This is kind of a rambling rant, but I hoping someone might be able to tell me if it sounds right or if I am misunderstanding the situation. I also had a question at the bottom if anyone knows:
I recently followed the directions in a BYO article to dechlorinate brewing water and after doing further internet research, I am pretty sure the BYO article is way off and I added way too much sodium metabisulfite to my brew.
Here is the article.
The dechlorination reaction requires 1.47 mg of sodium metabisulfite to reduce 1 mg of free chlorine. In practice, this ratio is increased twothree fold. In easy to use terms, a 1/2-ounce Campden tablet can be used to dechlorinate 20 gallons
Mr. Wizard states that 1/2 ounce of sodium metabisulfite will dechlorinate 20 gallons of water. He also stated that this was one tablet. After doing the market research I found that pretty much all campden tablets that you can buy are actually only 550 mg or 1/50 of an ounce. However, before I realized that, I assumed that the tablets came in multiple sizes and I adjusted for my brew water based off of the weight stated in the article. I estimated 1/5 of an ounce for 7 gallons. That should have been at least 10 tablets, but my scale might be off because I ended up measuring out and adding 7 tablets to 7 gallons. Interestingly enough, that is the ratio that the bottle recommends for sterilization.
After reading more HBT threads on the subject (and reading the label), it seems that I should have used less than one tablet total. And also, doing the math in the BYO article shows that Mr. Wizard may be contradicting himself and that the article might have a fairly significant typo. 1.47 mg of SMS per 1 mg chlorine is only 38 mg for 26 Liters or 7 gallons. Scaled up for chlorine content of 2 - 4 mg/l, that is still only 76mg to 153 mg total. Mr. Wizard is off by at least a factor of 10 when he says to use 1/2 ounce for 20 gallons. It looks like that could be a typo and should say 1/2 gram for 20 gallons. That would make sense and match closely to his 1.47 to 1 ratio.
Anyway, so yea I put in 7 tablets for 7 gallons which is 3850 mg; so its a little higher than the 150mg I actually needed. My first concern was that the yeast would not survive or have a very hard time in the wort. However, this morning it looks like the yeast are fermenting the wort quite well. There was about 8 hours of time from when the water was treated and the yeast was pitched and as I understand it, the metabisulfite will be removed from the solution after a certain amount of time (24 hours is how long wine makers go). Perhaps 8 hours was long enough for it to get to safe levels. The wort was of course also boiled and I'm not sure if that also has an effect.
So is that a technical error in the BYO article? They are actually calling for numbers higher than even what I put in which would be beyond sterilization levels and leaving out the fact that you are supposed to wait 24 hours.
I also had another question if anyone knows, I couldn't find anything specific on this:
What does the sodium metabisulfate leave behind; anything that affects your water profile? I would guess there may be a certain amount of sodium. I also read a HBT thread that stated that the sulfite gets converted to sulfate. If this is true, anyone know how much of these compounds are present? Would it affect your chloride to sulfate ratio?