Walter R. Carmody Seattle University Seattle, Washington
An Easily Prepared
Wide Range Buffer Series
Over the years, explaining to students the reactions involving strong and weak acids and bases and their salts, either by lecture demonstration or by laboratory experiment, has been handicapped by the lack of a simple and inexpensive means of indicating the pH of the experimental solutions used. The use of a pH meter is impractical mainly because of the cost but also because it involves principles of measurement not understood by the student. In recent years the development of mixed indicators capable of indicating the pH of solutions over wide ranges has greatly increased the possibility of designing new significant demonstrations and experiments. In his own classes, the writer has found that, the introduction of demonstrations and experiments using the mixed indicator of extremely wide range developed by Richardson1 produces very satisfying results. One requirement for demonstrations and experiments involving indicators is a wide-range buffer series of pH standards for comparison with the experimental mixtures. The preparation of one of the several wellknown series of buffer solutions is not a serious problem in the analytical laboratory of a college or university, but under different circumstances it may indeed be enough of a problem to discourage the average high school teacher and many college instructors from undertaking a particular experiment or demonstration. There appears to be a need for a wide-range buffer series that may be prepared with moderate accuracy from two or three common chemicals and that requires the use of equipment as available and inexpensive as a graduated cylinder and a laboratory trip scale. The original buffer series developed by S. P. L. Sorensen2 requires the use of eight chemicals in seven different stock solutions, including “carefully” standardized solutions of sodium hydroxide and hydrochloric acid. The more commonly used series of Clark and Lubs3 as well as the wide-range series developed by Prideaux and Ward4 5use fewer chemicals and stock solutions but still require standardized solutions. The well known series of Mcllvaine6 satisfies the requirements in that it uses only two chemicals, two stock solutions and no standardized solution. This series, however, is limited on the alkaline side to buffers with Other less known series also fail to a pH of 8.0 or less. meet the requirements in one way or another. 1
2
3
Richardson, F. R., J. Chem. Educ., 33, 517 (1956),
Sorensen, S. P. L., Ergeb Physiol., 12, 393 (1912). Clark, W. M., and Lubs, H. A., J. Biol. Chem.., 25, 479
(1916). 1
Prideaux, E. B. R.,
and
Ward, A. T.,./.
Chem. Soc., 125, 426
(1924). 5
McIlvaine, T. C., J. Biol
Chem.., 49, 183 (1921).
The goal of the present work was the development a series of buffers with pH values ranging from 2.0 to 12.0 that would require the preparation of only two stock solutions and would use only two or three common chemicals. Also it should be preparable with the desired precision (± 0.05 pH units) using ordinary graduated cylinders for measuring the stock solutions. A Model G Beckman pH Meter was used in determining the pH values of the experimental solutions. A Type E-2 glass electrode was used on the alkaline side. The meter was standardized and checked against buffer standards in the same region of pH as that of the particular experimental solution being measured. The goal has been realized by the use of three common reagent-grade chemicals: boric acid (anhydrous), citric acid (monohydrate), and tertiary sodium phosphate (12H20), dissolved in two stock solutions. Solution “A” is 0.200 M boric acid and 0.050 M citric acid. Solution “B” is a 0.100 M solution of tertiary sodium phosphate. These stock solutions (one liter or more of each) are prepared by weighing out (to the nearest 0.1 gram) the correct amounts of the components and diluting to volume in a volumetric flask. The volumes of the two stock solutions used in preparing 200 ml of buffer solution at different pH values are shown in Table 1. Investigators over the years have avoided the use of tertiary sodium phosphate as a component of accurate
of
Table 1.—Volumes of Acid Solution “A” and Basic Solution “B" Required for the Preparation of 200 ml of Solutions in Buffer Series pH 2.0 2.5
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0
Solution “A” volume (ml)
Solution “B” volume (ml)
195 184 176 166 155 144 134 126 118 109 99 92 85 78 69 60 54 49 44 33
5 16
24 34 45 50 66 74 82 91 101 108
115 122 131 140 146 151 156 167 183
17
Solution “A”: Solution “B”:
Boric Acid 0.2 M, Citric Acid 0.05 .1/. Tertiary Sodium Phosphate, 0.1 M.
