Copper sulphate
CuSO4, copper(II) sulphate pentahydrate
A last-resort treatment for hydrogen sulphide and light mercaptans.
Typical dose
A copper screen sets the trial; the minimum that clears the smell, typically fractions of a mg/L of copper.
When to add it
On a finished wine showing reduction, once a bench trial confirms copper helps.
Legal limit
Residual copper limit 0.5 mg/L (US TTB).
Vegan
Not a fining question for this one; nothing animal-derived is involved in a normal addition.
In practice
Binds hydrogen sulphide and simple mercaptans into odourless copper sulphides, but it does nothing for the heavier disulphides a neglected reduction becomes, which is why catching it early with a splash-rack beats treating it. It also strips desirable thiols, can throw a copper haze weeks later, and residual copper is capped, so it is a treatment of last resort measured in tiny amounts.
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 antimicrobial and reduction treatment