From Distillers Wiki
Tablets used in winemaking to sanitize equipment and fermentation media and add free SO2 to the must or wine. When crushed and dissolved, they provide sulfur dioxide (SO2) in a convenient form. Tablets must be crushed to use, but this ensures the proper dosage and assists in their dissolution. The active ingredient in Campden tablets can be purchased bulk from most winemaker suppliers under its chemical name, potassium metabisulfite.
For sanitizing bottles, primaries, secondaries, funnels and other equipment, two crushed tablets dissolved in 1 gallon of water will suffice. Do not rinse equipment after sanitizing.
For adding to must, use one crushed and dissolved tablet per gallon of must and wait 12 hours before adding yeast. Campden tablets come in various sizes and doses, so inquire if not packaged with instructions. Most tablets are intended to dose 5 US gallons (19 liters) or 5 Imperial gallons (23 liters).