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Eclectic
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[*] posted on 2-11-2007 at 06:55
SO2 Generator


Usually on this forum, we are obsessed with converting sulfur or sulfur dioxide to sulfuric acid and/or SO3. What if you want to go the other way? Anyone know of ways to use sulfuric acid as feedstock for a lab scale SO2 generator almost as convenient as an SO2 cylinder that can generate 1-40 lbs of SO2 in a reasonable amount of time without producing an excessive amount of heat or dilutant inert gasses?

Copper turnings in H2SO4 are supposed to work. Does anyone know if elemental sulfur reacts with hot H2SO4 the same way? Any other chemistry that might be useful? Catalysts?

(I've already thought about a sulfur/O2 burner. A 200 CF O2 welding tank could produce about 35 lbs of SO2 from burning sulfur, but getting rid of the heat could be a problem.)

[Edited on 11-2-2007 by Eclectic]
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[*] posted on 2-11-2007 at 08:07


Don't you have access to sulfites or (meta)bisulfites. A lovely generator can be made by mixing H2SO4 with an equal volume of water, and then adding sulfite or (meta)bisulfite to this. Slight heating gives a smooth and not too fast generation of lots of SO2. You don't need strong heating, and that is a pleasant thing.

If you don't have sulfites, then indeed copper metal could be used, but the problem with this method is that a large excess of sulphuric acid is needed. Use of elemental sulphur buring in oxygen also can be used, but the problem with this is that it does not give pure SO2, but SO2 mixed with a lot of air or oxygen. A practical problem with that approach is to lead it through something. You don't have a pressurized source, because formation of SO2 from S and O2 does not lead to formation of more volume of gas from solid (each SO2 takes one O2). When copper/concentrated sulphuric acid, or sulfite/medium conc. sulphuric acid, is used, then you can have a closed flask, in which SO2 is formed.




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[*] posted on 2-11-2007 at 08:48


I wanted to avoid the production of waste salts and excessive heat if possible. Does anyone know if hot H2SO4 reacts with elemental sulfur ?

A pure O2/sulfur burner is probably the way to go if not, but the cooling would be a bitch in order to be able to liquify the resulting gas.

Maybe I should wait until winter and use it as a space heater. :D

[Edited on 11-2-2007 by Eclectic]
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[*] posted on 2-11-2007 at 09:14


Late Halfapint of the former Hive once mentioned heating sulfur with sulfuric acid as a SO2 generator that was well regulated. I suppose the reaction would be:

H2SO4 + S --> 2 SO2 + H2

I've never read about this way in any other source, but the person was most trustworthy in his descriptions. Of course hydrogen gas might pose a problem depending on application. One solution is to absorb the SO2 in aqueous alkali to make a sulfite and neutralize that later to release SO2, another is indeed liquification.

[Edited on 11/2/2007 by trilobite]
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[*] posted on 2-11-2007 at 09:23


I was thinking in an excess of H2SO4, 2H2SO4 + S --> 2H2O + 3SO2 , but I don't know if it will actually work, or if there are nasty side reaction products to deal with. Maybe problems with thermal runaway? :o
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[*] posted on 2-11-2007 at 10:20


Hydrogen does sound a bit improbable for a reaction product, you may well be right. I don't think thermal runaway would be a problem since he emphasized that the rate of sulfur dioxide production was easily controlled by adjusting heating. To me this suggests an endothermic reaction with a large positive entropy term.
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[*] posted on 2-11-2007 at 13:30


My abridged Mellors mentions heating any suitable reducing agent, copper turnings, sulphur or mercury, etc. with concentrated sulphuric acid. Copper is normally used;

"" The flask is about one third filled with copper turnings and sufficient concentrated sulphuric acid is added not quite to cover the copper. On heating, sulphur dioxide is evolved and may be passed through a wash bottle of concentrated sulphuric acid to dry it""

It also mentions SO2 can be formed by the action of "moderately concentrated"??? sulphuric acid on sodium sulphite or bisulphite.

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[*] posted on 2-11-2007 at 14:54


steel shavings should work with hot H2SO4. If you use this or copper the salts can be harvested and purified for use or to sell on ebay.



