H_3PO_3 is diprotic acid. Explain.
H_3PO_3 has three H atoms and therefore, it is expected to be tribasic. However, in its structure, two hydrogen atoms are joined through oxygen atoms and are ionisable. The third H atom is linked to P and is non-ionisable.
H _3 PO _3 \rightleftharpoons HPO _3{ }^{2-}+2 H ^{+}