Sulfite and brewing water

[RPh_Guy]

I'm confused about the information in the sticky (https://www.homebrewtalk.com/forum/threads/campden-tablets-sulfites-and-brewing-water.361073/) and I have attached the article since the link in the sticky is broken.

My first question is about the amounts of products of the reaction.

For example:
Monochloramine: 51.45 g/mol
Sulfate: 96.06 g/mol

1 mol of monochloramine yields 1 mol of sulfate, so 1mg of monochloramine will yield 1.87mg of sulfate, NOT the 2.7mg listed in the article/sticky.

The amounts for other compounds also appear to be incorrect, or I've missed some step in the calculation. The stoichiometry and math seem pretty simple.

My second question is about the ascorbic acid. One "concern" listed about ascorbic acid is that it has a "finite shelf life" What is the shelf life of ascorbic acid and what is the shelf life of sulfite? Does the tablet form radically improve the shelf life? Most sources online seem to believe that ascorbic acid has a longer shelf life than sulfite.

@mabrungard @ajdelange

 

[Silver_Is_Money]

K2S2O5 + H2O = 2SO2 + 2KOH

2SO2 + 8NH2Cl + 12H2O = 2SO4-- + 8Cl- + 8NH4OH-

If the above is how the reaction actually progresses (of which I have serious doubt), then It looks to me like 1 mol of K-Meta evolves 2 mols of sulfate ions while in the process of reacting with 8 mols of monochloramine. No idea as to whether or not it progresses in this fashion, but it all weight and atom count balances if I summed things correctly. It certainly does not charge balance, thus my skepticism.

 

[RPh_Guy]

Well, here's my interpretation of the basic reaction of sulfite with monochoramine:
SO2 + 2 H2O + NH2Cl → H2SO4 + HCl + NH3

These acid products would dissociate and react, leaving us with chloride, ammonia, and neutralized alkalinity.

I'm using SO2 for simplicity, but if you substitute bisulfite into the equation the chemistry is the same.
SO2•H2O ↔ H+ + HSO3-

So 1 mol of monochoramine reacts with 1 mol of sulfite (0.5 mol of bisulfite salt) to yield 1 mol of ammonia, 1 mol of chloride, 1 mol of sulfate, and 3 mol of hydrogen ions.

This doesn't match up at all with the numbers in the article, so I'm messing something up or the article is wrong.

 

[Silver_Is_Money]

It's likely overall more complex. There are likely to be multiple reaction outcomes going on simultaneously for K-Meta in water with malts present, with each outcome path fractionally at some (unknown to me) percentage.

One of such potential paths:
In the presence of free H+ (wherein all malts are assuredly acidic), K-Meta reacts as follows:
K2S2O5 + 2H+ = 2K+ + H2O + 2SO2

This reaction path also evolves 2 mols of SO2 for every mol of K-Meta

All of the various potential and fractional reaction progression paths are also assuredly reversible to some degree, mitigated in reversal potential only by the evolution of SO2 as a gas. SO2's solubility in water is rather high though. The ability to reverse is a function of the various reaction components Ka and Kb dissociation constant(s), and these would also determine the potential reaction paths fractional percentages. Ultimately, due to reversals, the various of potential or possible reactions are all unlikely to ever go to full completion.

 

[RPh_Guy]

There are likely to be multiple reaction outcomes going on simultaneously for K-Meta in water with malts present, with each outcome path fractionally at some (unknown to me) percentage.

I'm interesting in the products of the reaction that occurs when adding a metabisulfite salt to chlorinated or chloraminated tap water.
Hopefully it's not that complex. I can't find the source of my calculation error. Maybe AJ got his calculations wrong? It's not clear who wrote that article since there's no name(s) on it.

Misinformation about sulfite is rampant and I'm trying to get all the facts into one place (I'm writing an article).

One of such potential paths:
In the presence of free H+ (wherein all malts are assuredly acidic), K-Meta reacts as follows:
K2S2O5 + 2H+ = 2K+ + H2O + 2SO2

Dissociation of metabisulfite salt in water:
K2S2O5 + H2O → 2 K+ + 2 HSO3-

Equilibrium of sulfite species:
SO2•H2O ↔ H+ + HSO3- ↔ 2 H+ + SO32-

Regardless of the sulfite species in the reaction, the number of moles is the same, so that shouldn't affect my calculations for the chlorine reaction
SO2 + 2 H2O + NH2Cl → H2SO4 + HCl + NH3 ↔ 3 H+ + [SO4]2- + Cl- + NH3


Right?

 

[Silver_Is_Money]

H2SO4 + HCl + NH3 ↔ 3H+ + (SO4)2- + Cl- + NH3.

This is not appear to be a balanced equation.

 

[RPh_Guy]

Looks balanced to me. It's hard without proper subscripts and superscripts. I used brackets to indicate the 2 minus charge on the sulfate ion.

Maybe I should have written it like this:
H2SO4 + HCl + NH3 ↔ 3H+ + [SO4]-- + Cl- + NH3.

1 sulfate, 1 chloride, 1 ammonia, 3 hydrogen

 

[Silver_Is_Money]

Looks balanced to me. It's hard without proper subscripts and superscripts. I used brackets to indicate the 2 minus charge on the sulfate ion.

Maybe I should have written it like this:
H2SO4 + HCl + NH3 ↔ 3H+ + [SO4]-- + Cl- + NH3.

1 sulfate, 1 chloride, 1 ammonia, 3 hydrogen

Ah, I took the brackets to mean the presence of two sulfate ions, as can be seen in my ill fated attempt at changing your brackets to parentheses.