# Making Phosphoric(v)acid

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From what I remember from my A2 chem course, phosphoric(V)acid can be formed from phosphorus pentoxide in the reaction [ce]P2O5 + 3H2 O -> 2H3PO4[/ce].

As i've been looking for a small amount of phosphoric acid, I wondered whether this reaction could be carried out at home by simply burning red phosporus to obtain the pentoxide then reacting with water? From what I have read, industrial methods involve dissolving the pentoxide in dilute phosphoric acid not water which suggest that the reaction my be slightly violent so I'm a little wary.

Anyway do you think this would work or is there a simpler/safer method to obtain the acid?

EDIT: this should probably be in the applied chemistry section actually.

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Inorganic chemistry also is a good place for this.

Making phosphoric acid from burning red P actually does work quite well, but you should assure a very good supply of air. If insufficient air is used, then lower oxides of phosphorous are produced and you also get phosporous acid, H3PO3. The reaction is somewhat cumbersome though. Collecting the smoke and leading it through water is a lot of a hassle. Also, you should be very careful not to burn too much red P at once. It is VERY flammable and burns very violently, with a LOT of smoke. Be careful!

The reaction of P2O5 (better: P4O10) with water is very violent, but the smoke of burning red P is consisting of small particles, spread over a large volume, and then the voilent reaction is not that problematic.

But on the other hand, isn't it a pity to burn valuable red P to make such a dull compound like H3PO4? Can't you buy chalk-remover for coffee machines? Many brands simply are dilute H3PO4. Over here in the Netherlands, a common chalk remover for coffee machines simply is 8% H3PO4, the rest is water. By boiling away the water, one could make fairly pure H3PO4. Go and look at hardware stores for chalk removers, maybe the situation is the same as for us and you also can buy dilute H3PO4 somewhere.

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Why not make phosphoric acid from a phopshate and sulfuric acid?

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yeah, thats how I make mine also, I start with Calcuim hydrogen phosphate and add sulphuric then filter of the ppt and evap the excess water off.

or you can add ammonium sulphate instead leaving ammonium phosphate, then heat this to decompose it and drive off the ammonia, leaving the phosphoric acid too.

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Thanks for the replies, i'll probably attempt the phosphate and sulphuric acid way first as I partially wanted to make the acid just for the satisfaction of making it, but if I fail i'll probably just buy some. I've seen bottles of rust remover beofre which I have heard are also dilute H3PO4 . Anyway I'm assuming you can buy relatively pure phosphates as fertalizers right?

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thats where I get mine from, as Super Phosphate of lime.

although a good 90% of by mass is insoluble and sand, when filteres and evaporated you get a reasonable yield of CaH2PO4.

it only costs about a quid for 2 kilos of the stuff too.

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ah excellent, I lost my job last week so I appreciate cheap things alot more than I used to.

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for a start, how much do you want?

I have some already crystalised CaH2PO4, and would be happy to send you a few grams of it to make some.

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wow I havn't seen generosity like that in ages, that would be awsome thanks. I'm not looking to make too much acid just enough to play around with and add to my collection, so yea I would expect 1-2 grams to be enough. Cheers i'll pm you my address.

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not a problem, and youre quite welcome, its also much easier to send Crystals than liquids too, just use your sulphuric acid and watch the calcium sulphate drop out of soln, filter and you have your acid

I may also send one or 2 other items to make the journey worthwhile.

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Its like christmas come early. Thanks again thats awsome.

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• 1 month later...

What is the ratio of Trisodium Phosphate and Sulfuric Acid to make Phosphoric Acid? How cold would I have to get the a solution of Phosphoric Acid and Sodium Sulfate to get the Sodium Sulfate to precipitate?

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I buy it in drums for a little over \$12 (US) per gallon. Why make something that's so cheap to buy?

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Chems which can be purchased at one place can be very hard to find at another place. For example, where I live, we can buy diethylether easily, but I know that in many countries that is a listed chemical, which cannot easily be purchased. So, sometimes there is no other option than making it yourself.

@KFC: The route through Na3PO4 is a very messy one. The problem is that all is soluble in water and you have no nice separation. Also, separation by means of distillation is not possible with this. You will have a very hard time making reasonably pure H3PO4 from this. The route through the calcium salt is easier, because in water, the calcium sulfate separates as a precipitate.

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for the tri sodium, Ide go the HCl route as opposed to the H2SO4 the maths are much easier the result is the same and excess HCl is easily driven off, AND youre left with only 1 salt to deal with.

react with 3 parts HCl the boil the product until no further water is given off, allow the liquid to cool, then filter out the NaCl crystals from the Phosphoric acid through rockwool.

that should leave you with a reasonably Pure product.

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YT, unfortunately that does not work. I have used the opposite way, making dry HCl gas from NaCl and H3PO4. For making HBr and HI, this even is the recommended route. Add solid NaBr or NaI (or KBr or KI) to H3PO4 and heat. Gaseous HBr or HI bubbles from the acid and what remains is sodium hydrogen phosphate. This latter preparation is better than the preparation with H2SO4, because H3PO4 is non-oxidizing and you do not obtain Br2 and I2 as byproduct.

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Thats remarkable! HI is a Very strong acid.

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The key to understanding this is the concept of equilibrium. There is an equilibrium, which is very much to the left:

H3PO4 + I(-)(solvated) <<<---> H2PO4(-)(solvated) + HI(solvated)

There is another equilibrium:

HI(solvated) <----> HI(gas)

However, the gas bubbles away and is taken out of solution. So, the second equilibrium is driven to the right and most important, HI disappears from the system! Because HI disappears from the system, the first equilibrium also goes to the right.

So, the high volatility of HI (and also of HBr, HCl) make it possible to drive off such strong acids by means of a weak acid. If HI were a liquid and did not escape from the mix, then one definitely could not make HI with H3PO4.

One can even make HNO3 with KNO3 and H3PO4 and heating to over 85 C. The HNO3 then is a gas and it escapes, leaving KH2PO4 behind.

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I just received a 1litre bottle of phosphoric. 83%. I got it from a hydroponics equipment supplier. They call it "ph down", not phosphoric, which is its hydroponic use. £8 per litre. Several suppliers in the U.K. It is also used, of course, in the motor trade as rust remover & preventer

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Yep, H3PO4 is usually sold in that kind of concentration. I have a 85% bottle. At this concentration it is a somewhat oily liquid. You can easily boil out that last amount of water and even continue heating, loosing more water, such that you get pyrophosphoric acid, H4P4O7, which also is an oily liquid. Ideal stuff for making HCl and HBr at high purity. Nice if you want to make 37% HCl.

Further heating makes it glassy, giving the very hygroscopic polymeric species (HPO3)n.

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itll save me wasting good sulphuric if I can just Buy it!

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Can I use Ammonium Phosphate and boil the Ammonia off, and how can I make Ammonium Phosphate from Sodium Phosphate?

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with much hard work!

Na is Much more basic than NH3, so youde have to make Phosphoric acid in the 1st place to get to the Ammonium phosphate stage.

although this isnt as crazy as it sounds, such things (intermediaries) are used all the time in purification and other synths.

its a lot of work though!

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If I already had Ammonium Phosphate could I boil off the Ammonia off to make Phosphic acid?

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it depends, Di Ammonium phos or Mono ammonium phos?

one will work the other won`t.

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