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Sodium and Potassium MetaBisulphite.

• Sulphur dioxide (SO₂) can be derived from the compounds sodium metabisulphite ( Na₂S₂O₅) and potassium metabisulphite ( K₂S₂O₅).
  
  
• Sodium and potassium metabisulphite are used as crystalline powders that have to be thoroughly dissolved and dispersed when added to juice, wine or water.
  
  
• The chemical abbreviation for sodium metabisulphite is Na₂S₂O₅
• The chemical abbreviation for sodium metabisulphite is K₂S₂O₅
  
  
• Na₂S₂O₅ or K₂S₂O₅ will form SO₂ when dissolved in acidified water as shown below.
  
  
  Na₂S₂O₅ + H₂O < == > 2KHSO₃
  
  2KHSO₃ + H⁺ < == > SO₂ + H₂O + K⁺
  
  The H⁺ (hydrogen ion) is contributed by an acid such as tartaric acid found in grape juice or wine.
  
  Sodium metabisulphite dissociates in the same way.
  
  
• To produce SO₂ from the metabisulphite salts in a water solution, the solution will require acidification.
  Tartaric acid is the acid of choice here, as it is the least problematic acid in wine production.
  
  
• Sodium metabisulphite releases 67.4% w/w as SO₂ with the potassium form releasing 57.7% w/w (see calculations).
  
  
• Measured volumes of pre-prepared, concentrated stock solutions of dissolved sodium or potassium metabisulphite, is a convenient way of adding SO₂.
  
  
  • Added in this dissolved form, thorough dispersion can be achieved, especially when added to musts with skins and large tanks.
    
    
  • Using stock solutions also reduces measuring errors.
    
    
• The addition of Na₂S₂O₅ or K₂S₂O₅ increases the sodium or potassium content of wine, with export regulation implications.