What is polyprotic acid ? Give examples of polyprotic acid and its ionization. 

Polyprotic acid : The acids which have more than one ionizable proton per molecule of the acid. Such acids are known as polybasic or polyprotic acids.

Diprotic acid: The acid which have two ionizable proton per molecule of the acid. Such acids are known as Dibasic or Diprotic acid.

e.g., $\mathrm{H}_{2} \mathrm{X}+$ (aq) $+2 \mathrm{H}^{+}+\mathrm{X}^{2-} \quad \mathrm{K}_{a}$

The ionization of the reaction is in two steps is as under,

$(i)$ $\mathrm{H}_{2} \mathrm{X}+\mathrm{H}_{2} \mathrm{O}+\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{HX}^{-} \quad \mathrm{K}_{a}$ $(i)$

$(ii)$ $\mathrm{HX}^{-}+\mathrm{H}_{2} \mathrm{O}+\mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{X}^{2-} \quad \mathrm{K}_{a}$ $(ii)$

In it, $\mathrm{K}_{a}$ $(i)$ $>\mathrm{K}_{a}$ $(ii)$ and $\mathrm{K}_{a}$ (i) $\times \mathrm{K}_{a}$ $(ii)$ $=\mathrm{K}_{a}$

Examples of diprotic acid : Oxalic acid $\left(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right)$, Sulphuric acid $\left(\mathrm{H}_{2} \mathrm{SO}_{4}\right)$, Carbonic acid $\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)$, Sulphurous acid $\left(\mathrm{H}_{2} \mathrm{SO}_{3}\right)$, Ascorbic acid etc.

Examples of triprotic acid : Phosphoric acid $\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)$, Cytaric acid.... etc.

In polyprotic acid solution constant mixture of acid like $\mathrm{H}_{2} \mathrm{~A}, \mathrm{HA}^{-}$and $\mathrm{A}^{2-}$.