Why is the third ionization of phosphoric acid, #H_3PO_4#, disfavoured?

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Answer 1 · 2018-01-22T07:19:32

Well, simply because phosphoric acid IS NOT a triacid.

At most it is a diacid in aqueous solution; and titration with sodium hydroxide would give an endpoint as #Na_2^(+)HPO_4^(2-)#.

This site reports the acidities of phosphoric acid...

#H_3PO_4 + H_2O rightleftharpoons H_3O^+ + H_2PO_4 ^-# #K_a1 = 7.5×10^-3#

#H_2PO_4^(-) + H_2O rightleftharpoons H_3O^+ + HPO_4^(2-)# #K_(a2) = 6.2×10^-8#

#HPO_4^(2−) + H_2O rightleftharpoons H_3O^+ + PO_4^(3-)# #K_(a3) = 2.2×10^(−13)#

The third dissociation DOES not occur to any extent in aqueous solution, as the second graph shows.....

ion.chem.usu.edu

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