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Calculations (preparation of Na₂S₂O₅ or K₂S₂O₅ stock solutions)

• The chemical abbreviations or chemical formulas for sodium and potassium metabisulphite are Na₂S₂O₅ and K₂S₂O₅ respectively.
  
  
• Each element (i.e. Na - sodium, S - Sulphur and O - oxygen) is assigned an atomic weight and can be used to calculate the molecular weight.
  
  
• The molecular weights of Na₂S₂O₅ , K₂S₂O₅ and SO₂ (sodium and potassium metabisulphite, and sulphur dioxide) are calculated to be 190, 222 and 64 respectively.
  
  
• One molecule of Na₂S₂O₅ or K₂S₂O₅ releases 2 molecules of SO₂ (sulphur dioxide) (see equations)
  
  
• Therefore 222 gm of K₂S₂O₅ (potassium metabisulphite) will release 128 gm (2 * 64 gm) of SO₂ when added to juice or wine.
  
  
  That is K₂S₂O₅ yields 57.7% w/w SO₂ (128/222*100)
  
  
• And 190gm of Na₂S₂O₅ (sodium metabisulphite) will release 128 gm (2 * 64 gm) of SO₂ when added to juice or wine.
  
  
  That is Na₂S₂O₅ yields 67.4% w/w SO₂ (128/190 * 100)
  
  
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  Let the yield of SO₂/X₂S₂O₅ be approximately 50 %w/w (allowing for loses of metabisulphite strength)
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  1 ml of stock solution @ 200 gm metabisulphite/L will contain 200 mg metabisulphite since -
  
  
  200 gm metabisulphite/L
  
  
  = 200,000 mg/1000 ml ( since 1 gm = 1000 mg and 1 L = 1000ml)
  
  dividing by 1000
  
  = 200 mg X₂S₂O₅/ 1 ml stock soln.
  
  @ 50 % w/w SO₂/X₂S₂O₅
  
  = 100 mg SO₂/1 ml stock solution (50 % of 200 mg = 100 mg)
  
  1 ml of stock soln. (100 mg SO₂) added to 10 L of juice or wine
  
  = 100 mg SO₂/10 L of juice or wine
  
  = 10 mg SO₂/1 L
  
  = 10 ppm SO₂ (1 mg/L = 1 ppm)
  
  
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  i.e. adding 1 ml of SO₂ stock solution to 10 L of juice or wine will raise it by 10 ppm of TSO2
  
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