Smooth203
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Sodium Metabisulphite & Magnesium
Okay, so I dissolved some sodium metabisulphate in water and dipped a magnesium strip in as I'm messing about with electrolytes and electodes for
electrolysis. But to my surprise, the magnesium gave of a foul smelling gas (Is this SO2?).
So I have a few questions that I've struggled to find an answer for:
1. What exactly happens to Sodium Metabisulphite when it is dissolved in water?
2. What is the magnesium reacting with in the solution?
3. What is the full equation for the reaction?
I'm using deionised water and ambient temperature being around 18 degrees C.
Any help would be much appreciated, thank you.
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Crowfjord
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Sodium metabisulfite in aqueous solution gives off SO2 gas all by itself. What did the gas smell like? Was it sharp and acrid, or more
like eggs or farts? Was the magnesium tarnished or deteriorated?
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Smooth203
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Quote: Originally posted by Crowfjord  | | Sodium metabisulfite in aqueous solution gives off SO2 gas all by itself. What did the gas smell like? Was it sharp and acrid, or more
like eggs or farts? Was the magnesium tarnished or deteriorated? |
The Solution on its own melt sharp, the gas produced by the magnesium smelt eggy. The magnesium is quite new and looks new, there might be traces of
oxide layers I don't know apart from that.
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Crowfjord
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Sounds like the magnesium might have reduced some SO2 to H2S.
Probably something like:
3Mg + SO2 + 4H2O--> 3Mg(OH)2 + H2S
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Melgar
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| Quote: Originally posted by Smooth203 | | Quote: Originally posted by Crowfjord | | Sodium metabisulfite in aqueous solution gives off SO2 gas all by itself. What did the gas smell like? Was it sharp and acrid, or more
like eggs or farts? Was the magnesium tarnished or deteriorated? |
The Solution on its own melt sharp, the gas produced by the magnesium smelt eggy. The magnesium is quite new and looks new, there might be traces of
oxide layers I don't know apart from that. |
Well, that's an easy one then. The bisulfite ions are being reduced to hydrogen sulfide by the magnesium.
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Smooth203
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| Quote: Originally posted by Crowfjord | Sounds like the magnesium might have reduced some SO2 to H2S.
Probably something like:
3Mg + SO2 + 4H2O--> 3Mg(OH)2 + H2S |
Ah okay, thank you!
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Smooth203
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| Quote: Originally posted by Melgar | | Quote: Originally posted by Smooth203 | | Quote: Originally posted by Crowfjord | | Sodium metabisulfite in aqueous solution gives off SO2 gas all by itself. What did the gas smell like? Was it sharp and acrid, or more
like eggs or farts? Was the magnesium tarnished or deteriorated? |
The Solution on its own melt sharp, the gas produced by the magnesium smelt eggy. The magnesium is quite new and looks new, there might be traces of
oxide layers I don't know apart from that. |
Well, that's an easy one then. The bisulfite ions are being reduced to hydrogen sulfide by the magnesium. |
Okay, but Crowfjord said it was SO2, not bisulfite ions, so which would it be?
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Melgar
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They're sort of the same thing in an aqueous solution, (ie, -OH + SO2 = -HSO3) but in this case, it's probably more accurate to say that it's
bisulfite ions being reduced, since that's the form that metals would react with.
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Smooth203
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| Quote: Originally posted by Melgar |
They're sort of the same thing in an aqueous solution, (ie, -OH + SO2 = -HSO3) but in this case, it's probably more accurate to say that it's
bisulfite ions being reduced, since that's the form that metals would react with. |
Okay! Thanks for that, I'm doing A-Level chemistry but as far as identifying reactions goes I'm rather inept! Anyway, many thanks for both of your
help!!
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Melgar
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| Quote: Originally posted by Smooth203 | | Okay! Thanks for that, I'm doing A-Level chemistry but as far as identifying reactions goes I'm rather inept! Anyway, many thanks for both of your
help!! |
I see. In that case, it might help if it were elaborated. Sulfur dioxide, much like carbon dioxide, will react reversibly with water to form
sulfurous acid, H2SO3. Carbon dioxide forms carbonic acid, H2CO3. Both of these reactions would normally be driven in the gas-producing direction,
since when gas forms bubbles and leaves the solution, it can no longer react. To keep these ions in solution though, a base can be added, like NaOH.
