H2C2O4 H+ + HC2O4-
HC2O4- H+ + C2O4-2
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.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/print-oxalic-acid-titration.shtml