What is the basicity of `H_(3)PO_(4)` ?

Explanation

The structure of `H_(3)PO_(4)` is tetrahedral. It has three P-OH and one P=O bond as shown below : Since it has three ionizable P-OH bonds, therefore, `H_(3)PO_(4)` is tribasic as shown below :
`H_(3)PO_(4)+H_(2)OhArrH_(3)O^(+)+H_(2)PO_(4)^(-),pK_(a_(1))=2.12" at "298K`
`H_(2)PO_(4)^(-)+H_(2)OhArrH_(3)O^(+)+HPO_(4)^(2-),pK_(a_(2))=7.2" at "298K`
`HPO_(4)^(2-)+H_(2)OhArrH_(3)O^(+)+PO_(4)^(3-),pK_(a_(3))=12.4" at "298K`
Since it is tribasic, it forms three series of salts with NaOH, viz., `NaH_(2)PO_(4),Na_(2)HPO_(4)andNa_(3)PO_(4)`.