Upsilon
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Synthesis of phosphoric acid from monoammonium phosphate
I am curious as to how I would go about creating phosphoric acid from cheap monoammonium phosphate. I am theorizing that a reaction with HCl would do
it:
NH4H2PO4 + HCl -> NH4Cl + H3PO4
However, I am not sure if this reaction would take place, and if it did, how I would extract the acid from the resulting NH4Cl solution. Could I
evaporate off the water, then melt the phosphoric acid and pour it out? Or would heat cause the acid to react with the NH4Cl?
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deltaH
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| Quote: | | Could I evaporate off the water, then melt the phosphoric acid and pour it out? |
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]
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Upsilon
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Is the "legendary" glassware necessary? Also, is it necessary to boil the acid, or could you just melt it to leave behind NH4Cl?
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deltaH
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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?
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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.
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woelen
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One thing to keep in mind is that hot concentrated H3PO4 or further dehydrates versions of that attack glass. The attack is not fast, but it is
noticeable and may make your glassware look ugly and it contaminates the acid with silicic acid.
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deltaH
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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.
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Nicodem
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Cross-linking threads as a favour to those too lazy to UTFSE: Prepublication » Orthosphosphoric Acid
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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MrHomeScientist
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I've had one of those Vision pots for quite some time for cooking. Does anyone else have experience with them? I never thought to use it for
chemistry, and had never heard any of the "legends" about them. I actually don't really like it for cooking because it's prone to superheat water
boiled in it and frequently causes boil overs.
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Paddywhacker
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This topic has come up before with excellent discussion of the method:
http://www.sciencemadness.org/talk/viewthread.php?tid=7370#p...
Addition of hydrochloric acid is superfluous and wasteful. Ammonium chloride will dissociate into ammonia and HCl gasses under these conditions.
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