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Author: Subject: Sulphurous acid metastability and catalysts
chornedsnorkack
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[*] posted on 31-10-2021 at 13:44
Sulphurous acid metastability and catalysts


Under which conditions is sulphurous acid stable, under which conditions unstable to dismutation, and under which conditions metastable for lack of good mechanism to dismute?

A likely equation to dismute might be:
3H2SO3=2H2SO4+S+H2O

Also, are any substances capable of catalyzing sulphurous acid dismutation?
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woelen
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[*] posted on 31-10-2021 at 13:55


Sulfurous acid is quite stable with respect to disproportionation of sulfur to sulfur(VI) and sulfur in a lower oxidation state. I do not know of any conditions in which sulfur(VI) is formed without an oxidizer.

H2SO3 is not stable in the sense of being present as a molecule or acid. The free acid is not known. In aqueous solution, it exists in minute quantities in solutions of SO2:

SO2 + H2O <<<---> H2SO3 (very strongly to the left)

If there is any H2SO3 in solution, then a small fraction of that is split as follows: H2O + H2SO3 <---> H3O(+) + HSO3(-).

When NaOH is added, then in the presence of excess SO2, sodium metabisulfite crystallizes:

2 Na(+) + 2 HSO3(-) ---> Na2S2O5 + H2O

In the solid state, two HSO3(-) ions combine, splitting off a molecule of water. So, solid NaHSO3 does not exist, this forms Na2S2O5 and water. This metabisulfite ion has structure (-)O2S-SO3(-), it has a sulfur-sulfur bond.

H2SO3, SO2, HSO3(-) are easily oxidized, however. Oxygen from air slowly oxidizes them. H2O2 does the job much faster and can be used to convert solutions of SO2 in water to dilute H2SO4.




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chornedsnorkack
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[*] posted on 31-10-2021 at 14:07


Looking at redox couples with similar potential to the supposed oxidant potential of sulphurous acid, I note iodide.

How sensitive is concentrated hydriodic acid to oxidation, if a weak oxidant causes even a small amount of reaction like:

SO2+4HI<->S+2I2+2H2O?
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[*] posted on 1-11-2021 at 00:07


HI is so easily oxidized or is hard to obtain as a clear liquid, it is usually stained with iodine.
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woelen
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[*] posted on 1-11-2021 at 00:08


SO2 usually acts as reductor, but it also can act as oxidizer. E.g. if you bubble H2S through a solution of SO2 in water, then you get sulfur (and many sulfurous oxy-compounds in side reactions): 2 H2S + SO2 --> 3 S + 2 H2O

The reaction you quote is not one, which I ever observed. It works the other way around. If you add iodine to a solution of SO2 in water, then you get HI and H2SO4. The iodine oxidizes the SO2.

I mixed acidified iodide solution and SO2 in several experiments and I never obtained sulfur in those experiments. There is a reaction though, SO2 and iodide form a yellow complex [I.nSO2](-), with n ranging from 1 to 4, depending on concentration and pH. You can nicely observe this if you mix a solution of KI in dilute H2SO4 or HCl with SO2 or an acidified sulfite or metabisulfite.

With anhydrous HI, or HI at very high concentration (e.g. azeotropic HI at 57%) there might be formation of sulfur with SO2. Non-dissociated HI is a stronger reducing agent than dissociated HI (which is H3O(+) and I(-) in aqueous solution). I never worked with concentrated HI though, so I do not know about that.

I do know, however, that concentrated (azeotropic) HI is a chemical, which is hard to keep around in a pure colorless state. It quickly turns brown, due to aerial oxidation. If you open a bottle of this, then fresh air gets in, and the oxygen in that air oxidizes HI to I2 and water. Commercial samples for this reason usually are brown.

An interesting observation, which may support your reaction equation, is that iodide does reduce thionyl chloride. If you add solid KI to SOCl2 (which is the acid chloride of sulfurous acid), then the liquid turns red/brown. Part of the iodide is converted to iodine. When this red liquid is added to water, then the SOCl2 quickly reacts to SO2 and HCl, but the liquid then becomes cloudy and has a sulfurous odor, of course much like SO2, but with some other component in its smell as well (more 'rotten').



[Edited on 1-11-21 by woelen]




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chornedsnorkack
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[*] posted on 1-11-2021 at 23:47


Fairly strong and reducing acids include HI, H2SO3 and H3PO2.
HI is stable to dismutation and strongest of the three.
H2SO3 might be capable to dismutation, but it is said not readily. At 20 C, sulphurous acid saturated with 1 bar SO2 (and SO2 has molar mass of 64, so denser than air) is said to have solubility of 104 g SO2 for 1000 g of water - but this solubility increases rapidly with lowering of temperature. From +20 Celsius to +0 Celsius, the concentration of sulphurous acid saturated with 1 bar SO2 increases from 9 % (as SO2) to 18 %, while the concentration of saturated HCl increases only from 42% to 45% over the same temperature range.
H3PO2 is known to be liable to dismutation on heating:
2H3PO2=H3PO4+H3P
3H3PO2=2H3PO3+H3P
These reactions cause rapid decomposition of H3PO2 on heating, in the range of 100 to 130 C. Also, direct drying of H3PO2 does not work because of dismutation - standard route to pure dry H3PO2 is through extracting H3PO2 to diethyl ether. Then dry H3PO2 melts at 26 Celsius.

Now, commercial clear, iodine-free HI commonly includes H3PO2 - if it does not interfere with the intended use of HI.

Are there any reducers which are capable of reacting with traces of I2 in concentrated HI so that neither the reagent not the reaction products dissolve in HI?
I suspect that Ag, Pb and Hg do not quite qualify. Although AgI, PbI2, Hg2I2 and HgI2 are poorly soluble in water, they are said to have a tendency to form soluble complexes in excess HI.
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[*] posted on 20-11-2021 at 04:02


H2SO3 not known as a molecule. There are only SO2 hydrates.
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[*] posted on 20-11-2021 at 04:10


Sulphur will react with hot concentrated sulphuric acid to produce sulphur dioxide (and water)
So it's unlikely that the reaction will go well in the opposite direction.
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yobbo II
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[*] posted on 20-11-2021 at 05:30



This may be of use

dissoc_2.jpg - 89kB
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Bedlasky
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[*] posted on 20-11-2021 at 08:05


Quote: Originally posted by chemister2015  
H2SO3 not known as a molecule. There are only SO2 hydrates.


H2SO3 has been detected in gas phase.




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chornedsnorkack
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[*] posted on 20-11-2021 at 14:26


Quote: Originally posted by chemister2015  
H2SO3 not known as a molecule. There are only SO2 hydrates.


I could not find actual phase diagram of SO2/water. Can anyone point it out? I do see it mentioned that only one solid hydrate forms.
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