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I have a friend who is allergic to sulfites in wine, so he sticks to drinking beer. I use campden tablets to dechlorinate my water, at a rate of one tablet per 10 gallons. How much sulfite makes it into the final beer when treating water this way? How does it compare to sulfite levels in wine?

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That would be hard to know accurately. A better way to remove the chlorine would be to use a Brita or Pur filter (an activated carbon filtering system), or expose the water in a colorless carboy or container to intense light for several hours with the top open to allow the chlorine to escape (this is a common way to allow the chlorine to escape when prepping for fish tanks). These do not add sulfur compounds.

At the rate that you are using the campden, the sulfite levels would be less than the typical ppm in wine. Before the metabisulfite reacts with the chlorine and chloramine, the concentration would be about 7 parts per million (using standard .44 g tablets). Sulfite levels of less than 10 ppm in wine are allowed for a wine to be called sulfite free ( see http://wineintro.com/glossary/s/sulfites.html). So even if none of the campden reacted, at the rate that you are adding it, the concentration would be below that accepted as "sulfite-free" in wine.

I use a brita filter and have experimentally determined that chlorine and other contaminants are reduced to sub part per million levels. BTW - the same filters also remove sulfite and sulfates. Be certain to use a relatively fresh filter.

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Using campden is less expensive, faster, and more effective than using a filter. Basically, there is no sulfite left. According to BYO "The reaction converts chlorine into chloride and the sulfite is converted to sulfate." (http://www.byo.com/stories/wizard/article/section/121-mr-wizard/475-clearing-chloramine-a-historical-hopping-mr-wizard).

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That article does not seem to say that no sulfite remains, just that the amount that does react with chlorine compounds in the water will be converted into harmless chemicals. This may or may not be all of the sulfite that you added to the water, it seems to me. –  Dustin Rasener Oct 1 '11 at 17:29
    
I think it says (maybe it was another article I read) that used at the recommended rate, all sulfite is converted. –  Denny Conn Oct 1 '11 at 21:25
    
Not true in terms of effectiveness of campden v filtering. I know this from my own analytical chemistry experiments. Faster, yes, if he were using it at the standard (.44 g per gallon) rate. He is using it at a much lower rate than the standard brewing rate, understandable because he is just using it to remove chlorine/chloroxides, so the article you link to doesn't really apply to his question. –  drj Oct 1 '11 at 21:51
    
The chemistry involved is that the bisulfite reacts with the chloramine to produce ammonium (Mr Wizard mentions this), sulfate (also mentioned by Mr Wizard) and chlorine gas, not chlorides. The reason some people boil the treated water (Mr Wizard mentions this) is to drive off the dissolved chlorine gas. So campden isn't really faster either. And there is always some remaining sulfite, but that is irrelevant (as I mention in my answer) because the original concentration is lower than what is allowed for "sulfite-free" wine. –  drj Oct 1 '11 at 22:12

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