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Loss of "strength" of Na₂S₂O₅ and K₂S₂O₅

• Na₂S₂O₅ and K₂S₂O₅ (sodium & potassium metabisulphite) will react with moisture in the air releasing sulphur dioxide (SO₂).
  
  
• Solid Na₂S₂O₅ or K₂S₂O₅ and solutions thereof, on exposure to moisture and carbon dioxide (CO₂) contained in the air around us, will react with CO₂, with losses of the metabisulphite strength.
  
  
• Over time, these losses can be quite significant in both the solid form and as a solution.
  
  
• It is important to test the "strength" of your sodium or potassium metabisulphite stock solutions on preparation and on further use.