1/4 tsp of Potassium Metabisulfite powder adds 50ppm to 5 gallons of must (wine or cider).
Powdered sulphite can be used in three different ways:
1) For an equipment and bottle rinse, mix 1 ounce of powdered meta with one gallon of good, clean water. This "diluted" gallon solution can be used to rinse bottles, carboys, tubing and anything else that you might want to give a good inhibiting rinse to before the wine comes in contact with it. The equipment can then either be left to air-dry, or used immediately after the excess drips out. (This leaves a residual sulphite layer as a protectant in your bottle... which is good!)
2) As a "direct-dosage" solution (for adding diluted meta-bisulfite directly into your wine) 1 ounce (weight) of meta powder should be mixed into 8 fluid ounces of water. Mix well, until dissolved. One teaspoon of this "concentrated" liquid solution is equal to one Campden tablet. Five teaspoons in 5 gallons of wine will give approximately 75 ppm sulphite, an average-to-high good dosage.
3) Or, for those that do not like to mix solutions, 1/4 TEASPOON of straight meta powder can be added into 5 gallons of wine and it will give a dosage of 50 ppm. DO NOT ATTEMPT THIS WITH SMALL AMOUNTS OF WINE. You cannot measure precisely enough. Use this method only when you have 5 gallons or more - preferably more.
Old Metabisulfite loses potency with time and should be replaced every year to assure that you are getting correct levels. You can use old Metabisulfite to make solutions for sanitizing equipment.
Potassium Metabisulfite, (often referred to as "SO2", "sulfites" "meta", or "meta-bi") has several uses in winemaking. At the crush, sulfites are generally used to help control the spoilage bacteria and indigenous yeast that may already be present both on the fruit and in the winery (i.e. on the picking bins, processing equipment, tanks, tubing, etc).
The amount generally used is enough to inhibit most of the unwanted organisms but not enough to hinder a cultured yeast, which has a higher tolerance to sulfites than most of the indigenous organisms do. This inhibition effectively "wipes the slate clean" for the cultured yeast to step in and rapidly colonize the must so that it can effectively dominate the subsequent fermentation.
In addition, sulfites also help to inhibit the enzymatic browning of both musts and finished wines so that all of their delicate complexities can be preserved. Later, during storage and in the bottle, sulfites at the proper levels will further protect a wine by continuing to inhibit spoilage organisms, as well as by scavenging oxygen.
Note that the exact amount needed to effectively do the job is determined by the pH of the wine.
In addition, it's important to keep in mind that free SO2 levels fall faster in wood cooperage than in glass or stainless, so if you are using a barrel you will most likely need to manage sulfite levels more closely.