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Sulphur dioxide and ascorbic acid

• Sulphur dioxide does not react fast enough with oxygen to act as an oxygen scavenger as is commonly believed.
  
  
• Ascorbic acid, however, does react fast enough with oxygen to be the preferred substrate for oxidation over most oxidisable juice and wine compounds.
  
  
• When ascorbic acid reacts with oxygen and is oxidised, it forms hydrogen peroxide (H₂O₂), a stronger oxidant then oxygen itself, and will result in the rapid oxidation of juice and wine.
  
  
• H₂O₂ (hydrogen peroxide), however, will react with any free sulphur dioxide (FSO₂) present, to form the stable compound, sulphuric acid (H₂SO₄), (see equation).
  
  The quantities of H₂O₂ and H₂SO₄ (sulphuric acid) involved are very small.
  
  
• Therefore, the use of ascorbic acid should only be used when free sulphur dioxide can be guaranteed (see ascorbic acid use in red wine).