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  Home > JCE Print > Journal of Chemical Education > Issues > 2003  > May  >
In the Laboratory
The Determination of the pKaof Multiprotic, Weak Acids by Analyzing Potentiometric Acid–Base Titration Data with Difference Plots
Arno Kraft
Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
Cover
May 2003
Vol. 80 No. 5
p. 554
Abstract
This paper discusses the pKa determination of mono-, di-, and triprotic weak acids with the help of difference (Bjerrum) plots, and the effect of strong acid–base concentration errors, ligand weight errors, and nonlinear electrode response. Experimental examples are given for the titration of an acidic heterocycle, as well as glycine, ethylenediamine, and tris(2-aminoethyl)amine (the last two after addition of excess HCl to ensure complete protonation) with standarized NaOH. The analysis procedure makes use of Microsoft Excel spreadsheets and nonlinear least squares curve fitting of the experimental data to the theoretical Bjerrum function. In addition to providing pKa values for mono- and multiprotic acids, this approach has been found suitable for detecting small errors in parameters, such as strong acid and ligand concentration, and corrections can often become necessary to achieve the best fit. Difference plots allow the pKa values of monoprotic and multiprotic weak acids to be determined rapidly and with good precision.
Supplement
Experimental details and a description of how Microsoft Excel Solver has been used to analyze titration data are available.
*  Contents JCE2003p0554W.doc (Microsoft Word)
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*  Citation
 Kraft, Arno. J. Chem. Educ. 2003 80 554.
*  Keywords
 Acid-Base Chemistry; Analytical Chemistry; Aqueous Solution Chemistry; Drugs / Pharmaceuticals; Quantitative Analysis
*  History
 Created:
Last Updated:		 March 28, 2003
February 28, 2005
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