Potassium Metabisulphite tablets are used in many wine recipes. Acts as a antioxidant and preservative in wine. Campden tablets are an easy to measure form of metabisulphite. Each tablet per 4.5 Litres give approximately 50 ppm free sulphur dioxide. 0.55 gram each. 48% SO2. One tablet per gallon yields about 75 ppm. We recommend 1/2 tablet per gallon. You need to crush the tablets before adding them to your wine.