Can I determine the pKa's for oxalic acid from a titration curve?

I am working on an experiment which will determine the concentration of oxalic acid and it's dissociation constants by performing a titration with the aid of a pH meter. If you can, please, help me with the theory for my experiment.
M. B. Wodzianski, Ferris State University

Oxalic acid is a diprotic acid (which means it can release 2 hydrogen ions per molecule). The ionization steps in your titration are

H2C2O4 H+ + HC2O4-
HC2O4- H+ + C2O4-2

The dissociation constant for the second reaction is always less than for the first reaction. You might expect to see two separate steps on your titration curve, corresponding to these two separate ionizations. But for oxalic acid, the two dissociation constants are not far apart. The rapid jumps in pH that correspond to endpoints will be very close to each other. You probably will see just a slight inflection corresponding to the first endpoint.

To clearly resolve the separate steps in the titration curve of a polyprotic acid, the successive Ka's ought to change by at least 3 or 4 orders of magnitude.

Here is an introductory discussion of endpoint detection. For a little more about find pKa's from pH measurements, see the next question.

Author: Fred Senese

General Chemistry Online! Can I determine the pK_a_'s for oxalic acid from a titration curve?

Copyright © 1997-2010 by Fred Senese
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