For a di-basic (or diprotic) acid, the dissociation occurs in two successive steps. For example, considering a general di-basic acid $H_2X$: Step 1: $H_2X \rightleftharpoons H^+ + HX^-$ (First dissociation constant, $K_{a1}$) Step 2: $HX^- \rightleftharpoons H^+ + X^{2-}$ (Second dissociation constant, $K_{a2}$)

It is more difficult to remove a positively charged proton ($H^+$) from a negatively charged ion ($HX^-$) due to electrostatic forces of attraction compared to removing it from a neutral molecule ($H_2X$). Therefore, the first dissociation constant ($K_{a1}$) is always greater than the second dissociation constant ($K_{a2}$). Thus, $K_{a1} > K_{a2}$ .