Potassium sorbate
sorbistat, the potassium salt of sorbic acid
Stops surviving yeast refermenting a sweetened wine. A preventer, not a killer.
Typical dose
Roughly 150 to 250 mg/L sorbic acid, which is about 200 to 340 mg/L of potassium sorbate (multiply the sorbic-acid figure by 1.34).
When to add it
At bottling, on a stable dry wine you are back-sweetening, always alongside sulphite.
Legal limit
Sorbic-acid limit 300 mg/L (US TTB), 200 mg/L (EU).
Vegan
Not a fining question for this one; nothing animal-derived is involved in a normal addition.
In practice
It stops yeast budding but does not kill an active ferment, so it cannot arrest one already running: it is only for a finished wine you are sweetening. Never add it without sulphite, because lactic bacteria turn sorbic acid into a compound that smells of crushed geranium, and that taint is permanent. Its own detection threshold is around 135 mg/L, so heavy doses are tasted.
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
Potassium metabisulphite
The antioxidant and antimicrobial the whole cellar leans on, and the usual source of SO2.
Sodium metabisulphite
An SO2 source like the potassium salt, but one that leaves sodium in the wine.
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.