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[*] posted on 2-11-2007 at 14:58


Well yeah, but if sulfur works to cleanly reduce H2SO4, then you have NO byproducts to deal with! Green chemistry approach to generate acid rain produceing gas. :D

[Edited on 11-2-2007 by Eclectic]
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[*] posted on 2-11-2007 at 23:36


Actually, water (not hydrogen!) is the byproduct of all methods utilizing reduction of H2SO4, and is troublesome because it dilutes the H2SO4 so that it may no longer reach reaction temperature. So if you make a SO2 generator with Cu or S and H2SO4, initially it will probably work well, but as soon as some SO2 has been generated, you will continue to boil the reaction mix and the SO2 production will get slower and slower despite the reagents being far from used up.

What you need is constant removal of water from the reaction mixture to keep the acid in the reaction mix concentrated. Running it in a distillation setup where the water condenses in the condenser and withdrawing the SO2 at the vacuum connection would be a good idea, for example.

If the H2SO4/S method actually works it would be a favorable method, simultaneous reduction of H2SO4 and oxidation of S to generate the SO2 and water as the only byproduct.
I could see some trouble though with evaporation of the sulfur, coating the inside of the distillation apparatus.
You better have some HNO3 at hand to distill in the apparatus (without water cooling, so that the hot HNO3 vapor reaches the whole inside surfaces) to oxidise the sulfur.
I used this cleaning method when my still was coated with sulfur on the inside from a synthesis of S2Cl2 and found it to be good and efficient. It emits vast amounts of red NOx vapors though.
Hot toluene can also work to dissolve the sulfur, and a wash with hot NaOH solution also dissolves sulfur (disproportionation to polysulfide and thiosulfate).

[Edited on 3-11-2007 by garage chemist]




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[*] posted on 3-11-2007 at 03:19


@Eclectic, see my PM. Hope this attachment helps.

[Edited on 3-11-2007 by Sauron]

[Edited on 3-11-2007 by Sauron]

Attachment: Pages from Mellor_ch57_1930.pdf (212kB)
This file has been downloaded 819 times





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[*] posted on 3-11-2007 at 05:34


Thanks for the info, guys. Has anyone actually tried this with H2SO4 and S? I haven't been able to find any references to this approach either. The practicality of the approach seems to be a matter of what temperature the reaction proceeds at a useful rate.
Below 100C, there would be water accumulation, but little sulfur fume coating the apparatus. At say 200C, I'd be reluctant to have a batch reaction where liquid reactants could convert to gas at an unknown rate. :o I'd be much more comfortable injecting liquid S at a metered rate into the hot H2SO4 if that is the case, although at that temp water dilution would not be a problem

I guess it's time for a proof of concept small distillation setup scale trial.

2H2SO4 + S --> 2H2O + 3SO2 ???

[Edited on 11-3-2007 by Eclectic]
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[*] posted on 3-11-2007 at 05:59


I tried the H2SO4 (96%) and S mix. It does not give SO2, not even when heated, such that the acid starts boiling (appr. 300 C). This simply does not work.



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[*] posted on 3-11-2007 at 06:13


Thanks Woelen, that saved me quite a bit of time...:(

Maybe with a catalyst? A dash of CrO3 or V2O5? Hg?

[Edited on 11-3-2007 by Eclectic]
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[*] posted on 3-11-2007 at 08:29


Too bad it doesn't work, thanks for trying it out.
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[*] posted on 3-11-2007 at 08:49


And yet, in the updated pdf provided by Sauron, there is a statement that it does, so maybe it WILL work with the appropriate catalyst? Test tube time...
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[*] posted on 3-11-2007 at 14:07


I understood (from private communication on this subject) that sulphuric acid is not capable of producing SO2 from S, and this is perfectly confirmed by my experiments.
You need oleum for that, with free SO3, dissolved in H2SO4. With that, you can dissolve S (getting a blue solution), which on further heating decomposes, giving SO2. It is the SO3, which oxidizes the S, not the H2SO4. When all SO3 is used up, then the reaction stops.

