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ronstark
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[*] posted on 10-10-2016 at 23:18
Polyphosphoric Acid


I am looking for a viable route to make it. Does someone point out in the right direction?

Thanks!

[Edited on 11-10-2016 by ronstark]
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woelen
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[*] posted on 11-10-2016 at 00:20


You can heat normal ortophosphoric acid (available in concentrations of 70% to 85%) for a prolonged time, boiling off water, and making this quite hot. What happens is that at a certain concentration, the acid molecules lose water and polymerize. First you get so-called pyrophosphoric acid (2H3PO4 --> H4P4O7 + H2O), but before this conversion is completed, you get further polymerization. Finally you end with a thick hygroscpoic syruppy mass, being a mixture of polyphospphoric acids. This sticky mass is not easy to handle. Also keep in mind that heating phosphoric acid in glass may slightly attack the glass (it becomes frosty, it is slightly etched) when heated to 250 C or so. Above that temperature the severity of the corrosion of the glass quickly rises with rising temperature.



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[*] posted on 12-10-2016 at 15:57


There is a Microwave procedure, posted somewhere on this site. It may involve microwaving under vacuum. Works for producing other acids too. Sometimes starting from mono-Ammonium salts. The Ammonia exits as gas, leaving behind the pure acid.

This is financially convenient because those ammonium salts can be quite inexpensive and available in ordinary commerce, whereas a single bottle of the pure acid, may run a hundred bucks or so.

Sadly, try as you might.....you won't be able to dehydrate your Phosphoric Acid all the way back to Phosphorus Pentoxide. At least, I don't think so , but perhaps someone here knows something I don't.

PS. Found a link to patent information ETC.
http://www.sciencemadness.org/talk/viewthread.php?tid=7370

[Edited on 13-10-2016 by zed]
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[*] posted on 12-10-2016 at 23:01


Indeed, you cannot dehydrate phosphoric acid all the way to P4O10. If you heat it very strongly, then you finally end up with a glassy and sticky mass, with empirical formula HPO3, buit in reality it is a polymeric species (HPO)3)n. On standing in contact with air, this slowly absorbs water and finally turns into H3PO4 again. It may take weeks or even months though before all of the polymerized phosphoric acid is turned into H3PO4 again. Even when dissolved in a large excess of water it takes months for such conversion.

Interstingly, if you add P4O10 to water, then you get a very violent reaction with production of a lot of heat, but the resulting product is not pure H3PO4. The resulting product also contains a lot of polymerized species and it takes months before all of it has converted to H3PO4.




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[*] posted on 13-10-2016 at 00:51


A quick warning about dehydrating phosphoric acid: although the process is conceptually simple (just put phosphoric acid in something and blast the shit out of it), the implementation is very hard to get right. Hot phosphoric acid eats through almost any material you could use to hold it in while heating: glass gets dissolved away to silicon phosphates, most ceramics are destroyed by a similar process, and metals simply react with the extremely hot acid. Only gold and platinum are safe, as far as I know.

There are a few threads on sciencemadness already discussing the dehydration of phosphoric acid, but as far as I know no one has reported a successful run.

However, pyrophosphoric acid is achievable by using simple glass as the vessel, as woelen mentioned: the temperatures required for this reaction are below that where the silicon phosphate degradation mechanism becomes significant.

[Edited on 13-10-2016 by Cryolite.]
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[*] posted on 13-10-2016 at 01:47


From EROS:
Quote:
Preparative Methods: by mixing x mL of Phosphoric Acid (85%, d 1.7 gmL−1) with 2.2x g of Phosphorus(V) Oxide (P2O5) followed by heating to 200 ◦C for 30 min.


Depending on your intended use, you may find Eaton's reagent as an alternative. Again, from EROS:
Quote:
The reagent was conceived as an alternative to the widely used, but often inconvenient, Polyphosphoric Acid (PPA) (see also Polyphosphate Ester, PPE). Eaton’sreagent successfully addresses the drawbacks of PPA’s physical properties. It is much less viscous, and is, therefore, easier to stir. Organic compounds are generally soluble in Eaton’s reagent, and the hydrolytic workup is less tedious.1 Reactions are run at ambient or slightly elevated temperatures. Standard aqueous workup is easy and clean. Eaton recommends quenching the reaction with
water; quenching in ice may cause methanesulfonic anhydride to precipitate and be extracted into the organic layer; quenching in aqueous base may cause extensive foaming. In addition to its ease of handling, yields obtained with Eaton’s reagent compare favorably with those obtained with PPA.

