In this note we discuss the chemistry of sulfur/sulfur dioxide/sulfite in wine.
Though the term sulfur, sulfur dioxide and sulfite are interchanged and used to describe the chemical species in the wine, because of the wine's acidity/pH, which is often in the range 3 to 4, the actual chemical species present in wine is bisulfite (HSO3-).
Therefore though sulfur dioxide (SO2) maybe added to wine or is naturally present, because of the acidity of wine the sulfur dioxide is often present as bisulfite (HSO3-).
Bound/unbound sulfite in wine
Wine is full of organic molecules, many of which are able to react with the sulfite in wine, the sulfite that has reacted with another molecule in the wine is defined as bound, whilst the sulfite left freely dissolved in the wine is termed unbound or free. The total of free/unbound and bound sulfite is termed the total sulfite.