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Author: Subject: Orthosphosphoric Acid
guy
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Orthosphosphoric Acid

For those who can't use the calcium phosphate and sulfuric acid method.

This method uses ammonium dihydrogen phosphate as the starting material to make pure phosphoric acid. It is heated until decomposition and no more NH3 gas is detected. A thick clear liquid is left behind (H3PO4) which cools down into a brittle solid which seems a lot like solidified sugar after it has been melted.

NH4(H2PO4) --heat--> NH3 + H3PO4

Materials:
A Teflon pan
Heat source

Note: I forgot to wiegh the reactants but the yield is quantitative due to the fact that H3PO4 cannot be boiled off.

Take some ADP and dissolve it in boiling water until the particles dissolve and break up; there will be a lot of black and green stuff. Filter with coffee filter paper AND either cotton balls (to get rid of fine particles). The solution should be much clearer but still yellow-greenish.
Boil off about half the volume and recrystallize the ADP. To the rest of the solution, add some alcohol to precipitate the rest of the ADP. Crystals should be mostly white.

Take the solid ADP in a teflon pan and heat it up until it melts (about 190 oC). Continue heating until the smell of ammonia is gone and a clear liquid is left behind. This liquid has a low melting point and can be easily melted in a couple of seconds (H3PO4 has a MP of 42 oC).

While it is cooling and solidifying, scoop it out so it doesnt stick to the pan.

This about 90g of solid H3PO4

Polverone
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Is it confirmed ammonia-free? If you heat the final product with an excess of sodium hydroxide, is it odorless?

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guy
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 Quote: Originally posted by Polverone Is it confirmed ammonia-free? If you heat the final product with an excess of sodium hydroxide, is it odorless?

I just found out that ammonium sulfate is also added in the mix. So when I was decomposing the salt, no more ammonia gas was produced. But when I put it in solution and added NaOH, ammonia was produced...

Of course this only applies if you get the 16-20-0 grade. The 11-52-0 grade is much more pure, and has little sulfate.

<b>So the solution is to heat it until no more ammonium sulfate smoke is produced. </b>

You could still skip that step if you don't care about ammonium sulfate impurity, such as if you where using it to produce HCl or use it in acid catalyst reactions.

[Edited on 1/8/2007 by guy]

garage chemist
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Ammonium sulfate does the same thing as ammonium phosphate when heated: it gives off ammonia and eventually forms sulfuric acid.
So you will have this in the acid as well.

I am also in doubt as to the validity of the procedure. You should acquire some phosphoric acid (pharmacy), make pure ammonium phosphate from that and heat to find out if it really forms phosphoric acid again. In am not sure if you product is not some polymerized ammonium metaphosphate or something like that.

[Edited on 8-1-2007 by garage chemist]

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guy
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 Quote: Originally posted by garage chemist Ammonium sulfate does the same thing as ammonium phosphate when heated: it gives off ammonia and eventually forms sulfuric acid. So you will have this in the acid as well. I am also in doubt as to the validity of the procedure. You should acquire some phosphoric acid (pharmacy), make pure ammonium phosphate from that and heat to find out if it really forms phosphoric acid again. In am not sure if you product is not some polymerized ammonium metaphosphate or something like that. [Edited on 8-1-2007 by garage chemist]

Phosphoric acid cannot boil off, it merely loses water, forimg pyrophosphoric acid, and then metaphosphoric acid, a solid. Sulfuric acid just boils away forming SO3 and H2O which combine with NH3 forming a cloud.

I tested my H3PO4 by putting an iron nail in there and H2 gas was liberated easily, ammonium diphosphate cannot do this.

One other reason why I know H3PO4 doesnt recombine with NH3 is that during the heating, only NH3 is given off, but after all the NH4(H2PO4) has reacted, no more NH3 was given off but only a white smoke of (NH4)2SO4.

[Edited on 1/8/2007 by guy]

guy
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Well, I finally got rid of the (NH4)2SO4...I dissolved the solid in water and added an equal amoount of EtOH...a very small amount of ammonium sulfate precipitated out. It was filtered and the solution was tested with NaOH. No smell of NH3 emited!

Now I am boiling the solution to really obtain pure H3PO4.