Prepared by adding Phosphorus(V) Oxide (P2O5, 36 g) in one portion to Methanesulfonic Acid (360 g) and stirring at rt until the P2O5 dissolves. Although Eaton recommends the use of freshly distilled methanesulfonic acid to allow for a clean workup and good yields, others report using the acid as purchased


Relevant references:
Eaton, P. E.; Carlson, G. R.; Lee, J. T., J. Org. Chem. 1973, 38, 4071

Alternatively, the solution may be heated during dissolution of the P2O5. See: Stradling, S. S.; Hornick, D.; Lee, J.; Riley, J., J. Chem. Educ. 1983,
60, 502.

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[*] posted on 13-10-2016 at 07:49


Quote: Originally posted by Cryolite.  
Hot phosphoric acid eats through almost any material you could use to hold it in while heating: glass gets dissolved away to silicon phosphates, most ceramics are destroyed by a similar process, and metals simply react with the extremely hot acid. Only gold and platinum are safe, as far as I know.



[Edited on 13-10-2016 by Cryolite.]


BromicAcid tried a few metals for this and found that copper worked. He used a large copper pipe end-cap (2 or 3 inches, I think), and reported success.

Others here have reported that Pyrex is attacked by the hot acid, but that a flask corrodes slowly enough that several runs could be done in the same flask before totally ruining it. (Marvin, Great, et al)

There has been some speculation about various stainless steel alloys working, but I don't recall any experiments. Likewise I don't recall anyone using quartz.

Nickel does NOT work. This was tried and eaten through during the reaction. (One of BromicAcid's tests, I think)

[Edited on 13-10-2016 by Maroboduus]
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[*] posted on 13-10-2016 at 09:38


From what I remember, copper does technically work, but it turns the acid a dark black color due to metal contamination. It's good for a curiosity, but I wouldn't trust it for synthetic purposes.
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[*] posted on 13-10-2016 at 13:44


Looking back at the pertinent thread, I see that BromicAcid did get black results, but his first test was with naval jelly(!), and his second was with Ammonium Phosphate fertilizer which seems to have had some nitrates in it and who knows what else.

Have you (or anybody else)heard of this being tried with a good grade of phosphoric acid? If so, were the results substantially the same? I'd been thinking of trying this out myself, and I'd rather not use Pyrex if I can avoid it. (But not so much that I'd buy a $100 quartz flask)

If copper just won't work I may look into finding a way to bond gold foil into a metal container. I've got enough Gold foil to re-surface Louis XIV's
bedroom, and wouldn't mind doing a few experiments along that line. (bonding foil to an appropriate metal, not re-surfacing Louis XIV's bedroom).

EDIT: anybody know offhand if silver stands up to hot phosphoric acid? Or if it's even reasonable to look into that? Sheffield plate can be found pretty cheap sometimes, and it's got a fairly thick silver layer which is mechanically bonded on as opposed to the thin electroplated silver you usually find.

[Edited on 13-10-2016 by Maroboduus]
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[*] posted on 20-10-2016 at 11:44


I tried boiling some H3PO4 in a testtube at school and a syroupy, milky liquid/suspension was a result. I don't know how damaged my test tube was or how well did I concentrate my H3PO4 and how pure it was, but I will surely repeat the experiment and examine what I have left now. Test tubes are cheap and we have few liters of pure 85% H3PO4, no one has any use for.

But next time I will be careful and I will not pour my product on a petri dish. This time I decided to do so (bad idea) to let it cool better but I cracked the petri dish and lost all material.

[Edited on 20-10-2016 by MeshPL]
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[*] posted on 20-10-2016 at 12:08


I once tried distilling some agricultural grade (green!) H3PO4 and nothing much happened, apart from a few drops of something (probably mostly water) and a sticky mess in the boiling pot.



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[*] posted on 20-10-2016 at 20:01


Quote: Originally posted by Maroboduus  
Quote: Originally posted by Cryolite.  
Hot phosphoric acid eats through almost any material you could use to hold it in while heating: glass gets dissolved away to silicon phosphates, most ceramics are destroyed by a similar process, and metals simply react with the extremely hot acid. Only gold and platinum are safe, as far as I know.



[Edited on 13-10-2016 by Cryolite.]


BromicAcid tried a few metals for this and found that copper worked. He used a large copper pipe end-cap (2 or 3 inches, I think), and reported success.

Others here have reported that Pyrex is attacked by the hot acid, but that a flask corrodes slowly enough that several runs could be done in the same flask before totally ruining it. (Marvin, Great, et al)

There has been some speculation about various stainless steel alloys working, but I don't recall any experiments. Likewise I don't recall anyone using quartz.

