Sciencemadness Discussion Board

Gases from sulphuric acid

chornedsnorkack - 29-4-2013 at 04:11

It is generally stated that "dilute" sulphuric acid reacts with metals giving off hydrogen, whereas "concentrated" sulphuric acid is itself reduced. The options to be formed on reduction of H2SO4 include SO2, S8 and H2S.

How easy is it to produce "pure" gases by reaction between metals and sulphuric acid? How wide is the concentration range where sulphuric acid forms appreciable admixture of both hydrogen and some reduced sulphur gases?

If SO2 contains even a few % of hydrogen, it can burn in admixture with air (which pure SO2 would not). If hydrogen contains even a few tens of ppm of either H2S or SO2 then it is smelly and poisonous, which pure hydrogen is not. So I am speaking of a six orders of magnitude range in terms of the speed ratio between competing reaction.

So any data as to the concentration of sulphuric acid? And does it depend on the temperature, or on the metal involved?

[Edited on 29-4-2013 by chornedsnorkack]

blogfast25 - 29-4-2013 at 05:34

Producing 'pure' gases from sulphuric acid will be quite hard, I'd imagine. In one industrial reaction I studied (at the plant), the digestion of Ilmenite [FeTiO3] with 98 % H2SO4, produced SO2, sometimes a bit of H2S and sometimes a bit of S. It was fairly unpredictable what each batch yielded. It appeared to depend on the composition of the ore.

To make pure SO2 at the lab level there are many other, better possibilities.

weiming1998 - 29-4-2013 at 06:33

To generate SO2, copper is a good metal to react with concentrated sulfuric acid. The reaction is non-existent at room temperature, but quite rapid at the boiling point of sulfuric acid. Due to how inert copper is, it is unlikely that much SO2 will become reduced further to other sulfurous products. No hydrogen will be formed. Due to the high heat, significant amounts of sulfuric acid mist will be mixed in with your SO2, but that can be condensed, with difficulty, leaving behind SO2 of a good purity.

Other metals however, are not as good. Magnesium metal (coarsely powdered) reacts violently on contact with concentrated sulfuric acid, producing enough heat to partially vaporise the acid within seconds. Some metals like aluminium (powder) will not appear to react with the acid at room temperature. However, when a slight heat is applied, the reaction suddenly starts and quickly gets out of hand, again vaporising the acid, producing solid (aluminium) sulfate. Mixed gases will be produced, due to the reactivity of these metals. So go for relatively inert metals like copper and avoid reactive metals if pure SO2 is what you want.

woelen - 29-4-2013 at 07:15

If you want pure hydrogen, then use HCl instead of H2SO4 and use Al-metal or coarse Mg-metal (NO POWDER!!). Bubble the gas through water to get rid of traces of HCl.

If you want pure SO2, then use sodium metabisulfite or sodium sulfite (the first is preferred) and add appr. 50% H2SO4 and slightly heat. The copper/H2SO4 method also works, but requires very high temperatures and is much more dangerous than the metabisulfite/50% acid reaction which runs already at 50 C or 60 C.

Fantasma4500 - 29-4-2013 at 07:54

i have heard conc. H2SO4 + Cu should produce SO2, but again this leads to not knowing the purity..

what uses of SO2 do you have anyways..?
if it is for NaHCO3 + SO2 to make sodium bisulfite i think it should be decent enough

for pure hydrogen theres many better alternatives, but dont try breathing this stuff as theres usually tiny particles of acid / base or whatever you decide to use for it

blogfast25 - 30-4-2013 at 09:10

It's an expensive way of making SO2 when you can simply burn sulphur.

ScienceSquirrel - 30-4-2013 at 09:29

Quote: Originally posted by Antiswat  
i have heard conc. H2SO4 + Cu should produce SO2, but again this leads to not knowing the purity..

what uses of SO2 do you have anyways..?
if it is for NaHCO3 + SO2 to make sodium bisulfite i think it should be decent enough

for pure hydrogen theres many better alternatives, but dont try breathing this stuff as theres usually tiny particles of acid / base or whatever you decide to use for it



You can buy sodium metabisulphite from home brew shops almost everywhere.

Eddygp - 30-4-2013 at 10:53

SO2 from conc. H2SO4 is not something I'd try. Also, you can get so many impurities that it's not worth the trouble.