Potassium metabisulphite
KMS, K2S2O5, potassium Campden
The antioxidant and antimicrobial the whole cellar leans on, and the usual source of SO2.
Typical dose
Yields a little over half its weight as SO2 (about 57%). Dose to a molecular-SO2 target for the wine's pH, not to a fixed free-SO2 number.
When to add it
At the crush, after malolactic, and before bottling; top up through ageing.
Legal limit
United States total SO2 limit 350 mg/L (TTB).
Vegan
Not a fining question for this one; nothing animal-derived is involved in a normal addition.
In practice
Releases sulphur dioxide, which scavenges oxygen and suppresses spoilage microbes. How much protection a given free-SO2 reading buys depends entirely on pH, because only the molecular fraction is active and that fraction shrinks as pH rises. Dose against pH: a wine at 3.7 needs several times the free SO2 of one at 3.2 for the same protection.
Linked faults are the ones this addition either treats or, used wrongly, causes. The fault page says which, and whether it can be undone.
Others in sulphite, sorbate and antioxidants
Sodium metabisulphite
An SO2 source like the potassium salt, but one that leaves sodium in the wine.
Potassium sorbate
Stops surviving yeast refermenting a sweetened wine. A preventer, not a killer.
Ascorbic acid
A fast oxygen scavenger for whites, used only alongside sulphite.
Dimethyl dicarbonate
A cold sterilant that kills yeast at bottling and then breaks down.