Titrations of polyprotic acids-- some examples.

Alanine, a naturally-occuring amino acid

Titration of 1.78 g (20 mmol) of [H₂A]Cl in 50.0 mL H₂O with 1.00 M NaOH (aq):

pK_a's = 2.35 and 9.87

Species distribution:

Some other titration curves of polyprotic acids

(These are taken from a page of real titration data determined by Chem 205 class in Fall 1997)

Glutaric Acid JD: 1 g glutaric acid +100 mL water; titrated with 1.000 M NaOH(aq) Note that the pKa values of glutaric acid are closely spaced (4.31 and 5.41 according to Lide, D. R., ed., Handbook of Chemistry and Physics, 71st ed., Boca Raton: CRC Press, 1990). Thus the shape of the titration curve looks pretty much like the titration of a monoprotic acid. Careful analysis might be able to tell the difference, but the data would need to be very accurate.

Phosphoric Acid DM: 25 mL "0.5 M H3PO4" + 75 mL water; titrated with 1.000 M NaOH(aq) pKa's are 2.12, 7.21 and 12.67 (Lide, D. R., ed., Handbook of Chemistry and Physics, 71st ed., Boca Raton: CRC Press, 1990). The first two pKa's can be estimated from the pH in the middle of each buffer region (this estimate is more accurate for the second pKa. The titration is stopped in the buffer region for the third pKa, but the pH is so high in this region that a third endpoint will not be seen. Careful calibration of the pH electrode along with analysis of the data is needed to extract the third pKa from the titration data.