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 (SO 2) 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 and may contain either sodium metabisulfite or potassium metabisulfite, so inquire if not packaged with instructions. Campden tablets were developed by the Fruit and Vegetable Preserving Research Station at Campden, Glos., England, as a durable replacement for the Campden Fruit Preserving Solution. The latter was developed circa 1920-1923. Also see Potassium Metabisulfite and Sodium Metabisulfite.
Also known as "Sorbistat K" and affectionately as "wine stabilizer," potassium sorbate produces sorbic acid when added to wine. It serves two purposes. When active fermentation has ceased and the wine racked the final time after clearing, 1/2 tsp. added to 1 gallon of wine will render any surviving yeast incapable of multiplying. Yeast living at that moment can continue fermenting any residual sugar into CO2 and alcohol, but when they die no new yeast will be present to cause future fermentation. When a wine is sweetened before bottling potassium sorbate is used to prevent refermentation. It should always be used in conjunction with potassium metabisulfite (1/4 teaspoon per 5 gallons of wine or 1 crushed and dissolved Campden tablet per gallon) and the wine will not be stabilized without it. It is primarily used with sweet wines and sparkling wines, but may be added to table wines which exhibit difficulty in maintaining clarity after fining. Also see Sodium Benzoate and Wine Stabilizer.
The above info is from Jack Keller's winemaking site: