calumscott
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Dechlorination
Sodium Metabisulfite and Potassium Metabisulphite are used for chemical dechlorination. They are also the two possible ingredients in the ubiquitous Campden Tablet and are used interchangeably. The only difference in terms of getting chlorine and chloramine out of your water is that one leaves Sodium behind, the other Potassium - and not very much of it either...
The first thing that happens is the production of Sulphurous Acid from the sulphur dioxide released when the Potassium/Sodium Metabisulphite dissoves:
That Sulphurous Acid then reacts with the various Chlorine residual species:
Taking each in turn then:
Chlorine
This is converting any chlorine (rather than chloramine) dissolved into the mains water into hydrochloric acid and sulphuric acid. Just acids, natural ones at that and have no real bearing on anything much, perhaps a very slight lowering of pH of your water but given the concentrations we homebrewers would probably struggle to even measure it.
Chloramine
This is converting Chloramine (the stuff that actually smells like what you think chlorine smells like!) into Sulphuric Acid and Ammonium Chloride - yeast nutrient! Handy that!
Dichloramine
This converts dichloramine into Ammonium Choride (more yeast nutrient), hydrochloric acid and sulphuric acid.
And finally:
Nitrogen Trichloride
This converts Nitrogen Trichloride into Ammonium Chloride, hydrochloric acid and sulphuric acid.
Stabilising
Campden tablets perform another vital role, that of stabilising worts and musts. The creation of the SO2 at the start of the process will effectively disrupt the ability of the yeast to function. For a while...
...they will get over it and start again though! This makes the Campden Tablet ideal for suppressing any wild yeasts or other nasties in musts for wine and cider (not really an issue for beer worts as they get boiled anyway) and letting your intended yeast become the dominant organism in your brew. Basically the Campden Tablet unlevels the playing field massively in favour of your chosen yeast.
Stopping Fermentation
This is where we find the limits of the Campden Tablet wonder pill. It won't actually kill off your yeast when you are through with fermentation. It is however, and essential part of the process. A crushed Campden Tablet added in conjunction with Potassium Sorbate will completely kill everything off and leave you with a safe-to-bottle brew.
Dosing Rates
For dechlorination of water one Campden Tablet will treat up to 20 gallons (46 litres).
For stabilising of musts, one Campden Tablet will treat one gallon (basically a demijohn be it a one gallon or five litre one)
For stopping fermentation one Campden Tablet in conjunction with 1/4 to 1/2 tsp Potassium Sorbate will treat a one gallon or five litre demijohn.
Who'd have thought that those little tablets were so clever? Campden Tablets for the win!
Oh, and if you'd ever wondered why "Campden" Tablets, well the original solution was invented in Chipping Campden, Gloucetershire and it seems that Boots thought it would be a good name when they developed the tablet form...
Sodium Metabisulfite and Potassium Metabisulphite are used for chemical dechlorination. They are also the two possible ingredients in the ubiquitous Campden Tablet and are used interchangeably. The only difference in terms of getting chlorine and chloramine out of your water is that one leaves Sodium behind, the other Potassium - and not very much of it either...
The first thing that happens is the production of Sulphurous Acid from the sulphur dioxide released when the Potassium/Sodium Metabisulphite dissoves:
Code:
SO2 + H2O --> H2SO3That Sulphurous Acid then reacts with the various Chlorine residual species:
Code:
H2SO3 + HOCl --> HCl + H2SO4
H2SO3 + NH2Cl + H2O --> NH4Cl + H2SO4
2H2SO3 + NHCl2 + 2H2O --> NH4Cl + HCl + 2H2SO4
3H2SO3 + NCl3 + 3H2O --> NH4Cl + 2HCl + 3H2SO4
SO2 = Sulphur Dioxide
H2O = water
H2SO4 = Sulphurous Acid
HOCl = Hypochlorous acid - formed when Chlorine dissolves in water
HCl = Hydrochloric Acid
H2SO4 = Sulphuric Acid
NH2Cl = Chloramine
NH4Cl = Ammonium Chloride
NHCl2 = Dichloramine
NCl3 = Nitrogen TrichlorideTaking each in turn then:
Chlorine
Code:
H2SO3 + HOCl --> HCl + H2SO4Chloramine
Code:
H2SO3 + NH2Cl + H2O --> NH4Cl + H2SO4Dichloramine
Code:
2H2SO3 + NHCl2 + 2H2O --> NH4Cl + HCl + 2H2SO4And finally:
Nitrogen Trichloride
Code:
3H2SO3 + NCl3 + 3H2O --> NH4Cl + 2HCl + 3H2SO4Stabilising
Campden tablets perform another vital role, that of stabilising worts and musts. The creation of the SO2 at the start of the process will effectively disrupt the ability of the yeast to function. For a while...
...they will get over it and start again though! This makes the Campden Tablet ideal for suppressing any wild yeasts or other nasties in musts for wine and cider (not really an issue for beer worts as they get boiled anyway) and letting your intended yeast become the dominant organism in your brew. Basically the Campden Tablet unlevels the playing field massively in favour of your chosen yeast.
Stopping Fermentation
This is where we find the limits of the Campden Tablet wonder pill. It won't actually kill off your yeast when you are through with fermentation. It is however, and essential part of the process. A crushed Campden Tablet added in conjunction with Potassium Sorbate will completely kill everything off and leave you with a safe-to-bottle brew.
Dosing Rates
For dechlorination of water one Campden Tablet will treat up to 20 gallons (46 litres).
For stabilising of musts, one Campden Tablet will treat one gallon (basically a demijohn be it a one gallon or five litre one)
For stopping fermentation one Campden Tablet in conjunction with 1/4 to 1/2 tsp Potassium Sorbate will treat a one gallon or five litre demijohn.
Who'd have thought that those little tablets were so clever? Campden Tablets for the win!
Oh, and if you'd ever wondered why "Campden" Tablets, well the original solution was invented in Chipping Campden, Gloucetershire and it seems that Boots thought it would be a good name when they developed the tablet form...