LSD25
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I only just found this, I have done this reaction many, many times - the polyphosphoric acid you end up with is as clean as the starting materials were. If for instance you start with fertilizer grade monoammonium phosphate, you will discover that the organic coating (some sort of lanolin based grease by the smell) is insoluble in water - but is soluble in most organic solvents, I strongly suggest washing the prills with something cheap (even petrol/turpentine/kerosene/toluene/acetone/paint thinner/degreaser/something like that - it doesn't matter, it won't get in the end product).

If any organic crud/polymeric crap is left, repeat the rinse procedure until it is gone.

Just be aware that water in the solvent will remove some of the the MAP (which is highly soluble in even cold water), so a modicum of care is appropriate.

Once the prills are a solid beige-cream-white colour it is time to dissolve them in a minimum amount of hot, deionized water (by all means used distilled, but deionized is a lot easier and OTC). Stir the hot solution then quickly decant the clear-ish supernatant through a large porcelain buchner - I suggest using washed sand instead of filter paper for the first couple of filtrations or you will be there for days. The fertilizer grade product (or the one most often encountered, such as in fire extinguishers) contains about 50% inert, insoluble material.

If you are concerned to extract as much as possible from a given quantity of fertilizer, by all means re-extract the filter cake from the filtrations using additional hot water.

Once you have your solution of MAP in water, proceed to crystallize it (fairly trivial actually, this shit is sold in crystal growing kits cos it is a cinch to crystallize). Do NOT allow the liquid to dry completely - you will simply pollute your product (quite possibly the ammonium sulfate mentioned above - the solution has a distinct yellow colour) instead filter the formed crystals (they will be in the form of well defined, water clear crystals) when the solution is down to about 10% of its original volume. Discard the filtrate, it contains numerous highly soluble impurities and ain't worth the work involved in purifying it (this shit is sooooo cheap).

I am currently working on the final rinse prior to the final recrystallization - from what I have seen, a rinse of the filter cake from the above step with cold 90-96% ethanol will remove most of the remaining crud, dissolving a minimal amount of product.

The polyphosphoric acid from this purification procedure should be essentially pure, as should the various phosphoric acids you decide to make from it. The old sayings tend to contain a lot of truth, in particular 'crap in equals crap out!'

PS You know what we need? A whole topic on purifying the various useful OTC chemicals (especially the hardware store and fertilizer grade variety). This is an amateur forum, that means not everybody has the money, the inclination or the access necessary to buy even Technical grade reagents

Whhhoooppps, that sure didn't work
Ritter
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 Quote: Originally posted by LSD25 I only just found this, I have done this reaction many, many times - the polyphosphoric acid you end up with is as clean as the starting materials were. If for instance you start with fertilizer grade monoammonium phosphate... Once you have your solution of MAP in water, proceed to crystallize it .... The polyphosphoric acid from this purification procedure should be essentially pure...

Maybe I'm missing something in having read through your procedure, but it looks like you have only succeeded in recrystallizing MAP & not in producing polyphosphoric acid (PPA).

I've toured an industrial phosphate plant & I've seen how much energy is required to make even pyrophosphates. I'm investigating the synthesis of PPA for a home lab-scale project but your procedure does not produce PPA.

Ritter
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chloric1
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Hi yield Ammonium Sulfate has some stinky impurities. Dissolving the fertilizer in an equal mass of tepid water and filtering leaves a clear yellowish liquid. Putting filtrate in a bottle with aquarium activated carbon gives a crystal clear water white solution.

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not_important
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Ritter, the recrystallization &ct was for cleaning up the phosphate before using in as in the process in the starting post of the thread, which does produce phosphoric acids.
Ritter
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 Quote: Originally posted by not_important Ritter, the recrystallization &ct was for cleaning up the phosphate before using in as in the process in the starting post of the thread, which does produce phosphoric acids.

Did you make PPA & how?

Ritter
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starman
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 Quote: PS You know what we need? A whole topic on purifying the various useful OTC chemicals (especially the hardware store and fertilizer grade variety). This is an amateur forum, that means not everybody has the money, the inclination or the access necessary to buy even Technical grade reagents

I think that's a fine idea."prepuification of subtechnical grades"With posts from members like LSD 25 who have written there own procedures from oft repeated home experimentation.
While a fair bit of info is readily available for purifying technical grades theres not much for commercial grades,multicomponent mixtures and the deliberately denatured.
Ebao-lu
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Actually, i believe neither H3PO4 nor PPA can result from heating of NH4H2PO4 as major products. Maybe some superphosphorous acids etc (that may still contain some ammonia, but lets consider it not critical)

 Quote: A thick clear liquid is left behind (H3PO4) which cools down into a brittle solid which seems a lot like solidified sugar after it has been melted.

