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Author: Subject: Hydrogen halides by phosphoric acids?
chornedsnorkack
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[*] posted on 27-2-2016 at 10:21
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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[*] posted on 27-2-2016 at 16:27


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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[*] posted on 28-2-2016 at 00:14


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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[*] posted on 28-2-2016 at 01:12


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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[*] posted on 28-2-2016 at 09:26


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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[*] posted on 2-3-2016 at 00:40


Quote: Originally posted by chornedsnorkack  

Interesting. The heating needed to distil HNO3 off could favour pyrolysis:
4HNO3<->4NO2+2H2O+O2


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.




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chornedsnorkack
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[*] posted on 5-3-2016 at 14:45


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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[*] posted on 6-3-2016 at 08:29


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.



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