Originally Posted by mbbransc
I bought some campden tablets today manufactured by Crosby & Baker. It states it equates to 30 ppl of SO2 in 1 gallon of water. I feel like that is less than most campden tablets (I'm guessing around 65-75ppm). Can anyone confirm how much I need to use in a 5gal batch?
1 mmol, 222 mg, of potassium metabite is equivalent to 2 mmol, (2*64 = 128 mg) of sulfur dioxide. So 30 ppm in 1 gal (3.785L) is 113.55 mg SO2 which came from 113.55*222/128 = 197 mg metabite. That does seem low. As I recall the typical Campden tablet weighs almost 700 mg and thus contains about 700/222 = 3.15 mmol K2S2O5 which would yield about 403 mg which spread over approximatley 4 L would give about 100 mg/L.
Based on the 700 mg/tablet 1 was enough for 20 gal. See the sticky at http://www.homebrewtalk.com/f128/cam...-water-361073/