Equations, showing the stoichiometry behind the disassociation of sodium and potassium metabisulphite salts in solution.

 

One molecule of Na₂S₂O₅ or K₂S₂O₅ (sodium or potassium bisulphite)

releases 2 molecules of SO₂ (sulphur dioxide)

 

Equations

 

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1 molecule of: plus 1 molecule of: yields 2 molecules of

Sodium metabisulphite Water Sodium bisulphite

 

Na₂S₂O₅ + H₂O ® 2NaHSO₃

 

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1 molecules of plus 1 Hydrogen yields

Sodium bisulphite ion

NaHSO₃ + H⁺ ®

 

 

1 molecule of 1 molecule of plus 1 molecule of a

Sulphur Sulphur

Dioxide Water Ion

SO₂ + H₂O + Na⁺

 

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Given the above equations it can be seen that it requires 1 molecule of K₂S₂O₅ (potassium metabisulphite) to yield 2 molecule of SO₂ (sulphur dioxide), given acidic conditions.