1. Removing chlorine / chloramine
2. Sanitizing fermentables
3. Sanitizing equipment
Anhydrous sodium metabisulfite (Na2S205) is typically the active ingredient in Campden tablets. Each Campden Tablet equals 1/16 teaspoon of sodium metabisulfite. When combined with water, the tablets provide an easy-to-measure delivery mechanism for adding approximately 75 parts-per-million (PPM) of sulfur dioxide (SO2) to each gallon of liquid and to the headspace in vessels used in fermenting.
In simple terms, dissolved Campden tablets kill microorganisms, but only for a while until the sulfur dioxide from the tablets dissipates to non-lethal levels for the little critters. At the proper levels they can be used to kill wild yeast, mold, bacteria and other microorganisms that might otherwise spoil what you're brewing; and after they're done you can add your own yeast to start fermentation. As an alternative you can also purchase potassium metabisulfite in bulk but then you need to measure tiny amounts of it instead of counting out a few tablets.
Small amounts of Campden tablets have the handy side-effect of precipitating chlorine and chloramine out of water. This is only a factor if you're using chlorinated water, such as a city water supply, and you're not using some other method of removing the chlorine such as reverse osmosis (RO), charcoal filtration or boiling. You want to remove the chlorine and chloramine because when left in the water, they can form smelly compounds called chlorophenols when they interact with beer.
I prefer Campden tablets to the other methods for dechlorination. Reverse osmosis is significantly more expensive. Filtering typically removes only a portion of the chlorine and chloramine, and it's also more expensive than Campden tablets. Both RO and filtering require additional equipment that you need to store and maintain. Boiling doesn't work on chloramine and it adds time to your brew day.
To use Campden tablets for Removing chlorine and/or chloramine, crush one tablet and add it to 20 gallons of water, or use 1/4 tablet for each five gallons. Don't over-think this: if you're somewhere near 1/4 tablet and somewhere near five gallons you'll be just fine. I toss a crushed partial tablet into the hot liquor tank before I start heating the water. Stir it up a bit and I'm ready to go.
To use Campden tablets sanitizing fermentables, add one or two tablets to each gallon of cider or must. Tablets should be crushed and dissolved in a small amount of water, then added to the must immediately. Wait 12 to 24 hours before adding your yeast to begin fermentation. For use with overripe and potentially spoiled fruit, go with two tablets and two days. For regular use, one tablet and one day. Allow the fermentable liquid to ventilate in an open container.
As far as I'm concerned, using Campden tablets sanitizing equipment makes no sense to me because StarSan and Iodophor are quicker, cheaper and easier to use. but if you have an bunch of tablets you want to use up, here's how to do it according to E.C. Kraus...
All equipment should be cleaned with soapy water first. Crush and dissolve sixteen (16) Campden Tablets per each gallon of water. Also add 1/2 teaspoon of Citric Acid. Sanitize fermentation vessels by putting in 2 to 3 inches of solution in the bottom of the vessel. Seal the vessel air-tight for 20 minutes to allow the fumes from the solution to permeate the inside walls. You can also put in the vessel other equipment such as hoses, hydrometer, air-locks, rubber stoppers to be sanitized at the same time.One Campden tablet can be added to each gallon of wine prior to bottling to preserve color and flavor. Crush the tablets and dissolve them in a small amount of the wine first. Potassium sorbate can also be added just before bottling time to eliminate re-fermentation.
Beware that beer, wine and other beverages treated with Campden tablets or sodium metabisulfite will retain trace amounts of sulfites after most of the SO2 has dissipated. Sulfites occur naturally in your body are generally not a problem except for a few people who are deficient in the natural enzyme to break them down. Allergic reactions have also been reported. Most commercial wines made in this country warn of sulfites on the label, except for the few that don't contain sulfites. Wines produced in other countries may not have the warning even though they contain the sulfites.