Klute - 1-9-2009 at 20:55
I want to prepare 100% H2SO4 to form alane solution from LiAlH4.. I don't have acces to SO3 or oleum, and don't particularily want to handle these
either.
So I was thinking of using P2O5 to remove the water remaining in concentrated H2SO4.. But a few things I am not sure of.. If using exces P2O5, I will
get polyphosphoric acid formed.. Hopefully this will just form a gum at the bottom, and not dissolve in the sulfuric acid. Because I'm not sure how
phosphoric acid would react with LiAlH4.
But could it be possible to use phosphoric acid to form the alane? In that case preparing 100% H3PO4 would be even easier, and it could be dissolved
in THF or ether to be added to the LiAlH4 suspension.. But I scared it might cause ring opening of THF.. Same as drying ethers with P2O5..
ANyone already tried forming SO3 from H2SO4 and P2O5? Did the P2O5 entirely dissolve in the sulfuric acid? How quick and complete is the reaction?
Eclectic - 1-9-2009 at 21:51
Dehydration H2SO4 with P2O5 works....The P2O5 dissolves completely,but distillation of SO3 out of the mixture requires very high temperatures,
approaching boiling point of H2S04, and an air cooled condenser. The idea of the glassware breaking from thermal stress while doing this was kinda
scary, so I used a hot air gun capable of melting solder as the heat source.
garage chemist - 1-9-2009 at 22:34
I have made SO3 from P2O5 and H2SO4 several times, and yes, the P2O5 will dissolve in the H2SO4 to a large extent already at room temperature.
Liberation of SO3 from this mix takes quite high temperatures, a sign that the SO3 is not present in the P2O5/H2SO4 mix from the beginning, but is
only formed upon heating. I don't think that you will be able to avoid preparing SO3.
Just go to Versuchschemie.de, the SO3 synthesis with P2O5 is discussed and documented there.
The water in the condenser should ideally have a temperature of ca. 30-35°C to avoid solidification of the SO3.
woelen - 2-9-2009 at 05:42
How well does the glassware hold when the mix of H2SO4/P4O10 is heated. I can imagine that the phosphoric acid formed in this reaction attacks the
glass at the high temperatures needed to drive off the SO3.
I once tried this reaction on a micro scale without good result, but I did not use enough P4O10. You must add quite a lot of P4O10 to the concentrated
H2SO4 in order to make SO3 (according to webpage 160 grams added to 230 grams of acid).
GC, did you mean this link: http://www.versuchschemie.de/topic,9438,-Schwefeltrioxid.htm... ?
This does not mention anything about attacking of glass by phosphoric acid, while many people tell that phosphoric acid and derived products should
not be strongly heated in glassware.
[Edited on 2-9-09 by woelen]
len1 - 2-9-2009 at 05:50
From the experiments I did on SO3 it comes out very clear that H2SO4 is not a compound per se, its a binary system with a complete spectrum due to the
total miscibility of the two compounds
H2O : SO3
Now you want an exactly 1:1 ratio. Wont 1.01:0.99 do? Why?
And you cant get it exactly 1:1 in practice either. The azeotrope is much easier to get.
In short I dont believe there are applications where 100% H2SO4 will do but 98% wont. To be sure there are applications where 98% wont do, but then
its 1.2:1 or 1.7:1, in short oleum that is required.
On a different note, I did not know that LiAlH4 will not react with H2SO4 as a base to liberate hydrogen - on the surface I would have expected it to
be explosive as it is with H2O - but I have not tried.
PS Ill add some more as it occured to me that perhaps its an extremely pure product you are after - so a 2% deviation from stoichiometry wont do.
In that case get 98% H2SO4, you can distil it at atmospheric to make sure it is the azeotrope. Then add dry sodium sulphate - it breaks the
azeotrope, as per my SO3 experiment. The b.p. drops and a mixture richer than 1:1 starts distilling - ie oleum. Collect enough, determine the
sulphate content - it will have to be at subpercentage accuracy - you cant avoid this hassle as its exact stoichiometry youre after - and add just
enough H2O to make 100% H2SO4 to as near as your measurement accuracy allows.
[Edited on 2-9-2009 by len1]
Eclectic - 2-9-2009 at 06:31
When I did this I did not see any glass etching, but I used an excess of H2SO4.
DJF90 - 2-9-2009 at 07:06
Len1: You are telling me that its possible to get oleum through the drying of azeotropic sulfuric acid with anhydrous sodium sulfate? At what
temperature does the oleum come over at?
Klute - 2-9-2009 at 07:10
Thanks for all the answers.. Looks like much more work that I anticipated.. Like I said I am not ready to prepare and handle SO3/oleum..
SO I will look into other possibilites to form AlH3.. I was thinking of BF3.ET2O, but apparently this forms diborane. Maybe 97% H2SO4 can be used
here, the water present just destroying part of the LiAlH4/AlH3...
I would still liçkz to see if 100% H3PO4 couldn't be used here..
DJF90 - 2-9-2009 at 07:17
I seem to recall adding a stoichiometric measure of AlCl3 to LiAlH4 will produce allane?
Edit: Find snippet attached
[Edited on 2-9-2009 by DJF90]
Klute - 2-9-2009 at 07:23
Yes, I think that is the most common method, after 100% H2SO4.. But I don't have any anhydrous AlCl3, and would like to do with what I have at hand.
I do have 85% H3PO4 and P2O5 though.
DJF90 - 2-9-2009 at 07:34
Surely it would be less hassle to make anhydrous AlCl3 from the elements then having to get the stoichiometry right for anhydrous H2SO4 (but no excess
of SO3)...
len1 - 2-9-2009 at 07:34
Im sorry but I dont understand this Klute. Youre willing to handle 100% H2SO4 but not 102 or 105%? Nothing magical happens when the 1:1 ratio is
exceed by a small amount.
@DJ90 These substances are ionic.
2NaHSO4 === Na2SO4 + H2SO4
Put this, the above formula with what we know from the SO3 prepublications, that NaHSO4 turns to pyrosulphate almost completely above about 400C and
voila result
Klute - 2-9-2009 at 07:41
Well it's more the SO3 I worried about, don't want to distill that stuff.. Just addign P2O5 to 98% H2SO4 and usign that as is requires minium handling
and would have been ok. Distilling H2SO4 and P2O5, and dissolving SO3 in the correct amount of 98% H2SO4 is another matter..
len1 - 2-9-2009 at 07:46
But as I said if you add dry sodium sulphate to 98% sulphuric acid youll start distilling not SO3 but low grade oleum, as you can see from above
formula.
I edited the bit about temperature because although low grade oleum boils in the range 220-280C, thats not the temperature at which it comes off from
the sodium sulphate.H2SO4 mixture. However this temperature is below about 500C by the following arguments
Na2SO4 + H2SO4 -> Na2SO4 + H2O + SO3.
From the SO3 results we know that above 500C essentially pure SO3 is given off. About 200C mainly H2O is given off. So the oleum lies somewhere in
between.
[Edited on 2-9-2009 by len1]