You could use a Campden tablet or mix a solution and use an amount of that. I don't have the solution info with me now but someone else may have it. I found it easier to just use the tablet for the 1-gallon carboys.
Josh for five gallons you would add 1/4 tsp. To help you out you need to offer more information. If you want to just go with the simple rule as five gallons every three months the easiest way to do that is add 5 ozs of warm water in a measure cup. Add 1/4 tsp of meta and stir to dissolve. Now add one ounce to your wine and dump the rest.
After a miscalculation on my first 1 gallon batch, I've switched to campden tablets for my smaller batches. Lets just say 1/8th tsp is way too much for a 1 gallon batch.
Well I feel dumb now, I knew campden tablets were k meta but for whatever reason I've only been using them for sanitation of equipment -.- thanks for the help guys, I think I need some sleep...