Sciencemadness Discussion Board

phosphorus pentoxide purification

Loptr - 7-8-2015 at 14:43

I have some phosphorus pentoxide that has been reacting with moisture. The mass is now a golden color with the acid pooled at the bottom of the container. Could someone provide a procedure to work this up and restore it to its previous glory?

I purchased it from Firefox-Fx, and it came in a bag that had been sitting around for a while before it went out. They forgot about my order, somehow, and two weeks later it shipped. I now have a credit on my account, so nothing lost, entirely.

My initial thought is to dissolve it in a dry solvent that dissolves the acid, but doesn't dissolve the phosphorus pentoxide. My searches didn't turn up a procedure, so I am wondering if you guys know of one.

gdflp - 7-8-2015 at 18:13

The 6th edition of Purification of Laboratory Chemicals specifies vacuum sublimation at 250°C. The only problem I can see with this is that the phosphoric acid may attack glass at that temperature. Brauer provides a method for sublimation on page 81 for removal of lower oxides of phosphorus, but it may work to purify it. You will need to remove a lower cut though, since polyphosphoric acid boils at 300°C at atmospheric pressure. I'm not sure how well it will work, but it's work investigating IMO.

Loptr - 8-8-2015 at 08:16

I appreciate the reply, gdflip.

I don't think I have that capability at this time. I was hoping to find a solvent that would dissolve the phosphoric acid, and not react with the phosphorus pentoxide. I don't care about purifying it so much, as just recovering the rest of the powder. Maybe put it into another container and let the acid go to the bottom, and then take the top?

careysub - 8-8-2015 at 09:05

Quote: Originally posted by Loptr  
I appreciate the reply, gdflip.

I don't think I have that capability at this time. I was hoping to find a solvent that would dissolve the phosphoric acid, and not react with the phosphorus pentoxide. I don't care about purifying it so much, as just recovering the rest of the powder. Maybe put it into another container and let the acid go to the bottom, and then take the top?


I have a suggestion about how to do this: make a perforated draining trivet out of card board on little cardboard feet that is cut to the size of your container. You could get even fancier and tape paper strips to lift the drained P2O5 out. A rig like this can be made in just a couple of minutes if you have cardboard, scissors or a razor knife, and tape.

A do shop projects a bit and one thing that all craftsmen (not that I claim so lofty a little) do is build a lot jigs to help with various tasks, many just once-ever things unique to a project. A jig is built out of cheap but serviceable materials, with a view as to its required durability. Throw-aways are common.

I think this extends to lab work as well - since many of us do not have well equipped labs and need to solve problems with what we have.

Loptr - 8-8-2015 at 10:14

That is a very good suggestion, careysub.

I have been eyeing an enclosable Büchner funnel, which would work really well here. Run the ingres gas tube from a sulfuric acid bubbler to the top of the funnel, charge the Büchner funnel with the P4O10, and then draw off the acid using negative pressure. This should separate the acid somewhat effectively.

chemrox - 8-8-2015 at 16:22

Don't you think cardboard might leach some dyes and glues? However you salvage the P2O5 learn to seal with wax and fill the bottle with an inert gas before closing it up again.

Loptr - 8-8-2015 at 16:27

Quote: Originally posted by chemrox  
Don't you think cardboard might leach some dyes and glues? However you salvage the P2O5 learn to seal with wax and fill the bottle with an inert gas before closing it up again.


Well, not so much the cardboard, but the stand to filter the acid off while in storage. Put the stand in the bottle and collect the acid off the bottom.