Potassium metabisulfite is one of the most important winemaking compounds.
It is an antioxidant and bactericide that releases sulfur dioxide into wine must.
Use 1/4 teaspoon per five gallons to add 50 ppm.
Or, mix 1/4 pound in 1 quart of water to make a stock solution;
1 teaspoon of stock solution in 1 gallon of must yields 50 ppm sulfur dioxide.