See Flgure)'~~ HOAC Ratio OACOrigi~l Add HCI 0.01 mole 0.015 mole 0.05 mole Add NaOH 0.01 mole 0.015 mole 0.05 mole
0.1810.018
pH
3.74
A
0.1910.008 0.19510.003
...
3.37 2.93
...
B 0.1710.028
3.96
0.1310.068
4.46
...
...
Ratio
HOAc -
pH
Ratio
4.74
1.011.0
0.1510.05 0.1910.01 0.2 M HOAc
4.26 3.47 2.72
D 0.05 r o l e 0.09 mole 0.10 mole
0.0510.15 0.0110.19 0.2 M O A F
5.22 6.02 9.02
0.05 mole 0.09 mole 0.10 mole
OAc-
0.110.1 C 0.05 mole 0.09 mole 0.10 mole F 0.05 mole 0.09 mole 0.10 mole
-
-
HOAc OAc-
Ratio
4.74
0.01810.18
1.0510.95
4.70
1.1010.90
4.66
0.9511.05
4.78
0.9011.10
4.83
...
.. .
F
...
HOAC -
PH
...
OAC-
G 0.01 mole 0.015 mole 0.05 mole H 0.01 mole 0.015 mole 0.05 mole
pH
5.74 0.02810.17
5.53
0.06810.13
5.02
0.00810.19 0.00310.195
8.12 6.55
...
...
...
...
a Owdratio equation not used in calculations.
'Lenen conespond to curves generated by data (see figwe).
Volume 62
Number 4
April 1985
337
Solutions 3 and 6: 1.0 M HOAcIl I) Y OAL- Wo 28.6: ml glarlal aeetw acid add 68.04 g NaOAc.3H20and dilute to XMl U ml with distilled H20.) Solution 4: 0.018 M HOAc/O.l8 M OAc- (To 9.0 ml of 1.0 M HOAc add 12.25 e NaoAc.3H10 and dilute to 500.0 ml with distilled HzO.) To solutions 1,2, and 3 add 15 drops each of bromophenol blue, and to solutions 4,5, and 6 add 15 drops each of bromotbymol blue. Use a buret and titrate, in order, flasks 1,2,and 3 with 1.0 M HCI. Use a second buret containing 1.0 M NaOH to titrate flasks4.5, and 6. Remarks
The following relationships between the different buffer solutions are normallv mentioned nrior to ~erforminathe titrations. The pH of thk two sets of buffers in flasks 2 i d 5 (0.1 M HOAclO.1 M OAc-) and in flasks 3 and 6 (1.0 M HOAcIl.0 M OAc-) are identical. In particular, the pH of these buffers eauals the numerical value of the DK. for acetic acid. 4.74. because the molar concentration of ace& acid and acetate ion nresent are eaual. The total molar concentration of acetic acid and acetate ion is the same (0.2 M ) in flasks 1, 2,4, and 5. Flasks 1 and 4 have initial pH values of 5.74 and 3.74, respectively. If the effective pH range of a buffer is taken to he the pK. (or pKh) value f 1, then the buffers in flasks 1and 4 delineate the limits of the effective pH range for the acetic acidlacetate ion buffer system.
338
Journal of Chemical Education
I'rior to performing any of these titrationu, the demonstrator may wish to illustrate the estimation of .pH by. means of indicot'rs by substituting 100 rnl of water plus the indirat o n in place oi the buffers and perfurrninr the same titrarions. This also serves to illustrate how rapidly the pH changes when the solution lacks any huffer capacity. We normally do not titrate the buffer in flask 1with base (curve B in the figure) or the huffer in flask 4 with acid (cuwe G in the figure). Neither titration would produce a significant color change. The tenfold excess of HOAc to OAc- in flask 1 and the tenfold excess of OAc- to HOAc in flask 4 account for the greater huffer capacity of the buffer in flask 1towards base and for the greater buffer capacity of the buffer in flask 4 towards acid. We have found this demonstration used in coniunction with the plots in the figure to he very helpful in ill&rating the concept of huffer capacity and in distinmishinp it from the most effective pH range for a buffer sys
