How much Chloride/Sulfate is added to water when using Campden Tablets? - Home Brew Forums

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Old 01-05-2010, 09:59 PM   #1
kal
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Hi guys,

I use one campden tablet (sodium metabisulphite) in 20 gallons of my city water to remove cholarmine. 1 tablet is supposed to be able to handle 20 gallons of water with 2 ppm chloramines. This is exactly the level of chloramine that I have.

In Brewing Classic Styles, John Palmer says the following about this:

"Both chlorine and chloramine are reduced to insignificant levels of sulfate and chloride ions (<10 ppm) within a couple of minutes at room temperature."

I also adjust my minerals (salts) based on style using www.ezwatercalculator.com.

My city's S04 and Cl are already very low (Cl=6, SO4=26). My chloramine is at 2ppm. If wanted to be more accurate with my salt additions should I add some points to my Cl and S04 counts because I use campden tablets or is really that insignificant? Adding 10ppm to my existing Cl count is nearly 3x as much!

Kal


 
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Old 01-07-2010, 08:48 PM   #2
martinworswick
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i'm also interested in the answer to this, less than 10ppm would raise my chloride and sulphate to around 11ppm and 18ppm

 
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Old 01-07-2010, 09:02 PM   #3
Scimmia
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It would depend on your specific water's level of chlorination, so I don't think anyone's going to be able to answer that for you. At that low of levels, though, the chloride/sulfate levels are pretty much useless anyway

 
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Old 01-07-2010, 09:09 PM   #4
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I'd further be curious how to calculate Sodium or Potassium contributions and sulfite addition.

 
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Old 01-07-2010, 09:21 PM   #5
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Quote:
Originally Posted by Scimmia View Post
It would depend on your specific water's level of chlorination, so I don't think anyone's going to be able to answer that for you. At that low of levels, though, the chloride/sulfate levels are pretty much useless anyway
Both good points. My water's chloramine level is 2 ppm which I believe is fairly standard.

Kal

 
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Old 01-07-2010, 09:34 PM   #6
martinworswick
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Quote:
Originally Posted by Scimmia View Post
It would depend on your specific water's level of chlorination, so I don't think anyone's going to be able to answer that for you. At that low of levels, though, the chloride/sulfate levels are pretty much useless anyway
i knew my levels were low and have been adjusting them, but until i read this i didn't know if i was "over adjusting" them

 
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Old 01-07-2010, 09:41 PM   #7
Scimmia
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Quote:
Originally Posted by kal View Post
Both good points. My water's chloramine level is 2 ppm which I believe is fairly standard.

Kal
It is when the water leaves the plant, and while chloramine is nowhere near as volatile as chlorine, it will very likely be different by the time it reaches your house. Chlorine/chloramine will also change quite a bit, as they detect different things in the water.

Of course, all of that could be said about most things we look at on a water report.

At 2 ppm, I wouldn't worry about it at all. I'm no chemist, but I can't imagine you'd end up with more than 2 ppm total chloride + sulfate from it; you're taking on some sulfite from the camden, but releasing some other molecules in the reaction as well.

 
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Old 01-07-2010, 09:47 PM   #8
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Thanks Scimmia.

What surprises me is how much my pH drops when I add one tiny campden tablet to 20 US gallons of water: It drops from 9.2 to 8.0. (Measured using a calibrated phep 5 pH meter).

Kal

 
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Old 01-07-2010, 11:49 PM   #9
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I don't for sure if this is how metabisulfite reacts, but here is my take on some reasonable redox reactions:

Eqn. 1: Na2S2O5 + H2O -> NaHSO3 (comproportionation)
Eqn. 2: NaHSO3 + NH2Cl + H2O -> NaHSO4 + NH3 + Cl- (reduction of chloroamine)

Eqn 3: 2NaHSO3 + O2 -> 2NaHSO4 (reduction of oxygen)
Eqn 4: NaHSO3 + H+ <-> SO2 + H2O + Na+ (SO2-SO3(2-) equilibrium)


The metabisulfite breaks down to sulfite in water, forming a strong reductant (eqn 1). The sulfite reduces the monchloroamine (eqn 2). From here you could estimate that your increase in chloride concentration will be equal the concentration of chloroamine in your water.

[Cl-] = [NH2Cl]

Determining the increase in sulfate concentration could be a little trickier. Every equivalent of chloroamine reduced generates one sulfate (eqn 2), but the NaHSO3 may also reduce oxygen (eqn 3). This is further complicated by the equilibrium between SO2 and HSO3- (eqn 4). At the very least you can set some upper and lower limits on the increase in the sulfate concentration. The sulfate concentration will be at least equal the concentration of chloroamine and less than or equal to twice the concentration of metabisulfite added.

The reduction of oxygen is where you could get a pH drop from. Assuming this is only reaction that generates an acidic species, NaHSO4 has a pKa of 1.9ish, a decently strong acid, it's possible to get a better estimate of the increase in sulfate concentration. If we knew the buffering capacity of the water (from carbonate and phosophate) and the corresponding pH change after addition of the metabisulfite, it's possible to determine the concentration of sulfate from the oxygen reduction. This value plus the concentration sulfate of chloroamine reduction should give an accurate estimate of the total increase in SO4(2-) if we make the assumption that any remaining sulfite is eliminated as SO2. A reasonable assumption considering the equilibrium would be skewed to SO2 as the pH decreases).

tl;dr Summary
[NH2Cl] = [Cl-] <= [SO4(2-)] <= 2 x [Na2S2O5]

Hope that's correct and helpful.


 
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Old 01-11-2010, 12:29 PM   #10
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In the final water episode of the Brewing Network podcasts with Jamil, John Palmer answers this question: it has a negligible effect and don't worry about it.
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