Pure H3PO4, afaik, destite is known to melt above zero(42,5C), never crystallyzes even in fridge. In an encyclopedia it is written that "in molten state, H3PO4 tends to supercool and at 15C it becomes a dense, viscous liquid and at -121C - glass-like body. As for my few experiments with H3PO4, i tried to concentrate the usual commercial solution (85%) to 100% by heating with weight-control. There seems to be no temperature, at which H3PO4 looses all the water(with a kinetically significant rate), and at the same time being stable to further dehydration. And of course, the resulted H3PO4 did not crystallize (i thought first it was a fail), and one more note - it does not affect glass significantly during the heating(usual tubes), its weight remains same..

Poluphosohoric acid also can't be done by this way. In organic chemistry PPA is known to be rather non-OTC compound, that is usually prepared beforehand by heating of P2O5 with H3PO4 or heating H3PO4 in vacuo. Both procedures are not as easy as it may appear. Probably, heating NH4H2PO4 in vacuo could give PPA, but i doubt.

The product of NH4H2PO4 heating should probably be boiled with water in order to make it H3PO4, and then the water should be evatorated until the temperature raises to the boiling temperature of needed concentration H3PO4 solution.
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I just did it...

Although I am quite new to home chemistry and began with it only for the sake of my kids, I got intrigued by the idea of getting phosphoric acid from fertilizer (which are among the very few chemicals I can actually buy). So I gave it a try.

My fertilizer is ammonium hydrogen phosphate, which gives off ammonia when heated only a little, and becomes then ammonium dihydrogen phosphate (as I suppose), so after that I am at the starting point of this thread.

The biggest issue I had at first was purifying the fertilizer. I could easily filter off a grey, muddy substance, but the remaining liquid was still dark brown, and on heating some greasy stuff came to the surface. However I went on heating it. When all the water had evaporated, it became a light-brown sugar-like solid. Upon further heating, it melted and gave off ammonia again. It turned then into a foamy, thick liquid which became darker while the reaction progressed, until it was almost completely black and only minimal amounts of ammonia came off. I stopped there, dissolved the whole mass with some water and filtrated. Now that whole black stuff remained in the filter, and out came a clear, light yellow liquid!

From a part of it I tried to boil off as much water as possible, but when it got thicker, it began to foam again. On cooling it formed a soft white mass; on touch it felt similar to wet sugar.

I also ventured to try a small drop of the dilute acid on the tip of my tongue (someone wanting to scold me now?) It tasted similar to citric acid.

I do not have the means to determine exactly what it is and how pure it is, but it is quite acidic. As far as I could determine without precision scales and without a pH-meter, one gram of the white stuff was neutralized by about 7,5 ml of a 1 M solution of baking soda (this is the only base I have of which I can prepare a solution of known concentration). Now this may mean different things, depending on what kind of acid(s) I have and how complete or incomplete the reaction was.

- After that I found some more information on Wikipedia where it says that H3PO4 dehydrates at relatively low temperatures to H4P2O7, and H4P2O7 when dissolved in water hydrates again to H3PO4. So that soft solid was most probably H4P2O7 ? - I mixed it with a small amount of water and heated again, but only gently. The resulting substance was softer and also more soluble; so at least a part of it may have hydrated back to H3PO4. (These are only my own interpretations as a beginner.)

I did not test if it is ammonia free (most probably it is not, since there was still a minimal reaction going on when I stopped).

At least I know I got *some kind* of phosphoric acid. And it seems not so important to get rid of that brown greasy stuff in the beginning, since it gets "thicker" during the reaction and can be filtered out at the end. Below is a photo of the filtration. (My filter paper is quite improvised...)

And this is what I got after boiling off most of the water:

Afterthoughts:

- That grey mud that gets filtered off at the beginning, can it be used for something useful apart from throwing it into the garden?

- In a future attempt I would like to capture the ammonia for further use. This would be easy in the first reaction (AHP to ADP), but not in the second one which would be difficult in a closed vessel because of the thick foam.

