Sciencemadness Discussion Board

100 % sulphuric acid

shadow - 7-8-2016 at 04:31

I want to dry my 98% sulphuric acid to 100%.
I might do this with oleum or so3, but can't I just remove the 2% with magnesium sulphate and filter?

[Edited on 7-8-2016 by shadow]

unionised - 7-8-2016 at 04:50

Sulphuric acid is a much better dehydrating agent than MgSO4 and also it reacts with sulphates to form bisulphates.

woelen - 7-8-2016 at 04:55

No, this will not work. Sulphuric acid is much more hygroscopic than magnesium sulfate. Suppose you add some hydrated magnesium sulfate to sulfuric acid, then the acid will dehydrate the magnesium sulfate.

And then there is another issue. What is 100% sulfuric acid? Pure 100% H2SO4 does not exist. H2SO4 quite strongly dissociates into HSO4(-), H3SO4(+) and more complex species like H2S2O7. In 98% sulfuric acid you will not find any free water. All water is bound as H3O(+) and H3SO4(+) and as counterion you will find HSO4(-). If you want something which on average is 100% H2SO4, then you just have a little more H2S2O7 in the mix. You can go further and go to "105%" H2SO4. This has even more H2S2O7 in the mix, and less HSO4(-) and H3SO4(+) and no H3O(+) at all. Above appr. 110% there also is free SO3 in the mix and the acid fumes.

You can only speak of 100% H2SO4 on average, where the weighted average of all present species is exactly equal to H2SO4 (e.g. one HSO4(-)-ion and one H3O(+)-ion and one molecule of H2S2O7 together average to H2SO4).

shadow - 7-8-2016 at 05:32

Thanks for your replies

Due to Len's article on SO3, he quotes:

" 100% SO3 (b=0) then corresponds to 100% oleum, while 100% H2SO4 is zero percent oleum - with no free SO3 being able to be distilled off."

I had the notion that 100% H2S04 was just that without ions and cations floating about.

Well, I'll make some oleum then.....

By the way, meant anhydrous magnesium sulfate, but I guess that wouldn't matter if there is no water to remove.

[Edited on 7-8-2016 by shadow]

[Edited on 7-8-2016 by shadow]

Dan Vizine - 7-8-2016 at 09:29

Oleum is one of those elusive chemicals that is essentially never available, anywhere. Ebay currently has ether, 67% HNO3, 96%H2SO4, 4% ethanolic picric acid, 5 micron Al, thermite, white P and As for sale, but no SO3 or oleum, ever. At least its straightforward and simple with good glassware. Just tedious. But you made the right choice, that's the way it's done 100% of the time.

SO3 is one of the most unique chemicals I've ever worked with. It is downright scary reactive!

[Edited on 7-8-2016 by Dan Vizine]

Praxichys - 8-8-2016 at 06:06

Oleum is pretty corrosive and probably expensive to ship. I have found P2O5 online though, and it wouldn't take much to set up a nice SO3 generation apparatus (by dripping sulfuric acid on warm P2O5) and lead it into some sulfuric acid to be absorbed as oleum, while monitoring the weight gain. The major expense here is getting creative with the glass since that's about the only common material you can use to contain the stuff.

I'm not sure why one would actually need oleum except for some extremely specialized reactions. Arylsulfonic acids come to mind but usually what can be done with oleum can also be done with conc. H2SO4 with prolonged heating and a small sacrifice in yield, or some creativity and a dean-stark apparatus. Often the alternative preparations are preferable anyway just out of sheer safety, not to mention expense.

woelen - 9-8-2016 at 01:52

Using P2O5 (better: P4O10) indeed is an option, but the yield of SO3 is very low. You need a lot of P4O10 to get the SO3 out of the H2SO4. I once tried to make SO3 by adding P4O10 to conc. H2SO4 and then heating the mix to drive off SO3. Only when I had added a lot of P4O10 I could drive off some SO3. I do not remember exactly anymore, but I think I needed almost 10 times as much of P4O10 (by weight) as H2SO4 to get a decent production of SO3. This makes oleum very expensive (P4O10 is not really cheap).