Sciencemadness Discussion Board

SO2 from Na2SO3

dome13 - 29-10-2019 at 08:12

Hi all!
I am looking for NaHSO3 synthetises. Exactly I want to make a SO2 generator and I found that is possible from Na2SO3. It would be cheaper for me instead of using Cu and H2SO4. Here is the link:

https://www.prepchem.com/synthesis-of-sulfur-dioxide/

My main question is there anybody did this reaction before?
Is it really a good source of SO2?

SWIM - 29-10-2019 at 10:06

Is there some reason you think it might be a problem?

What could be more straightforward without actually burning sulfur to make it?

H2SO4 is stronger, so the sulphurous acid is liberated.

The sulphurous acid levels increase until it exceeds its saturation point and SO2 bubbles out of the solution.

It is freed of entrained water by bubbling through the wash bottle full of H2SO4.

The mixing of sodium bisulfate and sodium sulfite to make it sounds like an easy route too if you need high purity gas.
It's listed below the other synthesis in your reference.

I'm actually kind of surprised they don't just use sulfur and H2SO4 (heated) to get SO2.
I suppose it's a reaction that might have serious clean-up issues.

woelen - 30-10-2019 at 00:58

Making bisulfite is easy.

Add half concentrated H2SO4 to Na2SO3 and gently heat. Lead the produced gas through a solution of Na2SO3, until that solution has a strong smell of SO2. Then evaporate to dryness. No need to dry the gas before leading in the solution, so it simply is a matter of making a solution of Na2SO3 in water and bubbling the produced gas through it. It helps a lot if you make fine bubbles. The SO2 then is absorbed more easily. You could use an aquarium air-stone to disperse the gas into small bubbles.

The reaction between sulfur and concentrated H2SO4 does not work. I tried this myself and even on strong heating (near 300 C, the acid starts fuming heavily) no appreciable reaction rate could be obtained. The sulfur simply melts and turns red and viscous when it becomes hotter, but I did not get SO2. On cooling down, the sulfur solidies and you get nice yellow spheres of sulfur. It does not dissolve in the H2SO4.

When you want to dry the sodium bisulfite, keep in mind that the material is fairly easily oxidized by air to bisulfate. It also fairly easily loses SO2. The solid is not NaHSO3, but Na2S2O5 (sodium metabisulfite). The same is true for the potassium salt. In aqueous solution you have HSO3(-), but on crystallization you get S2O5(2-) ions. The structure of these ions is (-)O2S-SO3(-).

[Edited on 30-10-19 by woelen]

AJKOER - 3-11-2019 at 10:12

Quote: Originally posted by SWIM  

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What could be more straightforward without actually burning sulfur to make it?
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Quote: Originally posted by woelen  

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When you want to dry the sodium bisulfite, keep in mind that the material is fairly easily oxidized by air to bisulfate. It also fairly easily loses SO2. The solid is not NaHSO3, but Na2S2O5 (sodium metabisulfite). The same is true for the potassium salt. In aqueous solution you have HSO3(-), but on crystallization you get S2O5(2-) ions. The structure of these ions is (-)O2S-SO3(-).

[Edited on 30-10-19 by woelen]


Based on the comments above, I think an interesting alternate path for small amounts of aqueous NaHSO3, albeit does not employ Na2SO3, but sulfur and results in a stable solution is as follows:

First, place burning sulfur in a large vessel until it no longer burns. Remove the sulfur burner.

Second, boil distilled water to remove any oxygen, then cool in a freezer absent air contact, and then add to the large vessel containing SO2.

Note, one may consider reversing Steps 1 and 2, to limit oxygen contamination.

Finally, shake the water/SO2 mix and periodically add dry NaHCO3 (Baking Soda) to the vessel.

Expected reactions:

S + O2 --> SO2

SO2 + H2O = H+ + HSO3-

H+ + HCO3- --> H2O + CO2 (g)

Net:

SO2 + H2O + HCO3- --> HSO3- + H2O + CO2

so you now have an aqueous NaHSO3 as required. Note, it is advised to employ a small excess of SO2.

I would leave the carbon dioxide in the vessel as a protective layer to block out oxygen and also store absent strong light.

Note, this process is limited by the size of the vessel employed as you will need over 22 liters of SO2 for each mole of product. It does, to its advantage, by periodic dosing with NaHCO3, perhaps obviate the SO2 solubility in water issue, as the alkaline NaHCO3 pushes the reaction to the right.

[Edited on 3-11-2019 by AJKOER]