Some examples of polyprotic acids

Sulfuric Acid (ref. 1)

H2SO4 + H2O HSO4- + H3O+	strong acid
HSO4- + H2O SO42- + H3O+	Ka2 = 1.2 x 10-2 M	pKa2 = 1.92

Phosphoric Acid (ref. 1)

H3PO4 + H2O H2PO4- + H3O+	Ka1 = 7.5 x 10-3 M	pKa1 = 2.12
H2PO4- + H2O HPO42- + H3O+	Ka2 = 6.2 x 10-8 M	pKa2 = 7.21
HPO42- + H2O PO43- + H3O+	Ka3 = 2.2 x 10-13 M	pKa3 = 12.67

Alanine Hydrochloride (ref .1)

H3NCH(CH3)COOH+ + H2O H3NCH(CH3)COO + H3O+	Ka1 = 4.5 x 10-3 M	pKa1 = 2.35
H3NCH(CH3)COO + H2O H2NCH(CH3)COO- + H3O+	Ka2 =1.3 x 10-10 M	pKa2 = 9.87

References

1. Lide, D. R., ed., Handbook of Chemistry and Physics, 71st ed., Boca Raton: CRC Press, 1990.