rocketman
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How to dry out phosphorus pentoxide?
I have some phosphorus pentoxide but over time it has absorbed water. How can I remove this water, how can I dry it out? 
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Detonationology
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Not much hope. Converting it back into phosphorus pentoxide from phosphoric acid would require some costly reagents.
“There are no differences but differences of degree between different degrees of difference and no difference.” ― William James
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BromicAcid
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From my experience the hydrated phosphorus pentoxide tends to form a sticky tacky mass on top of the good material underneath. If that is the case,
scrape that bit off and cut your losses, seal it up better next time. As Detonationology said, converting back to anhydrous material is not a trivial
matter since the P<sub>2</sub>O<sub>5</sub> is not simply picking up waters of hydration but reacting chemically.
Edit: But in the spirit of Sciencemadness, combine the hydrated bits with sufficient carbon (charcoal) to afford the reaction-
2P<sub>2</sub>O<sub>5</sub> + 10C ---> P<sub>4</sub> + 10CO
Take the reaction mass and put into a strong retort able to withstand ~1000C. Equip for distillation. Heat to red heat to liberate the phosphorus.
Take care that any water that was taken up by your phosphorus pentoxide will end up in your effluent gases as phosphine (highly toxic, pyrophoric
gas). Collect your elemental phosphorus under water (which certainly doesn't count as health food either). Finally, dry it off with a quick rinse in
alcohol and oxidize it directly with atmospheric oxygen (dry air) note that the reaction is highly exothermic, it may catch on fire and phosphorus
fires are known to burn 'like the fires of hell'. An apparatus for this is beyond the scope of this post. Your phosphorus pentoxide will be
collected as a 'smoke'. If you limit oxygen and feed it in slowly you will end up with sub-oxides of phosphorus. I debated on this depth of answer
since it seems the smart-ass response but I think it might be one of the only ways to get back to where you started.
[Edited on 2/23/2016 by BromicAcid]
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chornedsnorkack
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Azeotropic phosphoric acid is slightly towards P2O5 from HPO3, and boils slightly over 800 degrees.
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Sulaiman
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I would keep the partially hydrated gel to use as phosphoric acid
because I have none 
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clearly_not_atara
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Normal dehydrating agents won't work, but... what about sulfuryl chloride? This is even more dehydrating than sulfur trioxide:
SO2Cl2 + H2O >> SO3 + 2HCl
At high enough temperature the reaction H6P6O18 + 3 SO2Cl2 >> 3SO3 + 6 HCl + 3 P2O5 should be favored by entropy considerations alone.
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UC235
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Quote: Originally posted by clearly_not_atara  | Normal dehydrating agents won't work, but... what about sulfuryl chloride? This is even more dehydrating than sulfur trioxide:
SO2Cl2 + H2O >> SO3 + 2HCl
At high enough temperature the reaction H6P6O18 + 3 SO2Cl2 >> 3SO3 + 6 HCl + 3 P2O5 should be favored by entropy considerations alone.
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Sulfuryl chloride isn't a dehydrating agent, but a chlorinating agent. It decomposes to chlorine and SO2.
P2O5 is an insanely powerful dehydrating agent. It will turn sulfuric acid into SO3, and nitric into N2O5.
While polyphosphoric acid can be made by throwing phosphoric acid in a furnace, P2O5 is exclusively made by burning phosphorus.
For OP, I would take Bromic's advice. Scrape off the goop. Congrats. That's probably in polyphosphoric acid territory and is still a useful
dehydrating agent.
[Edited on 24-2-2016 by UC235]
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clearly_not_atara
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| Quote: | | Sulfuryl chloride isn't a dehydrating agent, but a chlorinating agent. It decomposes to chlorine and SO2. |
It
depends on conditions; SO2Cl2 will convert triphenylphosphine oxide to triphenylchlorophosphonium chloride, which was what inspired me in this case.
See:
http://researchrepository.ucd.ie/bitstream/handle/10197/4940...
Unless you know the reaction won't proceed you're not telling me anything I don't know by paraphrasing Wikipedia. I'm well aware of the reactivity of
P2O5 but that doesn't mean there isn't anything that will dry it; rather, anything that will dry it is too expensive to be practical.
