It is a gas resulting from the combustion of sulfur. It has been used since antiquity (by Romans and Egyptians) to sanitize barrels.

Added as sodium metabisulfite, both antiseptic and antioxidant, SO2 is the working horse of winemakers. Some use it from the very beginning (to avoid heating honey) and then again at each racking. It is added at least at bottling to ensure the stability of mead during ageing.

The chemical equilibrium is [V6 p 202]:

SO2 + H2O <-> HSO3- + H+ <-> SO32- + 2H+

K1 = 1,7 10-2       K2 = 5 10-6

One can notice that the quantity of SO2 -- the only efficient form of the three (figure below, left) -- depends on the pH (figure below, right). If the pH is higher than 4, the efficiency of SO2 is down to nothing. After the chemical equilibria above (using the law of mass action) or looking at figure 9, one can know how much free SO2 is necessary to get some amount of active SO2 (table).

SO2 as bound, free et active SO2
SO2 can be under the forms of free SO2 (including active SO2) and combined SO2 [V4]
active SO2 as a function of pH
Concentration of active sulphite (called H2SO3here) as a function of pH [V6]

One can notice that at a pH of 4, ten times as much sulfite is necessary to get the same result as when the pH is 3. In order to dose SO2 properly, the pH must be known. If it is not we are likely to add 2-3 times as much as necessary or not enough. Some recipes say to add such quantity of sulfite without saying anything about pH. Such a number is almost worthless. Unfortunately, when one does not have a pH-meter, there is no other choice. pH paper is quite cheaper than a pH-meter but it is not precise enough to be of real use.

pH free SO2 active SO2 / free SO2
3.01815.52 %
3.22823.55 %
3.44422.26 %
3.67001.43 %
3.811160.90 %
4.017950.56 %
4.229180.34 %
Free SO2 to use to obtain 1 ppm of active SO2 as a function of pH.

Sulfite must be kept in a dry place, away from air. Its efficiency decreases when it gets old. Note that some people do not tolerate SO2.

White wine needs 15-20 mg/L of free SO2 and sweet white 40-50 mg [Peynaud in V4].

References : Peynaud in V4 (chapter 6), chapter 6 of V2.

May 28th 2002