Potassium metabisulfite is added to wine to inhibit bacteria and yeast growth, as well as slow down oxidation. It may leave an unpleasant aftertaste in wine if the dose is too high. This chemical is also used in a water solution as an antiseptic rinse to sanitize equipment. It is identical to, but better than, Sodium Metabisulfite, because it does not add sodium to one's diet. CAUTION: Some people, particularly asthmatics, can have a severe allergic reaction to this substance.
Use 1/8 teaspoon of powder per gallon of wine provides 150 ppm free SO2. A little bit goes a long way, so be careful! Always test the free S02 content of your wine (using Titrets and Titret holder) to determine the proper amount to add. Generally speaking, the target free SO2 for red wines is 20-30 ppm and 25-40 ppm for white wines. The exact target depends upon the pH of the wine.