Since the acid is diprotic, there are two equilibrium dissociation reactions for it. Consequently, there will also be two expression for equilibrium constants, Ka₁ and Ka₂.
First reaction:
H₂SO₃ ⇄ 2H⁺ + HSO₃²⁻ ------> Ka₁ = [H⁺][HSO₃²⁻]/[H₂SO₃]
Second reaction:
HSO₃²⁻ ⇆ H⁺ + SO₃²⁻ ------> Ka₂ = [H⁺][SO₃²⁻]/[HSO₃²⁻]
