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1999
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March
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In the Laboratory
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A Simplified Method for Finding the pKa of an Acid-Base Indicator by Spectrophotometry
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George S. Patterson
Suffolk University, Chemistry Department, 41 Temple Street, Boston, MA 02114-4280
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March 1999
Vol. 76 No. 3
p. 395
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| Abstract |
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Experiments on determining the pKa of an acid-base indicator by visible spectroscopy are often found in analytical and physical chemistry lab texts. The procedure described here is a modification suitable for general chemistry lab students. The students first verify the lmax of the highest-absorbing form of an indicator, usually the base form, on their spectrophotometer. Next, they adjust the pH of five aliquots of a buffer-indicator solution to values centered around the pKa of the indicator and then measure the absorbances, A, of the resulting solutions. The students bring one of these solutions to low pH to generate the pure acid form of the indicator, HIn, and measure its absorbance, AHIn. They take another solution to high pH to generate the pure base form, In{, and determine its absorbance, AIn-. Using the equation pKa = pH + log10 [(A - AIn-) / AHIn - A)], they calculate the pKa 's of the solutions, or, equivalently, they plot the logarithmic function data vs pH to find the x-intercept, which is the pKa value. Results for eight common indicators are reported. The wastes from the experiment can be disposed by neutralizing and pouring them down the drain.
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| More Information |
 Citation |
Patterson, George S. J. Chem. Educ. 1999 76 395.
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Keywords |
Introductory / High School Chemistry; Laboratory Instruction; AcidÐBase Chemistry; Equilibrium; UV-Vis Spectroscopy
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History |
Created:
Last Updated: |
June 15, 1999
June 22, 2005
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| Home > JCE Print > Journal of Chemical Education > Issues >
1999
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March
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395
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