- Why, after all, do we have to decompose what the chemical industry has synthesized, instead of them selling us their stuff (for example phosphoric acid) directly? I suppose it is just for the sake of teaching chemistry to my kids...
Drunkguy
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The ammonia has separate uses, but I am more interested in how the phosphoric acid can be prepared.
unome
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Here is the patent that LSD25 thought he got the idea first from, apparently it had already been thought of by someone else...

I've done this, and must say the purification of the fertilizer grade material is ESSENTIAL, otherwise the organic coating (and insolubles - I suspect calcium phosphates) will interfere and cause local overheating and glassware failure (the REALLY nasty kind - loud crack followed by flying shrapnel).

It can be done in a flask as a source of almost dry ammonia with heat on a hotplate... Cooling must be gradual or the flask will crack. Please don't do it in the home MW, the clouds of ammonia produced will leave stains on stainless steel.

The polyphosphoric acid product can indeed be used to prepare phosphoric acids, but the hydrolysis is slow (it is increased appreciably by the addition of iodine containing solutions - even PVP-I2 - the smell produced is distinctive).

Attachment: US5951831.Condensation.Polymerization.Phosphoric.Acid.pdf (67kB)

JohnWW
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Ammonium phosphate is sold by farm supplies stores and large department stores as a soluble N and P fertilizer under the name "Ammphos". I am not offhand certain whether the stuff is (NH4)3PO4 or (NH4)2HPO4, but it should be on the packaging. Another variant sold as a fertilizer is "Magamp", believed to be (NH4)MgPO4, but this would not produce H3PO4 or H4P2O7 or HPO3 on heating.

[Edited on 26-10-09 by JohnWW]
unome
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To make a long effort short - the rust converter version of phosphoric acid available pretty much everywhere could be neutralized with ammonia (g) to make a much less difficult to purify ammonium phosphate, also one in which there is NO chance of added sulfate/insoluble calcium/other salts. Perhaps use the fertilizer grade stuff as the source of ammonia and the polyphosphoric acid produced as a dessicant (works wonderfully as that) and make your "PURE" reagents using something you can control?
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Apparently, the product formed qualifies to be called Polyphosphoric acid.

From EP0996587, courtesy of the folks at the Vespiary.

Monoammonium Phosphate....

When monoammonium phosphate was subjected to to microwave radiation, the phosphate heated rapidly. Almost immediately after turning on the microwave oven, the monoammonium phosphate melted and began to boil. Analysis of the gas that was released, showed it to be a mixture of of both ammonia and water. Monoammonium phosphate is composed of 14.7% NH3 and 61.69% P2O5. After microwave heating for 10 minutes, the glassy material that was formed contained 4.1% NH3 and 77.2% P2O5. In heating the the monoammonium phosphate, temperatures are reached that exceed the decomposition temperature of ammonia in the presence of phosphoric acid. Thus, ammonia is released, forming phosphoric acid. This is polymerized to polyphosphoric acid. The melting point of monoammonium phospate is 190C and this temperature was reached very quickly.
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Ah! Just read the above PDF. It's the mother paper, from which my little excerpt came. Further, dehydration/condensation is possible.
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Personally, I think the P2O5 content is a little skimpy for the product to be considered true polyphosphoric acid. But, what do I know?

I'd like to try the reaction with stirring and/or vacuum, in an effort to get the rest of that ammonia out. That might boost the P2O5 content to over 80%. Which would be about equal to the content found in many commercial PPAs.

I don't know of any real problems with buying polyphosphoric acid. Though it isn't otc. But, price would be one consideration.

Nowadays, unless you are buying bulk, there is a huge screw-factor to overcome, when you are purchasing through regular chemical companies.

Spectrum's regular price for PPA is $40.00 for 100grams. Buy 2.5 kilos, and the price is about$200.00. Buy 250 lbs, about 110 Kilos, and you can get the whole shebang, for less than \$1000.00..... Less than ten bucks a kilo.

[Edited on 31-8-2010 by zed]

[Edited on 31-8-2010 by zed]

[Edited on 31-8-2010 by zed]
yobbo II
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Phosphoric acid can be purchased at farm stores for washing milking machines in Europe.

 Sciencemadness Discussion Board » Special topics » Prepublication » Orthosphosphoric Acid Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication   » References Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Whimsy   » Detritus   » The Moderators' Lounge