ave369
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Sulfamic acid decomposition
Hello, it's me again, with another crazy idea how to prepare sulfuric acid.
Sulfamic acid is OTC as a descaling agent. I've read somewhere that among its thermal decomposition products are SO3 and H2O (along with SO2 and N2),
hence, if you decompose it in a retort and condense the results, you will get sulfuric acid.
But I can't get the equation of the decomposition reaction anywhere. Does anyone know?
Smells like ammonia....
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Hawkguy
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2H3NSO3 -> N2, SO2, H2SO4, 2H2
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Metacelsus
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There are many ways sulfamic acid could decompose. Most simply,
H3NSO3 -> NH3 + SO3 (basically, reverse of formation, so therefore unfavored)
My guess as to the approximate stoichiometry of the actual reaction would be:
3 NH3SO3 -> 3 H2O + N2 + 3 SO2 + NH3
I don't see any way hydrogen gas could be formed, mechanistically.
[Edited on 12-10-2015 by Cheddite Cheese]
[Edited on 12-10-2015 by Cheddite Cheese]
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softbeard
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I agree with C. Cheese.
The final products would probably be a mix of N2, H2O, SO2, and maybe some SO3 (as
H2SO4 with the H2O); ie. a mess.
You'd probably be better off trying to heat dried NaHSO4 and condense the SO3 as described in the oleum threads elsewhere on the
board. Of course, ideally, you need some ~95%+ H2SO4 to absorb the SO3.
Back to electrolyzing CuSO4 solutions...
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ave369
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Quote: Originally posted by softbeard  |
You'd probably be better off trying to heat dried NaHSO4 and condense the SO3 as described in the oleum threads elsewhere on the
board |
Already done that. Found out I was running out of test tubes too fast. That's why I'm looking for alternative methods so frenetically.
Smells like ammonia....
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deltaH
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Sulfamic acid reacts with oxidants to make sulfuric acid. When the oxidant is something like nitric or nitrous acid, then sulfuric acid is cleanly
formed with evolution of nitrous oxide and nitrogen gas, respectively, but such reagents are of course not practical.
However, the lesson is that you need to oxidise the amine group away.
What about electrolysing hot concentrated sulfamic acid solutions, hypothetically generating nitrogen gas and sulfuric acid at the anode and hydrogen
gas at the cathode? You wouldn't even need a seperator for this because I don't think you can reduce sulfamic acid easily to anything.
Lead would NOT be a suitable metal for the anode because lead sulfamate is very soluble  A lead cathode, however, should work well though in terms of compatibility with the acid, but it would have a high overpotential for
the hydrogen evolution reaction, but so what for the home chemist?
Manganese dioxide anode? Might also be a good electrocatalyst for the oxidation of the NH2- group?
Anyway, all hypothetical, but give it a try if you want.
[Edited on 13-10-2015 by deltaH]
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softbeard
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Copper would probably work ok as a cathode, but selecting the anode material would be tricky. Platinum, as usual, is the no. 1 contender; until you
see how much it costs. Graphite would work, but you'll need to filter off the graphite gunk suspended in your product. Maybe MMO anodes (coated Ti),
as used for chlorate cells, would work best.
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deltaH
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MMO cannot be used below pH 1... one big drawback of it. The coating dissolves off in strong acids quickly.
Graphite would be good, filtering is not a problem, but you'd get no catalytic effect with the graphite and might just end up making oxygen on the
anode instead of the desired oxidation of sulfamic acid, but it's worth a shot considering availability and cost!
[Edited on 14-10-2015 by deltaH]
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ave369
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I'm okay with graphite gunk. It's for a Zintl-Karyakin distillation anyway, graphite will not be distilled.
Smells like ammonia....
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softbeard
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| Quote: Originally posted by deltaH | | MMO cannot be used below pH 1... one big drawback of it. The coating dissolves off in strong acids quickly. |
Thanks Delta, I didn't know that. Something to keep in mind when using these MMO anodes. I had in mind the TiO2 in the MMO would be like a
fired ceramic; and so be virtually insoluble in acid. But that's not the case...
Seems like every one of these anode materials, except platinum, has a weakness.
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deltaH
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There are some specialised MMO's that might handle it, made from 'platinum oxides'. I have two small such plates and a 5A power supply, so I think I
might try this with a dilute sulfamic acid solution. The electrodes are too precious to me to try concentrated solutions 
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softbeard
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My guess is that 'platinic oxides' should be ok in dilute sulfamic acid... notice the 'should' emphasis. You just never know if some
kind of soluble Pt(sulfamic acid)++++ complex doesn't form (+4 for oxidation state is guess).
You'd hate to see your 'platinic oxides' anode dissolving before your eyes 
(nah, kidding, I doubt it... kinda...
On that note, not even solid Pt is completely safe. One of the parameters in perchlorate production is mg's of Pt loss per ton of perchlorate.
[Edited on 15-10-2015 by softbeard]
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deltaH
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Yeah, I feel the same, I love the electrodes too much to try more than 0.5M solution.
I don't trust that nature understands the theory 
That would ultimately yield something close to pH 1 and so should be fine.
I can dissolve some lime in enough vinegar to make a clear solution and then mix this with the product of electrolysis. If the solutions turns white,
then I'd say it's evidence you made some sulfuric acid.
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teodor
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I think the main result of decomposition is some adduct of sulfur trioxide.
Experimental:
An evaporation flask was charged with 15.87 g of sulfamic acid. Long arm and Dimroth cooler were attached. The receiving flask was placed in an
ethanol bath cooled to -20C. The same coolant had been continuously pumping through the cooler. In the opening of the receiving flask a drying tube
(CaCl2) was attached.
The acid was melted and immediately the decomposition was started. The heatgun was adjusted to give air with the temperature 580C.
The snow immediately started to form on a spiral of the cooler directly from gaseous phase.
The decomposition was not particularly fast and I had no patience to decompose all the charge, at the same time I decided that I should not increase
the temperature. I will be able to provide an additional information about how much acid was decomposed in the next report.
The arm was disconnected. Every time I opened apparatus a white cloud of irritating smoke was always formed, similar to that of oleum by its visual
appearance. Every time when I opened the apparatus a bit of oily liquid appeared on glass surface near the joints.
The "snow" didn't melt at the room temperature (12 - 17C). No slightly change after one day was noticed.
Next day I started to cool ethanol again, so now I kept the receiving flask at -35C and I supply warm water (approx 40C) through the spiral. All
“snow” sublimed to the bottom of the receiving flask without any visible liquid phase.
I attached a dropping funnel charged with distilled water.
Any drop of water was making a small “explosion” filling the flask with a white cloud and raising the temperature. The oily liquid was formed on
the bottom. It will be subject for further chemical analysis.
I noticed SO2 smell going from drying tube after every water contact.
My assumption that it could be some SO3 adduct consisted of SO3, SO2 and the nitrogen which is decomposing by water loosing SO2 part. But it could be
also some polymeric form of more or less pure SO3 also.
There is one unsolved problem yet. After melting the sulfamic acid is always liquifying in a form similar to epoxy resin. It adheres to glass and
unavoidable cracks the flask upon cooling even being spread when is still liquid as a thin layer.
Probably adding some high boiling point solvent to the flask before cooling could help, otherwise some acid resistant boat is probably needed to reuse
the flask.
So, that's all for now. And I express my thanks to woelen for his old present - the bottle of the sulfamic acid I used for this experiment.
P.S. As I already suggested many times, no information from the books with the words "industrial" or "technology" should be taken as granted. It's
obviously that no any water was distilled off from the decomposition.
[Edited on 23-12-2024 by teodor]
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