chornedsnorkack
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Hydrogen halides by phosphoric acids?
Is it possible to produce hydrogen halides from halides by hot concentrated phosphoric acids?
A standard way to produce HF and HCl is by cold (or warm?) concentrated sulphuric acid:
2NaF+H2SO4->2HF+Na2SO4
NaCl+H2SO4->HCl+NaHSO4
With HCl, presumably HCl solubility in concentrated sulphuric acid is poor enough to permit removal of HCl.
HF condenses at 20 degrees and is presumably miscible with sulphuric acid, so I suppose heating is indispensable to distill off HF.
This does not work with HBr and HI, because both are oxidized by concentrated sulphuric acid. (And in dilute sulphuric acid they would just dissolve).
Now, can phosphoric acids be used to produce hydrogen halides?
H3PO4 is very much weaker acid than H2SO4 and the hydrogen halides HCl, HBr and HI.
On the other hand, phosphoric acids are very hard to reduce.
Or boil.
Azeotropic sulphuric acid, about 98 % H2SO4 and the rest water, boils at 330 degrees. Sulphur trioxide condenses rapidly at 45
degrees, and once fully polymerized, evaporates somewhere between 60 and 70 degrees.
Azeotropic phosphoric acid is slightly towards P2O5 from HPO3 - and boils over 800 degrees. Phosphorus pentoxide
condenses rapidly, to P4O10, around 430 degrees, and once fully polymerized, boils somewhere between 600 and 800 degrees (hard
to find exact data!).
So, are there any conditions where heating phosphoric acids with halides could cause evaporation of hydrogen halides?
Yes, HBr is very much stronger than phosphoric acid, so the equilibrium of the reaction
KBr+H4P2O7<->HBr+KH3P2O7
should be firmly far to the left in solution. But could evaporation of gaseous HBr permit the reaction to proceed anyway, say at 300 degrees?
Also, what about HI?
Phosphoric acids resist reduction. But HI is itself unstable. The reaction
H2+I2<->2HI
is an equilibrium reaction, whose equilibrium is established in the region of 400...500 degrees.
Would it be possible to use phosphoric acids to produce gaseous HI under conditions where HI does not undergo pyrolysis, like under 400 degrees?
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solitanze
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Aqueous phosphoric acid (often in 75-85% concentration) + a iodide salt is the standard method for preparing HI. HI is relatively stable if the
temperature isn't too high. You may obtain a slightly brown colored product though, probably because of oxidation by air or even decomposition.
HI and water form an azeotrope, with 57% HI. It's the same with aqueous HBr, this forms an azeotrope @ 48% HBr.
Be aware that hot phosphoric acid attacks glass.
Azeotropic data may be found in Perry's chemical engineering handbook.
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ave369
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Yes, phosphoric acid works for all halide salts. In fact, even for all salts of volatile acids: I've successfully prepared nitric acid from phosphoric
acid and nitrate.
Smells like ammonia....
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chornedsnorkack
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Quote: Originally posted by ave369  | | In fact, even for all salts of volatile acids: I've successfully prepared nitric acid from phosphoric acid and nitrate. |
Interesting. The heating needed to distil HNO3 off could favour pyrolysis:
4HNO3<->4NO2+2H2O+O2
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Magpie
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This is a nice discussion. I can add my experience in making HI according to the method of Argox (Rhodium archive). As I recall I produced HI as
advertised. The only downside was the removal of a stubborn phosphate polymer in the reaction vessel. But this had a facile resolution once the
right solvent/reagent was tried.
http://www.sciencemadness.org/talk/viewthread.php?tid=23257#...
[Edited on 28-2-2016 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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ave369
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This depends on the amount of water in the reaction mixture. More water means higher temperature of distillation and more decomposition. I used 85%
phosphoric acid and the nitric acid came out yellow, but not brown.
Smells like ammonia....
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chornedsnorkack
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I see that phosphoric acid becomes significantly oxidizing to modestly inert reducers like H2 and C from 350...400 degrees.
HI is weaker reducer than H2 - but less inert.
How much heating will HI/phosphoric acids stand before they liberate any iodine?
100 % H3PO4 is said to boil at 261 degrees - where the vapour is almost pure water.
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ave369
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When I was making HI through this method, it came out noticeably brownish-yellow. This means some iodine was liberated and reacted with HI to make
HI3. I heated it until the boiling point of the HI azeotrope.
Smells like ammonia....
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