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FSO₂

• The curves or graphs show the different forms of FSO₂ as an approximate percentage of total FSO₂ at different wine/juice pH values.
  
  
• Note that within the wine's/juice's pH region of 2.9 - 4.0, the predominant FSO₂ form is the bisulphite ion (HSO₃^-) and for this reason HSO₃^- represents the total FSO₂ concentration in juice and wine.
  
  
• The sulphite ion (SO₃⁼) however, is so small, that for all practical purposes can be ignored.
  
  
• The molecular sulphur dioxide (MSO₂) concentration, although small at wine pH values, is very significant and becomes increasingly significant with only a small decrease in pH values (see table or formula).
  
  
• Note that at a pH of 1 all FSO₂ exists as the MSO₂ form. The addition of phosphoric acid, during SO₂ determination, lowers the pH to 1, shifting the equilibrium of all the FSO₂ to the molecular form of SO₂.