Just use Campden then don't worry about it. I keep a 5-gallon jug filled with Campdenated tap water at all times (and if I brewed bigger batches, I'd keep 10 or 15 gallons). When it's emptied, I fill it back up and add 1/2 crushed Campden tablet. Cheap 'n' easy. The reaction is said to be instantaneous, but I figure it's easier to just have it all ready for next time.