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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 4 Issue 2 (1999) pp 68-72

Spectroscopic and Potentiometric Investigation of a Diprotic Acid: An Experimental Approach to Understanding Alpha Functions

Dale D. Russell,* Jared Potts, Robert M. Russell, Curtis Olson, Martin Schimpf

Department of Chemistry, Boise State University, Boise, ID 83725

Published online: 2 April 1999

Abstract. A laboratory experiment suitable for second- or higher-year chemistry students uses thymol blue, an organic dye, for investigating the fraction of each colored species present as a function of pH. Visible color changes in the vivid, highly absorbing dye permit students to “see” relative amounts of acid, base, and ampholyte species during the titration, while potentiometric measurement of pH and spectroscopic determination of concentrations permit quantitation of the species present. A standard Beer’s law calibration curve is constructed for known concentrations of the pure forms of each species at a pH where that species is present as nearly 100% of the total analytical concentration of the dye. Using the molar absorptivities of the three species computed from the calibration curves, simultaneous determination of concentration by species is performed on a set of solutions having constant total concentration of dye such that each solution represents one point in the range pH 0 to pH 13. A mathematical software application is used to solve the simultaneous equations and determine the fraction of each species present. Students then use a spreadsheet application to plot the fractions as functions of pH (the alpha functions). The significance of the isosbestic points in the spectra is also explored.

Key Words: Laboratories and Demonstrations; quantitative analysis

(*) Corresponding author. (E-mail:

Article in PDF format (176 KB ) HTML format

Supporting Materials:

An Adobe Acrobat PDF file of the Student Handout (30 KB) 10.1007/s00897990068b

A WinZip file of Excel files containing sheets, charts, and calculations of student data (41 KB) 10.1007/s00897990068c

Issue date: April 2, 1999

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