Unfortunately I have no oleum (and if I had, I would not use it for making something simple as SO2). Using oleum for making SO2 is insane, oleum is very expensive and a very hard to obtain chemical (also difficult and dangerous to make yourself). If you can get oleum, then you certainly can get NaHSO3 or Na2SO3 and use that for making SO2.




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[*] posted on 3-11-2007 at 14:55


Woelen, did you do any tests with small amounts of Cr,Fe,V,Hg?

You of all people I would think would willing to actually try a quick experiment. (I don't have any test tubes, V2O5, or Hg with me right now).

Who told you H2SO4 is not capable of producing SO2 from S, and why would you believe hearsay? (I trust your experiment with pure acid and sulfur, I'm just questioning "someone informed me")

"Sulphur dioxide is also formed when sulphuric acid is heated with sulphur: 2H2SO4 + S=3SO2+2H2O, as shown by J. B. A. Dumas, and C. F. Anthon. H. L. F. Melsens
found it to be an advantage to mix the acid and sulphur with pumice-stone.
E. Hart recommended warming sulphur with fuming sulphuric acid, containing
30 per cent. SO3, when the blue soln. so formed is heated, sulphur dioxide mixed
with some trioxide is given off. The evolution of the dioxide ceases when all the
trioxide has been acted upon, and the sulphur melts. J. Knezaurek obtained
sulphur dioxide by heating sulphuric acid with charcoal: 2H2SO4+C=2H2O
+2SO2+CO2, for purposes where the admixed carbon monoxide and dioxide
will do no harm. W. L. Scott recommended using acid of sp. gr. 1-824 containing
74 per cent. SO3. If a more cone, acid is used, a portion is reduced to sulphur;
and if a more dil. acid is employed, some hydrogen sulphide is formed. The
washing liquid should be mixed with lead sulphate or coarsely powdered charcoal.
One of the commonest methods of preparing small quantities of sulphur dioxide
is to heat cone, sulphuric acid with copper: Cu+2H2SO4=SO2+2H2O+CuSO4
—vide 3. 21, 6. According to G. J. Warner, sulphuric acid is reduced to sulphur
dioxide at 160°, and S. Cooke showed that the reaction is accelerated by the
presence of platinum."

This is from Sauron's posted pdf above, page 2.

[Edited on 11-3-2007 by Eclectic]
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[*] posted on 3-11-2007 at 15:58


What I said in my previous post is exactly the same as what you now tell me in this quote. SO2 is formed from oleum (fuming sulphuric acid, 30% SO3, according to your quote) and as soon as all SO3 is used up, no SO2 is formed anymore and the sulphur only melts.

So, again, pure H2SO4 is not capable of producing SO2 from sulphur, you need oleum (=fuming sulphuric acid with SO3 in it) for that. Your quote is not about H2SO4 alone, but about oleum.

----------------------------------------------------------------------------------------------------------------------------

Maybe the addition of metal salts changes things, but for one, I already know that it will not work:

CrO3 will not work. I did an experiment with this in sulphuric acid (CrO3 from K2Cr2O7), but when this is heated strongly, then it decomposes, giving oxygen and an insoluble chromium(III) compound (which is a riddle for me on its own): http://woelen.homescience.net/science/chem/exps/raw_material...

I can try with V2O5, but I also have doubts on that. I'll come back on that one after I did the experiment, but right now, I expect that either it does not react at all, or the sulphur reduces this to a vanadium(IV) species.

[EDIT: changed URL of link so that it works again]

[Edited on 9-5-12 by woelen]




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[*] posted on 3-11-2007 at 15:58


Quote:


E. Hart recommended warming sulphur with fuming sulphuric acid, containing
30 per cent. SO3
, when the blue soln. so formed is heated, sulphur dioxide mixed


um, yeah. that would be oleum
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[*] posted on 3-11-2007 at 16:12


"Sulphur dioxide is also formed when sulphuric acid is heated with sulphur: 2H2SO4 + S=3SO2+2H2O, as shown by J. B. A. Dumas, and C. F. Anthon. H. L. F. Melsens
found it to be an advantage to mix the acid and sulphur with pumice-stone."

