This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
Neither SO2 nor sorbate kills yeasts; they inhibit them, and can
prevent microbial activity, but only if cell counts are low. If you
have a mounting problem, they won't do a good job in controlling it.
The amount of sulphite needed depends on the pH of the wine -- the
lower the pH the less you need (at pH 3.2, you need 21ppm (21mg/L) free
SO2; at pH 3.5, you need 50ppm (50mg/L) free SO2.) This has to do with
A) the fact that the active form that inhibits bacteria forms better at
lower pH's and B) the lower the pH, the better the acidity in the wine
is in itself able to protect the wine. The following is the pH
dependant equilibrium. The forms depicted in the left are favoured by
higher pH's; the right by lower pH's.
SO2 + H2O <---> HSO3- + H+ <---> SO3-- + H+
5gal*4.5L/gal = 22.5L 21mg/L*22.5L = 472.5mg
pH Red White 2.90 7ppm 11ppm 3.00 8 13 3.20 13 21 3.40 20 32 3.60 31 50 3.70 39 63