Copper sulfate solution is used to reduce Hydrogen Sulfide (H2S, or rotten egg smell) in wine. The copper sulfate binds with H2S and causes it to precipitate out as a black residue, the wine can then be racked off the residue.
Usage recommendations: Varies depending on the amount of copper sulfate. Proper levels should be determined by running bench trials.
As a rough guideline, 0.5 mL per gallon of 0.1% solution will give about 0.3 PPM of copper sulfate.