It's a litany of different methods purported to produce SO2 from the 1850's. Just because oleum works, doesn't mean other method's DON'T. What's up with the latching on to one sentence and ignoring everything else?!?

Woelen, I'm fairly sure that acidic CrO3 is going to oxidize sulfur, probably all the way to SO3. The question is whether or not the resulting Cr2(SO4)3 will in turn be oxidized by hot H2SO4 to a higher oxidation state, producing SO2 and becoming available to attack more elemental sulfur, hence acting as a catalyst. In your experiments with Cr(III) in hot H2SO4, were you in a position to smell if any SO2 was evolved? You didn't indicate that there was bubbling, so probably not. But maybe other transition elements would catalyze the reaction?

If Dumas and Co. got sulfur to react with H2SO4, and you didn't, it's likely that their H2SO4 had a contaminant that was acting as a catalyst, yes? Dumas was a contemporary of Faraday in the 1800's., and I doubt they were using a reagent grade acid.

A trace of HNO3, Br, or Cl might facilitate the reaction. Eye of newt, hair of dog...


[Edited on 11-3-2007 by Eclectic]
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[*] posted on 3-11-2007 at 17:48


No. You're not going to like what I'm going to say, BTW.

My first thought when I saw this thread was: "What is so hard about mixing S and H2SO4 in a test tube and finding out for yourself?"

Then I thought about all the old literature available to anyone with the slightest inclination to read it, and I thought some other things that are probably best left unsaid.

I also thought that it would make sense that the recommended methods for making it are the recommended methods for good reason.

Now it's just not Mellor that gives that quote, it's all over.

I have no idea why it does not work for woelen. Like I said this is easy to test and it sure is. Quicker than writing this post. Over here with colorless 96% technical acid and sulfur powder, it works fine. The acid soon turns blackish the evolution of gas, easily noted by bubbles at the exit tube just under the surface of water in a test tube. The was no boiling or anything in the flask (it was more convenient for me to use a jointed flask with gas tube and a mantle than dig though my stuff for stoppers and tubing) No analysis of the gas was made, but it is plentiful enough and it contains an unpleasant amount of SO2.

I don't see why Melsens would talk of 12 L cast iron flask scale if it didn't work.




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[*] posted on 3-11-2007 at 18:07


Am I bovvered? Do I look bovvered? (Obscure reference to the Catherine Tate Show)

No test tubes handy today, nor my 50 lb bag of sulfur prills. ;)

I'm all for experimentation, and figured I'd ask and try and stimulate some. Cast Iron Flask may point to iron as a catalyst?
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[*] posted on 4-11-2007 at 04:09


Quote:
I have no idea why it does not work for woelen. Like I said this is easy to test and it sure is. Quicker than writing this post. Over here with colorless 96% technical acid and sulfur powder, it works fine. The acid soon turns blackish the evolution of gas, easily noted by bubbles at the exit tube just under the surface of water in a test tube. The was no boiling or anything in the flask (it was more convenient for me to use a jointed flask with gas tube and a mantle than dig though my stuff for stoppers and tubing) No analysis of the gas was made, but it is plentiful enough and it contains an unpleasant amount of SO2.

What flask are you using? It really does not work for me. I only see the sulphur melting and nothing else. At a certain point the liquid becomes so hot, that the acid starts emitting white fumes, at that point I stopped. The acid remains colorless, the sulphur forms droplets in the acid.

First, the droplets are viscous and yellow:


At much higher temperature, the droplets have a deep red color:


I also cannot understand why the acid would become black-ish, as S.C. Wack describes. My acid does not change color, it may become slightly darker, but that's all. Even after 10 minutes of heating bloody hot, things are not changing.

[EDIT: Changed URL's of picture so that they work again]

[Edited on 9-5-12 by woelen]




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[*] posted on 4-11-2007 at 05:36


I suspect that the reaction of S and H2SO4 may be catalysed by transition metals or NOx, both of which would not be unexpected in 19th century H2SO4. A quick test of this would be to add the smallest crystal of KNO3 to the non-reacting S+H2SO4, or a trace of Fe, V, or Mn. I'd try it myself, but it's late enough that the neighbors might complain about infernal visitations.
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