This makes it so there are a lot more -OH ions in solution than there are +H ions. A free SO2 molecule that's about to leave the solution would
instead react with a -OH ion, forming a bisulfite ion again.
In fact, a similar reaction with CO2 is the reason you can smell sulfur dioxide in the first place. CO2 will slowly dissolve in the solution from the
air, and when it does, that creates more H+ ions and uses up -OH ions, acidifying the solution. Even though sulfurous acid is a stronger acid than
carbonic acid, because the concentration of bisulfite ions would be so much higher than bicarbonate ions, the species that ends up leaving solution is
more likely to be SO2. Additionally, you notice SO2 more because you can't smell CO2. If you wanted to eliminate the smell completely, you could add
NaOH to your solution, which would capture ANY acidic species entering the solution and keep it there.
Yet another chemical that behaves like this is chlorine. It reacts with water to form hypochlorous acid and hydrochloric acid. (HCl and HOCl) Both
of these acids can be neutralized by sodium hydroxide, which keeps them in solution.
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Smooth203
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| Quote: Originally posted by Melgar | | Quote: Originally posted by Smooth203 | | Okay! Thanks for that, I'm doing A-Level chemistry but as far as identifying reactions goes I'm rather inept! Anyway, many thanks for both of your
help!! |
I see. In that case, it might help if it were elaborated. Sulfur dioxide, much like carbon dioxide, will react reversibly with water to form
sulfurous acid, H2SO3. Carbon dioxide forms carbonic acid, H2CO3. Both of these reactions would normally be driven in the gas-producing direction,
since when gas forms bubbles and leaves the solution, it can no longer react. To keep these ions in solution though, a base can be added, like NaOH.
This makes it so there are a lot more -OH ions in solution than there are +H ions. A free SO2 molecule that's about to leave the solution would
instead react with a -OH ion, forming a bisulfite ion again.
In fact, a similar reaction with CO2 is the reason you can smell sulfur dioxide in the first place. CO2 will slowly dissolve in the solution from the
air, and when it does, that creates more H+ ions and uses up -OH ions, acidifying the solution. Even though sulfurous acid is a stronger acid than
carbonic acid, because the concentration of bisulfite ions would be so much higher than bicarbonate ions, the species that ends up leaving solution is
more likely to be SO2. Additionally, you notice SO2 more because you can't smell CO2. If you wanted to eliminate the smell completely, you could add
NaOH to your solution, which would capture ANY acidic species entering the solution and keep it there.
Yet another chemical that behaves like this is chlorine. It reacts with water to form hypochlorous acid and hydrochloric acid. (HCl and HOCl) Both
of these acids can be neutralized by sodium hydroxide, which keeps them in solution. |
Oh wow, thanks, that makes it clearer, there's me thinking it's so strange and different but it's so similar to how CO2 works  .
So, thank you for all your help, I'm sure I'll be back with another question sometime 
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PirateDocBrown
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Na2S2O5 (aq) + 6 Mg (s) + 9 H2O (l) = 2 H2S (g) + 2 NaOH (aq) + 6 Mg(OH)2 (s)
The precipitation of the Mg(OH)2 and evolution of H2S gas drives the reaction forwards.
In the absence of Mg, it's much simpler:
Na2S2O5 (aq) + H2O (l) = 2 SO2 (g) + 2NaOH (aq)
This has no solid precipitation to drive it, and SO2 is more soluble than H2S, which allows more reverse reactions.
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Smooth203
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| Quote: Originally posted by PirateDocBrown | Na2S2O5 (aq) + 6 Mg (s) + 9 H2O (l) = 2 H2S (g) + 2 NaOH (aq) + 6 Mg(OH)2 (s)
The precipitation of the Mg(OH)2 and evolution of H2S gas drives the reaction forwards.