Nickel does NOT work. This was tried and eaten through during the reaction. (One of BromicAcid's tests, I think)

[Edited on 13-10-2016 by Maroboduus]


Quartz vessels ares readily etched by boiling phosphoric acid in them. Don't bother. I tried dehydrating phosphoric acid (85%, syrupy) in a quartz test-tube. Result: etched (frosted) quartz test tube.
Aside from noble metals like gold & platinum, maybe zirconium would stand up to it. Maybe Palladium?
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[*] posted on 20-10-2016 at 21:52


Good to know. But as I said above, I wasn't going to.
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[*] posted on 21-10-2016 at 23:27


The composition of azeotropic phosphoric acid is reported to be slightly towards P2O5 from HPO3, and boiling point around 850 degrees.
What is the glass point of HPO3?
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[*] posted on 22-10-2016 at 08:57


Actually how much harm does a little salts dissolved in PPA do? If you have lots of testtubes, than you can sacrifice some to get some impure polyphosphoric acid. Depends on what do you want do do with your acid.

[Edited on 22-10-2016 by MeshPL]
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[*] posted on 23-10-2016 at 08:50


Quote: Originally posted by woelen  
Indeed, you cannot dehydrate phosphoric acid all the way to P4O10. If you heat it very strongly, then you finally end up with a glassy and sticky mass, with empirical formula HPO3, buit in reality it is a polymeric species (HPO)3)n. On standing in contact with air, this slowly absorbs water and finally turns into H3PO4 again. It may take weeks or even months though before all of the polymerized phosphoric acid is turned into H3PO4 again. Even when dissolved in a large excess of water it takes months for such conversion.

Interstingly, if you add P4O10 to water, then you get a very violent reaction with production of a lot of heat, but the resulting product is not pure H3PO4. The resulting product also contains a lot of polymerized species and it takes months before all of it has converted to H3PO4.

I'd been neglecting buying phosphoric acid, because I had quite a bit of P2O5 and figured I'd just add water to it after I'd used it for something that wouldn't contaminate it, if I needed phosphoric acid for anything. Of course, reading this, I'm glad I finally bought some.

Incidentally, because P2O5 forms a viscous, sticky mess that stops reacting as soon as it absorbs any water, the way to overcome this seems to be to increase its surface area by dry-mixing it with silica gel or alumina beads. Since those are desiccants, they probably have a bit of water adsorbed onto their surfaces, which makes P2O5 sticky, which results in a coating of P2O5. Anyway, this reactant is useful for a crazy number of things, and in fact can do Friedel-Crafts acylation using only carboxylic acids and the corresponding aromatic substrates! No need for acid chlorides or aluminum chloride, or even a single chlorine atom. Apparently, the P2O5 dehydrates the acids into their anhydrides, then acts as a Lewis acid itself for the acylation. You don't even end up with waste acids like you would if you used an anhydride in a regular Friedel-Crafts reaction, because the P2O5 apparently just dehydrates them back into their anhydrides. Considering P2O5 can be obtained for about $20 a pound, this seems very promising.



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[*] posted on 26-10-2016 at 07:39


Quote: Originally posted by woelen  
Also keep in mind that heating phosphoric acid in glass may slightly attack the glass (it becomes frosty, it is slightly etched) when heated to 250 C or so. Above that temperature the severity of the corrosion of the glass quickly rises with rising temperature.

So, is the exercise of 30 minutes heating at 200 degrees safe for glass?
The boiling point of 100 % phosphoric acid is quoted as 261 degrees (vapour being almost all water). The viscosity of 100% phosphoric acid drops from 140 centipoises at 25 degrees (supercooled) to just 9 centipoises at 100 degrees, and further at further heating.
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[*] posted on 26-10-2016 at 22:51


Heating at 200 C for 30 minutes will be safe, in the sense that the glass will not be dissolved and you do not have to fear cracking of your glass vessel (at least, not more than heating any other high boiling liquid at 200 C).

There may be slight etching. Even if your acid only is contaminated with trace amounts of silicate, then you can see that on the glass very well. It becomes somewhat frosty. It is amazing how little glass needs to be etched away to make a big difference in appearance.
This frosty appearance only becomes visible when the glass becomes dry. When wetted, it hardly is visible anymore.

[Edited on 27-10-16 by woelen]




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[*] posted on 27-10-2016 at 02:42


The more concentrated is the polyphosphoric acid, the more it etches glass. 200 C is somewhere at borderline between the vessel being stable and being quickly etched, small hot spots having temperature of 250 C may lead to a failure of your vessel in an hour or two.
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