Ascorbic acid
vitamin C
A fast oxygen scavenger for whites, used only alongside sulphite.
Typical dose
Around 50 to 100 mg/L at bottling, with adequate free SO2 already present.
When to add it
At bottling, into whites and roses about to meet oxygen.
In practice
Mops up oxygen faster than sulphite can, which protects delicate white aromatics at the moment of bottling. But once it is spent it turns pro-oxidant, so it is safe only with enough free SO2 to carry the job on. Never used alone.
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.
Potassium sorbate
Stops surviving yeast refermenting a sweetened wine. A preventer, not a killer.
Dimethyl dicarbonate
A cold sterilant that kills yeast at bottling and then breaks down.