[Edited on 24-2-2016 by clearly_not_atara]
[Edited on 24-2-2016 by clearly_not_atara]
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UC235
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| Quote: Originally posted by clearly_not_atara | | Quote: | | Sulfuryl chloride isn't a dehydrating agent, but a chlorinating agent. It decomposes to chlorine and SO2. |
It
depends on conditions; SO2Cl2 will convert triphenylphosphine oxide to triphenylchlorophosphonium chloride, which was what inspired me in this case.
See:
http://researchrepository.ucd.ie/bitstream/handle/10197/4940...
Unless you know the reaction won't proceed you're not telling me anything I don't know by paraphrasing Wikipedia. I'm well aware of the reactivity of
P2O5 but that doesn't mean there isn't anything that will dry it; rather, anything that will dry it is too expensive to be practical.
[Edited on 24-2-2016 by clearly_not_atara]
[Edited on 24-2-2016 by clearly_not_atara] |
Don't give me shit based on a single line in a paper as reference. I dug up the referenced material. Here is Citation 30:
https://www.jstage.jst.go.jp/article/cl1972/6/2/6_2_151/_pdf
Triaryl/alkyl phosphine oxides are deoxidatively chlorinated by oxalyl chloride (same as the paper you link) or diphosgene, then reduction by thiols.
No sulfuryl chloride.
No sulfuryl chloride in US Patent 4,301,301 either which is lumped in with reference 30. This concerns reduction of R3PCl2 with hydrogen gas.
Here is reference 32: http://pubs.rsc.org/en/Content/ArticleLanding/2012/CC/C1CC14...
It's behind a paywall but I had a look and there is no mention of sulfuryl chloride either. They use oxalyl chloride followed by NaBH4 to produce
phosphine-borane adduct in a one-pot reduction of phosphine oxides. Alternately, they preserve stereochemistry using MeOTf or trialkyloxonium
tetrafluoroborates as the activator for the phosphine oxide before reduction.
I assume the mention of SO2Cl2 as an agent in the paper you link is simply an error and belongs in the previous sentence since it is expected that
sulfuryl chloride can halogenate a trialkyl/arylphosphine to the dichloride. I expect that this is contained in one of the references (33,34,35,or 36)
but I don't particularly care enough to look.
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chornedsnorkack
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| Quote: Originally posted by clearly_not_atara | Normal dehydrating agents won't work, but... what about sulfuryl chloride? This is even more dehydrating than sulfur trioxide:
SO2Cl2 + H2O >> SO3 + 2HCl
At high enough temperature the reaction H6P6O18 + 3 SO2Cl2 >> 3SO3 + 6 HCl + 3 P2O5 should be favored by entropy considerations alone.
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Which way should the equilibrium of the reaction
3SO2Cl2+P2O5<->3SO3+2POCl3
go?
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clearly_not_atara
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Use the enthalpies of formation to find your answer. You can estimate entropy crudely from data on similar compounds and usually get away with it. I'm
pretty sure the equilibrium is hugely in favor of P2O5 + SO2Cl2.
In the case of metaphosphoric acid I was hoping HCl formation would drive the reaction. From this paper the delta-Hf of HPO3 is -226.7 kcal and 1/2 of P2O5 is -135ish. HCl is -92, so if SO3 is at least as stable as SO2Cl2 there's a prima
facie thermodynamic basis for the idea (entropy favors forming HCl), but by contrast the phosphorus halides are extremely dehydrating relative to
their sulfur cousins. That doesn't mean the reaction will proceed but it's certainly not impossible.
Other possibilities might include SiCl4, thionyl chloride, TiCl4, S2Cl2, etc. Some of these have downsides (like turning into glass).
[Edited on 25-2-2016 by clearly_not_atara]
[Edited on 25-2-2016 by clearly_not_atara]
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chornedsnorkack
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So:
POCl3: -610 kJ/mol*4=2440
P4O10: -3010 kJ/mol
SO2Cl2: -355 kJ/mol *6=-2130
SO3: -396 kJ/mol*6=-2376
What I seem to get is: whole left -5140; whole right -4816.
Indeed - POCl3 should chlorinate SO3
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
[Edited on 26-2-2016 by chornedsnorkack]
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rocketman
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Drying Phosphorus pentoxide
Thanks guys. Had no idea it would be so difficult.  Would cry but that would
only add to the water.
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