In the absence of Mg, it's much simpler:
Na2S2O5 (aq) + H2O (l) = 2 SO2 (g) + 2NaOH (aq)
This has no solid precipitation to drive it, and SO2 is more soluble than H2S, which allows more reverse reactions. |
Oh I see, alright, thanks for that 
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AJKOER
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More likely and simply, the known reaction of Mg with H2SO3 forming hydrogen gas:
Mg + H2SO3 → MgSO3 + H2 (g)
Any sulfide on the surface of the magnesium metal forms trace amounts of correspondingly H2S as well.
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More complex and less likely, assume the presence of excess SO2 (from metasulfite acting on water), creates a disulfite:
HSO3- + S02 (aq) → HS205- (See source, equation (3) )
Then, with magnesium metal, dissolved oxygen in the water and in the presence of H+ (created by SO2 acting on water) forms a battery cell (with O2 and
Mg electrodes, see discussion at https://en.m.wikipedia.org/wiki/Magnesium_battery). The resulting electric current then is presumed to reduce a small amount of the disulfite as
follows:
HS205- + e- → HS- + .SO5- (See source, related reaction (5a) )
Source: "Free-radical chemistry of sulfite " by P Neta, link: https://www.google.com/url?sa=t&source=web&rct=j&...
which, in the presence of acid, forms H2S:
H+ + HS- = H2S
[Edit] Note: if the Mg source is an alloy, the same argument above holds without the required presence of oxygen. Also, the opening thread mentions
the presence of an electrolyte and electrodes.
--------------------------
To test the validity of sulfide formation on the surface of the magnesium metal, pre-treat the Mg with say a short vinegar bath, and see if H2S is
still formed.
Pre-boiling distilled water to remove dissolved gases and repeating the experiment in a closed vessel absence any O2 should eliminate the second
scenario.
Check the purity of the magnesium source with respect to possible alloy metals.
[Edited on 22-5-2017 by AJKOER]
[Edited on 22-5-2017 by AJKOER]
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Melgar
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The above explanations seem confusing to me. Magnesium would most likely react with -HSO3 to produce -OH, 2MgO, and MgS. Then MgS would react with
water to produce another MgO and H2S. MgO may or may not react with water to produce Mg(OH)2, like CaO would, since MgO is less reactive.
| Quote: | | The precipitation of the Mg(OH)2 and evolution of H2S gas drives the reaction forwards. |
No! This is not right at all. The massive amount of energy released when magnesium is oxidized is what drives the reaction forward. The other
reaction isn't right either, and doesn't account for the fact that the main dissolved species is bisulfite, not SO2. This is the real reaction:
Na2S2O5 (aq) + H2O (l) = 2NaHSO3 (aq)
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Boffis
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Zinc reduction produces at least some dithionite. Dithionite is not stable in acid solutions, however. So as the Mg dissolve the pH will rise if I
understand the equations above so dithionite may still be present in the final solution.
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woelen
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The reaction of Mg with bisulfite in solution is not a clean simple reaction. There are competing reactions. Bisulfite solution is quite acidic. One
reaction which occurs slowly is formation of hydrogen gas and production of Mg(2+) ions in solution. You can see this formation of H2, it forms
bubbles of gas. Another reaction is reduction of SO2 and H(+) from the acidic solution to H2S, water and Mg(2+) ions. This reaction cannot be seen,
because the H2S dissolves in the water. You can smell it though, some of it escapes with the bubbles of H2.
The pH rises, because both reactions consume H(+).
If you let the reaction run for a long time, then the solution becomes cloudy. H2S and SO2 further react with each other in a very complicated way,
producing many different foul-smelling sulphur compounds, but the cloudiness is due to formation of elemental sulphur. A very strongly simplified
equation is:
2 H2S + SO2 --> 2 H2O + 3 S
Reality is much more complicated. What is formed is called "Wackenroderschen Lösung", which can be translated as "Wackenroder solution".
Unfortunately there is only a stub page in Wikipedia about this. I myself have a book, which contains a deep description of this solution, which is
very interesting.
See: https://en.wikipedia.org/wiki/Wackenroder_solution
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Elemental Phosphorus
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If you want to be absolutely sure about whether or not the gas is hydrogen sulfide, you can bubble some of the gas that results from the reaction
through some copper sulfate. If a black precipitate starts to form, you have hydrogen sulfide, at least as a part of the gases.
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Smooth203
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Woah, thanks guys, all of this has helped me a lot!
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