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buffer series not only because of the poor quality of the commercially available salt but because of the problem of preparing a product of a definite composition. Bell15 has obtained some evidence for the existence of two hydrates of the salt mixed with sodium hydroxide. On the other hand, Bates, Pinching, and Smith6 report that a stable product with a definite water content of between zero and 12 moles per mole of salt would be “difficult or impossible to obtain.” The quality of the commercially available tertiary sodium phosphate has improved materially during the past few years and at the present time at least two manufacturers are furnishing a “reagent grade” salt with the maximum sodium hydroxide content (approximately 2.0%) indicated on the label. To test the consistency of the brands of “reagent grade” tertiary sodium phosphate available, four different sets of buffers were prepared according to the figures given in Table 1 using two lots of one brand manufactured two years apart and one lot each of two brands of other leading manufacturers.8 The pH readings of the four series agreed within 0.03 units throughout. 7
Buffer Capacity of Mixtures
The seven weak acids involved in this series of buffers listed in Table 2 according to decreasing ionization constants. As Prideaux and Ward4 have pointed out, the production of an effective buffer series of wide range depends upon the inclusion of a sufficient number of weak acids differing by relatively small amounts in their pK’s so that the effect of successive acids overlap. On the other hand if the pK’s of the acids are too far removed from one another the stronger acid is completely neutralized, or nearly so, before the neutralization of the weaker acid commences. It is to be noted that the differences between the successive pK’s of the acids in Table 2 are relatively small with the exception of the last two, which differ by 2.8 units. One should expect that a buffer series which involves the acids listed would have consistently good capacity with the possible exception of those buffers with pH values in the region approximately half-way between pH 9.2 and pH 12.0. are
Bell,
Russell N., Ind. Eng. Chem., 41, 3901 (1949). Bates, R. G., Pinching, G. D., and Smith, E. R., J. Research Natl. Bur. Standards, 45, 418 (1950). 8 Samples were supplied by J. T. Baker Chemical Company, General Chemical Company, and Mallinckrodt Chemical Works. 6 7
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Journal of Chem real Education
The buffer capacity of selected buffers of the series determined by measuring the change of pH caused by successive additions of relatively small volumes of standard HC1 solution to 100 ml of the buffer solution. The buffer capacity was found to fluctuate between limits of 0.025 and 0.030 moles per liter per pH unit for mixtures in the region between pH 2.0 and pH 10.0. Above pH 10.0 the buffer capacity of the mixtures was found to diminish to a low of 0.020 at pH 10.5 and then, above pH 11.0, to increase rapidly to a high of 0.09 at pH 12.0. The addition to the system of an acid with a pK in the neighborhood of 10.5 would undoubtedly increase the buffer capacity of the mixtures in this area, but any improvement so gained would not be worth the trouble of buying and weighing out a fourth component for the mixtures. was
Table 2.—Ionization Constants of Weak Acids Included in Buffer Series Acid
h3po( H3C6H5O7 H2C6H5O7"'
HCsH.,07-7
1LPU4h3bo3 HPO,-5
Kc
pK
8 X 10'5 8 X 10
2.1
X 4 X 6 X 6 X 1 x 2
10-s 10~8 10~8
10-“ 10-15
3.1 4.7 5.4 7.2 9.2 12.0
Precision Requirements
A consideration of the differences in the volumes of solution “A” and/or solution “B” required to prepare successive buffers, as shown in Table 1, indicates that the pH is most sensitive to such volume changes in the range of buffers from pH 10 to pH 11. At such sensitive points the precision with which the pH of a buffer may be reproduced is poorest. In this region a change of 10 ml in the volume of solution “A” corresponds to a change in the pH of the buffer solution of 1.0 unit. In order to produce a buffer with pH precise to the nearest tenth unit (=fc 0.05) it would be necessary to measure the volume of the acid to the nearest 1.0 ml. This is possible using a clean 100-ml graduated cylinder. It should be noted that any error in the measurement of solution “B” also introduces an error of the resulting mixture; however, this error would be relatively smaller in this region, since the volume of solution “B” to be measured is from three to four times as great as that of solution “A.”
