Explain polybasic acids and polyacidic bases with examples.

Explanation

Acids having more than one ionisable proton per molecule are known as polybasic or polyprotic acids. Carbonic acid, phosphoric acid, oxalic acid, hydrogen sulphide and sulphurous acid are polybasic weak acids. Polybasic acids ionise in stages. Therefore, they have more than one ionisation constant values.
Example:
(i) `H_(2)CO_(3)`, carbonic acid is a dibasic acid. It has two ionisation constant values.
`H_(2)CO_(3)hArrH^(+)+HCO_(3)^(-),,K_(a_(1))=([H^(+)][HCO_(3)^(-)])/([H_(2)CO_(3)])=3xx10^(-7)` First ionisation constant.
`HCO_(3)^(-)hArrH^(+)+CO_(3)^(2-),K_(a_(2))=([H^(+)][CO_(3)^(2-)])/([HCO_(3)^(-)])=6xx10^(-11)` Second ionisation constant.
(ii) `H_(2)S`, hydrogen sulphide is also dibasic in nature
`H_(2)ShArrH^(+)+HS^(-)`
`K_(a_(1))=([H^(+)][HS^(-)])/([H_(2)S])=9xx10^(-8)` First ionisation constant.
`HS^(-)hArrH+S^(2-)`
`K_(a_(2))=([H^(+)][S^(2-)])/([HS^(-)])=1xx10^(-15)` Second ionisation constant.
(iii) `H_(3)PO_(4)`, orthophoshoric acid is a tribasic weak acid.
The stepwise ionisation of it is shown below
`H_(3)PO_(4)hArrH^(+)+H_(2)PO_(4)^(-)" "K_(a_(1))=1.1xx10^(-2)`
`H_(2)PO_(4)^(-)hArrH^(+)+HPO_(4)^(2-)" "K_(a_(2))=2.0xx10^(-7)`
`HPO_(4)^(2-)hArrH^(+)+PO_(4)^(3-)" "K_(a_(3))=3.6xx10^(-12)`
(Similar discussion and conclusions hold good for polyacidic bases also.)