William E. Brown
and J. A. Campbell Harvey Mudd College Clarernont, California
I I
Acid-Base Indicators An experiment in aqueous equilibria
W e have used with great success the experimental approach outlined by Tohey1 and by Ramettez,3 for studying the acid-base equilibria of indicator dyes. As a result we have undertaken further study of the common acid-base indicators as to suitability for such an experiment. This is consistent with our feeling that students like to work unknowns, enjoy having "their own compound" to study, and find that the slight variations in problems and their solutions found with different compounds tend to minimize the cookbook nature of the experiment. The table gives the wavelengths a t which maxima occur for the acid and base forms of each indicator, the wavelength of the isosbestic point (all in millimicrons), and the value of e a t each of the three wavelengths for each of the indicator forms, where c is defined by the equation l~g,~(Io/I)= eel = A , the absorbance. The molar concentration is represented by c , and the tbickness of the sample in centimeters by I. The table also lists the pK, the stock concentration in grams of indicator per liter of alcohol-water solution, and the working concentration in terms of the dilution ratio of the stock solution with water. All of our work was done in buffered solutions with absorbances measured both on a Spectronic and a Cary Model 13. Figures 1 through 6 show absorbance versus wavelength in millimicrons for the five indicators which prove to have suitable properties. An intermediate curve is given in each case from which values of pK can be calculated a t various points including the two maxima. The variations a t the isosbestic point are the maximum one finds with careful work and indicate the accuracy with which the experiment can be performed by students.
Figure 1. Abrorbonce venm wavelength for the ocid form. HA, the base form, A-, and on intermediate solution for crcsol red-acid form.
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Figure 2. Absorbonceversus wove. length for the arid form, HA, the base form, A-, and an intermediate ~ o i u t i afor crew1 red-bare form.
Journol of Chemical Education
Absorbtivity Data for Six Acid-Base Indicators
Cresol Red
OrangeIV
-.*...Rmm-
phenol Blue
Acid Form
Isosbestio
Ami
hmsi
519 528
emmi
280 473 615 471
436 213 498 MethylRed 528 3110 468 432 229 478 434 130 484
PhenolRed CresolRed
Baae Form Ami
er.ri
ernsi
78.6
434
130 399
75.0 1130 110 57.6
592 432
495 1470 643 336
363
448
558 573
PK
Working cono. (diluStock tion Cono. of ( d l ) stock)
1.59 1.000 1.89 0.500
1/50
4.04 4.99 7.72 8.06
1/10
2.400 0.0800 0.500 1.000
1/20
1/50
Our students do not usually use the numerical concentration data, nor do they calculate values of e. Rather, they make up all their solutions carefully to the same dilution so that the total concentration of indicator is identical in each of their working solutions, allowing direct determination of concentration ratios (base form to acid form) from the experimental absorbances. They are asked to use their data a t the isosbestic point to check on the constancy of the total concentration. We notice with methyl red, as did Ramette,=but not Tobey,' a shoulder a t about 555 mfi presumably due to HA+, though possibly an impurity. But good values of K are still obtainable in methyl red systems, so the data are included here. Our commercial samples of thymol blue were so contaminated that i t was not possible to get reproducible results, and the indicator was
' TOBEY, STEPHEN W., J. CHEM.EDUC.,35, 514 (1958). a
RAMETTE, RICHARD W., J. CHEM.EDUC.,40, 252 (1963). RAMETTE, RICHARD W., J. CHEM. EDUC.,44, 647 (1967).
Figure 3. Absorbance versus wavelength for the ocid form, HA, the bore form, A-, ond on intermediate solution for orange IV.
Figure 4. Absorbance venur wavelength for the acid form, HA, the bare form, A-, ond an intermediote solution for bromophenol blue.
eliminated for th'k reason. It may pay to call spccial attention to the fact that cresol red provides an interesting example of an indicator with two ranges both of which are readily accessible to study in this experiment. Bromothymol blue, phenolphthalein, thymol blue, methyl orange, alizarin yellow, and thymolphthalein give imprecise results due to poor resolution of the peaks. We are most appreciative of a grant from the National Science Foundation Science Course Improvement Program which supported this work in development of laboratory experiments, and wish to thank Steven W. George and Julie Kaya for some of the preliminary work. Figwe 5. Absorbance versus wavelength for the ocid form, HA, the bore form, A-, and on intermediate solution for methyl red.
Figure 6. Absorbonce venur wavelength for the ocid form, HA, the bore form, A-, ond an intermediate rolvtion for phenol red.
Volume 45, Number 10, October 1968
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