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Equations showing the equilibrium between the free sulphur dioxide (FSO₂) species, molecular sulphur dioxide (MSO₂), and the bisulphite (HSO₃^-) and sulphite ions (SO₃⁼). (see graph also)

• An increase in the hydrogen ion (H⁺) concentration will shift the equilibrium of the equations to the left.
  
  
• That is, a decrease in pH will increase the bisulphite ion(HSO₂) concentration but more importantly the molecular sulphur dioxide (MSO₂) concentration.
  
  .

  SO2

  +

  H2O

  < == >

  HSO3-

  +

  H+

  
  

  HSO3-

  < == >

  SO3=

  +

  H+