Synthesis of phosphoric acid from monoammonium phosphate

WARNING: Not for a novice!

Be aware that phosphoric acid readily forms polyphosphoric acids upon concentration and dehydration. These makes the liquid very viscous!

I've myself 'boiled down' phosphoric acid in corning VISION glassware, you know the amber coloured glass dishes.



These pots are alchemical legends

At some point, it just stops boiling because the phosphoric acid get's so dehydrated it doesn't want to dehydrate more. It's probably almost entirely a polyphosphoric acid at this stage, (HPO3)x. At that point, it's thick but I think it's still somewhat pourable because of the high temperatures.... you might have a chance to decant it from crystals that settle at the bottom if you work carefully.

You must work in an extracted environment (e.g. fume hood) as the polyphosphoric acids makes nasty fumes

BEWARE, such extremely hot polyphosphoric acid if it gets on you will give you terrible terrible burns in an instant! I only did this because the amber corning glass cooking ware is legendary and I had all the safety bells and whistles: thick apron, arm length welders gloves and face shield.

In theory, if you carefully decant this out at this stage into a second heat and acid resistant container, let it cool, then add water to get 75% solution and simmer it for a few hours if necessary to give it time to hydrolyse back to H3PO4, you might have achieved your goal.

[Edited on 28-10-2013 by deltaH]

Quote: Originally posted by Upsilon

Is the "legendary" glassware necessary? Also, is it necessary to boil the acid, or could you just melt it to leave behind NH4Cl?

lol, of course Back to a serious note, when boiling concentrated acids that have a high boiling point, I want to be as certain as possible that my glassware doesn't break on me for obvious reasons. To be honest, these corning vision pots are the only thing I've ever trusted in this respect.

Boroglass in general is just as good I suppose, except that these things are almost indestructable, beakers and flasks break more easily in day to day use, especially the large ones.

I really advise the serious amateur chemist to get their hands on one of these (they lurk on ebay and such, no longer in production if I am not mistaken?)

You ask why is this necessary? Because you started with conc. HCl which has a lot of water, so you will have quiet a bit of residual NH4Cl dissolved in that. By removing as much water as possible, you maximisise the amount of NH4Cl you could potentially crystalise.

Thanks woelen, I had a look but they look the same as always, even little micro scratches are still there on the base, but yeah... I wouldn't want them spoiled, so I will keep an eye on it to see if anything changes. What I like about them is that they are so hard to break. I have a big corning vision pot that I do nearly all of my larger preparations in... wait let me find a photo:

Something like this:



It's really excellent for large preps and smaller ones as well where a lot of frothing is expected.

If I'd bought 5l pyrex beakers, I can guarantee I'd have broken them by now, you just lightly tap one of those against something and they break. The problem is the wall thickness on large beakers are just too thin!

These dishes I trust with my life... literally.