The Chemistry of Chlorine Removal

Campden tablets are commonly used in winemaking to provide a controllable amount of sulphur dioxide, which is an anti fermentative, and helps to slow bacterial growth. They consist of the metabisulphite (more properly pyrosulphite) ion in combination with either potassium or, more commonly in the UK, sodium, and for our purposes can be considered to be more or less 'pure'. Metabisulphite is an excellent reducing agent and when it does so the end products are sulphate and hydroxide ions. When a campden tablet is dissolved in water it produces sulphur dioxide according to the following reaction.

Na2S2O5 + H20 -> 2SO2 + 2Na+ + 2OH-

If there is any chlorine present in the water (in the UK this is more than likely) then sulphur dioxide reduces the chlorine as follows

SO2 + 2Cl(*) + 2H2O -> 4H+ + 2Cl- + SO42-

With the overall reaction (for free chlorine) being

2Na+ + S2O52- + 4Cl + 3H2O -> 2Na+ + 2SO42- + 6H+ + 4Cl-

Where chloramine is involved then the reaction becomes

2Na+ + S2O52- + 2H2NCl + 3H2O -> 2Na+ + 2SO42- + 2Cl- + 2NH4+

(That's the chemistry over with :D, unfortunately the maths bit is still to come)

The usual recommendation of 1 Campden tablet per 20 (US) gallons (~77.5l or 17 UK gallons) is intended to deal with a worst-case scenario of 3mg/l chlorine as chloramine. This would add 1.9mg/l of sodium; 3mg/l of chloride, 8mg/l of sulphate, 1.5mg/l ammonium ions and the alkalinity would be reduced by 4.2mg/l as calcium carbonate. If you consider that we normally boost calcium or chloride levels to 100-150mg/l, the extra 8mg/l sulphate or 3mg/l chloride is pretty insignificant. In most cases less metabisulphite will be required, which means some sulphur dioxide will be left over once the chlorine has been removed. Some will be driven off when the liquor is heated prior to mashing and sparging, and the rest will react with organics in the mash reducing them, and forming reductones, which are considered desirable as they help prevent oxidation and staling during storage.

Of course if the level of Chlorine/chloramine is known (say from a water company quality report, or from using a test kit), the amount of sodium metabisulphite can be precisely determined from the following table (courtesy AJ deLange)

Per mg free ChlorinePer mg free Chloramine
mg sodium metabisulphite added1.3372.674
mg sodium added0.3230.646
mg sulphate added1.352.70
mg ammonia added00.51
mg chlorine added1.01.0
Alkalinity neutralised / mg/l as CaCO32.111.43

I would like to say a big thank you to AJ deLange for providing me with a copy of his work, which forms a significant portion of this article.

Adding 1/2 a campden tablet to 10 Gallons of brewing liquor will disperse any chlorine or chloramine, and while it will add chloride and sulphate to the liquor, it does so in such incredibly small levels that it can be effectively ignored, any unreacted sulphite acts as a reductant, preventing later oxidation of the wort/beer. For brewers using standard mains water which routinely have chlorine, and chloramine, added this is a cheap, effective and simple way of removing the chlorine and can only be considered a good thing.

(*) Chlorine does not exist as Cl, but for the purposes here it can be considered so. The actual reaction is

Cl2 + H2O -> H+ + Cl- + HOCl

and at higher pH it is

Cl2 + 2OH- -> Cl- + OCl- + 2H2O

The two forms are in equilibrium as

HOCl <--> H+ + OCl-

Thanks to Aleman for the text on this page.

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