Potassium & Sodium Metabisulfite
Potassium or Sodium metabisulfite act as an anit-oxident(keeps your wine from turning to vinegar), helps preserve the color of the wine, and encourages the development of esters that give wine the smooth flavor.
As a Sanitizing Solution
Either are the most commonly used and the most effective sanitizing agents for wine making. The name is usually shortened to metabisulfite, or sulfite, and it can only be used to sanitize. It is not a cleaning agent.
When preparing a solution, dissolve 2 oz powder to one gallon water. This sanitizing solution can be used over again if properly stored in a sealed container. It remains effective for a few months. Always use in a well ventilated area.
When sanitizing your equipment with this solution, make sure to thoroughly clean any utensils that will be coming in contact with you wine.
Once you have sanitized equipment, be sure to rinse. Any leftovers may inhibit your fermentation.
To Kill Wild Yeast
When using to kill wild yeast and bacteria in your "must" PRIOR to starting fermentation, the dosage required is 1/4 tsp per 6 gallons of wine.
Dissolve the powder in water, then add to your "must". Please adhere to the waiting period(24 hrs)before starting fermentation. If you don't, you will kill your yeast and not be able to get fermentation started.
Many wine recipes call for Campden tablets. These are a form of Sodium Metabisulfite in a pre-measured dose. The usual requirement is one tablet per gallon of wine. This can be a little strong, so I would advise cutting it in half or 3/4.
The purpose of using campden tablets in your "must" BEFORE starting fermentation, is that you are killing any wild yeast or bacteria in the must that may create any off tastes.
When adding campden tablets to your must, crush and dissolve them in water, then add to your wine. You are recommended to wait for at least 24 hours before adding the yeast to start fermentation. This waiting period allows the solution to dissipate.