Why does dechlorinator lower oxygen in a tank?

[CT_]

Why does dechlorinator lower oxygen in a tank?  I've heard this a lot and I don't get why.  I tried to look up the reaction between sodium thiosulfate and chlorine and I found two reactions depending on if its cl2 or chlorite:

Na2S2O3 + Cl2 + H2O → S + 2HCl + Na2SO4
Na2S2O3 + 4HOCl + H2O -----> 2NaHSO4 + 4HCl

Neither of these seem to consume any O2.  Is there an downstream reaction with one of the products?

[Betsy]

I think it's essentially a redox reaction (reduction-oxidation)?

The sodium thiosulfate (Na2S2O3) reduces the chlorine and as a result of that process becomes oxidized (with the dissolved oxygen in the water and maybe also oxygen from the water molecule itself?) to form the sodium sulfate compound (Na2SO4).

That may be an over simplification...and it might also be completely wrong, haha!  Following this, because #sciencerules! 🤓

Edited March 24, 2021 by Betsy

[Hobbit]

Found something! From an EPas pamphlet:

Quote

Dosing in excess must be avoided because excess sulfite can react with dissolved oxygen (four parts sulfite to one part oxygen) in the wastewater to produce sulfates, potentially leading to reduced dissolved oxygen concentrations and low pH levels in the finished effluent for high levels of overdose (WEF, 1996).

https://www3.epa.gov/npdes/pubs/dechlorination.pdf

So any extra SO3/sulfite (or S2O3/thiosulfite I presume) can react with oxygen to form SO4/sulfate. 

[RockMongler]

The thing to remember about chemistry, especially in complex systems like a fish tank, is the 'ideal' reaction (which are the ones you look up for sodium thiosulfate and chlorine/chloramine) isn't the only possibility.  Sometimes the excess sulfate that isn't binding up the stuff you want out of your water, will react with the oxygen.  Its the same way that burning methane ususally doesn't generate carbon monoxide, it ususally makes water and carbon dioxide.  But, if you are burning methane under lower oxygen conditions, you can start getting carbon monoxide forming instead of carbon dioxide.

[CT_]

Thanks guys! I knew I was missing something!

 

15 hours ago, Hobbit said:

Found something! From an EPas pamphlet:

https://www3.epa.gov/npdes/pubs/dechlorination.pdf

So any extra SO3/sulfite (or S2O3/thiosulfite I presume) can react with oxygen to form SO4/sulfate. 

So does this mean once we open our Prime bottle or whatever we use that it will eventually go bad because of atmospheric O2?  I wonder if there's a preservative that reacts to the O2 first and makes oxygen depletion in the tank worse.

[Philip]

18 hours ago, RockMongler said:

The thing to remember about chemistry, especially in complex systems like a fish tank, is the 'ideal' reaction (which are the ones you look up for sodium thiosulfate and chlorine/chloramine) isn't the only possibility.  Sometimes the excess sulfate that isn't binding up the stuff you want out of your water, will react with the oxygen.  Its the same way that burning methane ususally doesn't generate carbon monoxide, it ususally makes water and carbon dioxide.  But, if you are burning methane under lower oxygen conditions, you can start getting carbon monoxide forming instead of carbon dioxide.

Exactly. In your example you are more often than not getting incomplete combustion, resulting in CO. In most cases you want extra O2 to make sure you are using up all the carbon.  The reactions are not really complex nor unexpected, it's just so many different things going on at once it's hard to simplify. 

[RockMongler]

1 hour ago, Philip said:

Exactly. In your example you are more often than not getting incomplete combustion, resulting in CO. In most cases you want extra O2 to make sure you are using up all the carbon.  The reactions are not really complex nor unexpected, it's just so many different things going on at once it's hard to simplify. 

That's kind of specifically what I mean by complex.  It is not a simple system.  It's complex because there are a lot of things going on at once in the same system, and a chemistry layperson (which is most people in the world, and the hobby) is going to assume everything always works out as nicely as you learned in a high school chemistry course.  There are a lot of 'moving parts' in the chemistry of our tanks.  The kind of chemistry most people get taught in school doesn't necessarily dig into systems where there are tons of things going on, and only look at clean, closed, ideal conditions.  As someone who teaches high school chemistry, I wouldn't want to even start talking about partial combustion considering the hard time most people have with the super straight forward basic chemistry concepts.

The big take away for most fish keepers is that the ideal reaction you look up for something usually isn't the whole picture.  If you run out of one thing that the reaction is relying upon, different things are very likely to happen with the remaining materials.  They won't always just sit there and do nothing.

[ChefConfit]

Just another thought. A lot of t3dechlorinators do more than just dechlorinate. I know several detoxify ammonia, nitrite and nitrate, and bind heavy metals. Could any of those reactions consume O2? 

[Philip]

On 3/25/2021 at 7:33 AM, RockMongler said:

That's kind of specifically what I mean by complex.  It is not a simple system.  It's complex because there are a lot of things going on at once in the same system, and a chemistry layperson (which is most people in the world, and the hobby) is going to assume everything always works out as nicely as you learned in a high school chemistry course.  There are a lot of 'moving parts' in the chemistry of our tanks.  The kind of chemistry most people get taught in school doesn't necessarily dig into systems where there are tons of things going on, and only look at clean, closed, ideal conditions.  As someone who teaches high school chemistry, I wouldn't want to even start talking about partial combustion considering the hard time most people have with the super straight forward basic chemistry concepts.

The big take away for most fish keepers is that the ideal reaction you look up for something usually isn't the whole picture.  If you run out of one thing that the reaction is relying upon, different things are very likely to happen with the remaining materials.  They won't always just sit there and do nothing.

So true and not just for Chemistry, but for life in general. 

[Philip]

On 3/25/2021 at 8:23 AM, ChefConfit said:

Just another thought. A lot of t3dechlorinators do more than just dechlorinate. I know several detoxify ammonia, nitrite and nitrate, and bind heavy metals. Could any of those reactions consume O2? 

Anything which there is oxidation taking place there will be a gain of oxygen, but that doesn't necessarily mean you will gain O2 in the form of free oxygen gas. You may have 3CO absorb the O2 and become 3CO2. 

I think the point of the original question was asking if some reactions resulted in more O2 in an aquarium. The answer is probably yes with most of these examples, but not enough to really matter.  At least in the long term balance of the aquarium.