Sodium metabisulphite
SMS, sodium Campden
An SO2 source like the potassium salt, but one that leaves sodium in the wine.
Typical dose
A similar SO2 yield to the potassium salt; the same molecular-SO2 targeting.
When to add it
As potassium metabisulphite.
In practice
Works identically as a sulphite source but adds sodium rather than potassium, and enough Campden tablets over a wine's life is a measurable salt load. Most winemakers standardise on the potassium salt for anything that touches the wine, and keep the cheaper sodium form for sanitising solutions, which never do.
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.
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.