[karbon]

Does anyone have any experience with generating hydrogen iodide from hydrogen and iodine? There are of course lots of references in the litterature, but they usually involve platinum and hydrogen/iodine gas at very high temperatures. Like with hydrogen chloride, it should be possible to generate hydrogen iodide through some photodissociation mechanism, but should this also be done in the gasphase? Or is it possible to do under high pressure in a solvent?

[Yakimikku]

I can't remember clearly at the moment, but I believe my labmate generated HCl gas for a reaction by dropping concentrated sulfuric acid onto NaCl. My first guess would be to look into seeing if the same could be done with NaI. You could then pass the HI gas directly to your reaction flask or whatever. Hope that helps!

[karbon]

Thank you, but the problem here is actually the generation of HI from hydrogen and iodine, and under what conditions this is possible.

[Yakimikku]

Oh my bad! I see so you want to generate HI from H₂ and I₂, but you want to avoid the extreme temperatures? Wikipedia claims irradiation at 578 nm increases the rate of the reaction significantly, but it doesn't say if you can get away with lower temperatures. In any case in standard pressure you'd need at least ~200 degrees C to keep the iodine in the gas phase (bp ~180). Maybe with the right catalyst you could get lower temperatures to work with iodine as a liquid or dissolved in a solvent. Why do you want to make it from H₂ and I₂? for purity?

[Borek]

Can't help it, whenever I read about HI synthesis, I start to think about drugs.

[Arkcon]

Just by their relative positions in the periodic table, you've got to realize that chlorine is more reactive than iodine.  So that's a strike against your plan.  Yakimikku: has already mentioned, if you want a gas phase photo reaction, you at least have to get your reaction vessel up past 180 C to vaporize the solid iodine.  And if you want a reaction with hydrogen, a platinum metal as a catalyst always helps.

[karbon]

Oh my bad! I see so you want to generate HI from H₂ and I₂, but you want to avoid the extreme temperatures? Wikipedia claims irradiation at 578 nm increases the rate of the reaction significantly, but it doesn't say if you can get away with lower temperatures. In any case in standard pressure you'd need at least ~200 degrees C to keep the iodine in the gas phase (bp ~180). Maybe with the right catalyst you could get lower temperatures to work with iodine as a liquid or dissolved in a solvent. Why do you want to make it from H₂ and I₂? for purity?

Mainly because its an economical and clean way to make HI. I read the wikipedia article also, but its references are pretty bad, and I havent actually seen any litterature describing the irradiation procedure and therefore I have no clue as to the difficulties of this reaction.

[karbon]

Just by their relative positions in the periodic table, you've got to realize that chlorine is more reactive than iodine.  So that's a strike against your plan.  Yakimikku: has already mentioned, if you want a gas phase photo reaction, you at least have to get your reaction vessel up past 180 C to vaporize the solid iodine.  And if you want a reaction with hydrogen, a platinum metal as a catalyst always helps.

Yeah I know, the reaction is even supposed to be endothermic. But do you think it would be necessary to carry out the reaction in the gas phase as opposed to having the iodine in a solvent at say 150C with 10 bar of hydrogen?

[Yakimikku]

Mainly because its an economical and clean way to make HI. I read the wikipedia article also, but its references are pretty bad, and I havent actually seen any litterature describing the irradiation procedure and therefore I have no clue as to the difficulties of this reaction.

In that case, it might be better to try a different route to making HI. H₂ + I₂ requires high temperatures, making it impractical. This and your other ideas about using higher pressures are both dangerous--risk of an explosion. Ultimately, do you want to use HI as a gas or as an aqueous solution? If you want the latter, than I wouldn't bother with the gas phase reaction and directly make it in another method. If you still want the gas, there are still other options. Industry apparently uses I₂ + N₂H₂. The nitrogen gas byproduct is nothing to worry about, but hydrazine is quite nasty/dangerous. Since this is the industrial process, I'm sure there is something economical about it. Just some thoughts. I don't want to completely deny your ideas however--there may be an effective way that's not commonly known or yet to be discovered.

[Honclbrif]

http://www.youtube.com/watch?v=NN82GoBG98s

Iodine would probably have less of a kick, but I'd still not recommend trying to make HI from H₂ and I₂ if you're a fan of having things like a face and hands.

If you need small quantities of a hydrogen halide your best bet is, as Yakimikku said, to add conc. H₂SO₄ to the sodium halide and pass the evolved gas through a drying tube.

[AndersHoveland]

If preparing anhydrous hydrogen iodide from the iodide salt, you cannot use concentrated sulfuric acid. It will oxidize anhydrous HI, especially at elevated temperatures. Many chemists make this mistake. Usually phosphoric acid is used in the distillation instead. HBr is also vulnerable to oxidation, but can still be obtained from conc. sulfuric acid, although in lower yields.