Sciencemadness Discussion Board

Sulfamic acid decomposition

ave369 - 12-10-2015 at 14:57

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?

Hawkguy - 12-10-2015 at 15:25

2H3NSO3 -> N2, SO2, H2SO4, 2H2

Metacelsus - 12-10-2015 at 15:54

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]

softbeard - 12-10-2015 at 17:36

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...

ave369 - 13-10-2015 at 01:53

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.

deltaH - 13-10-2015 at 09:17

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 :mad: 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]

softbeard - 13-10-2015 at 11:10

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.

deltaH - 13-10-2015 at 21:27

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]

ave369 - 14-10-2015 at 03:27

I'm okay with graphite gunk. It's for a Zintl-Karyakin distillation anyway, graphite will not be distilled.

softbeard - 14-10-2015 at 11:52

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.

deltaH - 14-10-2015 at 12:19

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 :(

softbeard - 15-10-2015 at 04:20

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 :o (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]

deltaH - 15-10-2015 at 08:00

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 :D

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.