Sciencemadness Discussion Board

Concentrating phosphoric acid

madmanhere - 6-3-2012 at 17:23

Hi,
Is there a way to concentrate phosphoric acid in home/lab conditions?

The lab supplier had two types of phosphoric acid:
1) unlabelled acid with no information on conc, costing just $2/ 500ml
2) labelled one from a reputed manufacturer, conc 88%, $9/500ml

It's no longer an issue as I bought the latter, still it'd be good to keep myself informed. Searched on Google, couldn't find anything useful.

Thanks.

Magpie - 6-3-2012 at 17:27

Evaporate it.

madmanhere - 6-3-2012 at 17:43

At what stage should one stop the evaporation? Any indicators?

For eg, at 70% concentration sulphuric acid produces heavy fumes on boiling.

zoombafu - 6-3-2012 at 18:30

You'll just have to do a titration. Preform a titration on the initial unknown to determine the concentration. Then from there you can work out the volume of water that needs to be evaporated to achieve the concentration you want.

Magpie - 6-3-2012 at 19:00

Use vacuum distillation:

http://www.sciencemadness.org/talk/viewthread.php?tid=2923&a...

weiming1998 - 7-3-2012 at 01:39

Evaporate it under the sun. After three days, take it inside in a cool place for a few hours. If you get a solid, then it is phosphoric acid hemihydrate, which melts at 29 degrees celsius. It is virtually impossible to make anhydrous phosphoric acid by drying, though, because it easily decomposes at high temperatures (just over 100 degrees) to pyrophosphoric acid, so it is difficult to drive the water off. You won't need anhydrous phosphoric acid in most applications anyway.

disulfideprotein - 7-3-2012 at 02:30

If you can find a desiccant that works with water but does not react with phosphoric acid that that might work.

madmanhere - 7-3-2012 at 02:57

Cool. Will try it out someday. :)

Talking about the hemihydrate, last week a few drops of conc phosphoric acid fell on the cement floor. Couple of hours later it solidified and became white in colour. Now regardless of what I do, the stain doesn't go away. :(

weiming1998 - 7-3-2012 at 03:30

Quote: Originally posted by madmanhere  
Cool. Will try it out someday. :)

Talking about the hemihydrate, last week a few drops of conc phosphoric acid fell on the cement floor. Couple of hours later it solidified and became white in colour. Now regardless of what I do, the stain doesn't go away. :(


As cement is Ca(OH)2 combined with other substances, I would bet that the white substance is calcium phosphate. To clean that, I would try some HCl to etch the layer of white away and react with the calcium phosphate if you aren't too concerned about your concrete being slightly etched and imperfect.

[Edited on 7-3-2012 by weiming1998]

Adas - 7-3-2012 at 08:55

I wouldn't pour HCl on that - it will also make the cement white.

Nicodem - 7-3-2012 at 09:59

Quote: Originally posted by weiming1998  
Evaporate it under the sun. After three days, take it inside in a cool place for a few hours. If you get a solid, then it is phosphoric acid hemihydrate, which melts at 29 degrees celsius.

Seriously? You can make concentrated phosphoric acid by just letting it evaporate at ambient temperatures? Do you have a reference for this?

Lambda-Eyde - 7-3-2012 at 10:22

Quote: Originally posted by Nicodem  

Seriously? You can make concentrated phosphoric acid by just letting it evaporate at ambient temperatures? Do you have a reference for this?

I once used a small amount of 15% phosphoric acid to clean a porcelain mortar which I had used to grind rust. After letting it stand for a few weeks, the volume had decreased considerably. I didn't try to measure the concentration, I just threw it away. I don't know if this proves anything, though.

Magpie: I'm very interested in your vacuum distillation process. I was under the impression that phosphoric acid couldn't be distilled. Could you provide some more details? Did you etch your glassware? This is IMO a potential topic for Prepublication.

Magpie - 7-3-2012 at 10:45

Quote: Originally posted by Lambda-Eyde  

Magpie: I'm very interested in your vacuum distillation process. I was under the impression that phosphoric acid couldn't be distilled. Could you provide some more details? Did you etch your glassware?


The phosphoric acid itself is not being distilled, but stays in the pot. This is really just a vacuum evaporation in a distillation apparatus. The ebulliator is needed to control bumping.

I don't recall any etching. I think I would have mentioned it if my good Kontes RBF would have been harmed.

If you have any more questions just ask.

Lambda-Eyde - 7-3-2012 at 11:40

Quote: Originally posted by Magpie  

The phosphoric acid itself is not being distilled, but stays in the pot. This is really just a vacuum evaporation in a distillation apparatus. The ebulliator is needed to control bumping.

I don't recall any etching. I think I would have mentioned it if my good Kontes RBF would have been harmed.

If you have any more questions just ask.

I see. When you said "Use vacuum distillation" I misunderstood. The acid you distilled, was that just a dilute solution, or was it prepared from a phosphate? Hot, concentrated phosphoric acid will etch glass - I'm glad to hear your Kontes flask was untouched. ;)

Magpie - 7-3-2012 at 11:57

Quote: Originally posted by Lambda-Eyde  
The acid you distilled, was that just a dilute solution, or was it prepared from a phosphate? Hot, concentrated phosphoric acid will etch glass -


If you go to the thread using the link I posted above I think you will find the answers to these questions. But, if not, feel free to ask.

Probably the key to no etching is the low temperature allowed by the use of vacuum.

Formatik - 7-3-2012 at 12:13

Phosphoric acid can also be concentrated by heating it in copper vessels.

weiming1998 - 8-3-2012 at 00:57

Quote: Originally posted by Nicodem  
Quote: Originally posted by weiming1998  
Evaporate it under the sun. After three days, take it inside in a cool place for a few hours. If you get a solid, then it is phosphoric acid hemihydrate, which melts at 29 degrees celsius.

Seriously? You can make concentrated phosphoric acid by just letting it evaporate at ambient temperatures? Do you have a reference for this?


References? Phosphoric acid is much less volatile than water right? And it should not form azeotropes with water at low concentrations(even though I can find no data about the azeotrope of phosphoric acid and water).
Phosphoric acid does not bond to water chemically (like a hydrate, or a dessicant such as P2O5), so adding all this up, why can't it be evaporated to the hemihydrate, or if an azeotrope exists, to the azeotrope?

Edit: What I meant is that phosphoric acid does not bond with water chemically above the hemihydrate concentration.

[Edited on 8-3-2012 by weiming1998]

woelen - 8-3-2012 at 01:08

Phosphoric acid cannot be simply evaporated until you get a hemihydrate. It is very hygroscopic and if you leave dilute phosphoric acid in contact with air it will concentrate somewhat but you will keep quite some water in the phosphoric acid. The vapor pressure of the water will become very low, I think even well before a concentration of 80% H3PO4 is reached.

If you really keep on heating to drive off all water, then the acid starts to polymerize and you end up with a thick syruppy mass which eventually forms a very sticky nasty soft glassy mass. First you get pyrophosphoric acid, H4P2O7 and on further heating you finally get something like (HPO3)n. All this happens at temperatures far above 100 C and you should not try to do this in glass. Glass is attacked by the hot (pyro)phosphoric acid and simply dissolves in the glassy stuff formed at higher temperatures. I'm not sure which metal withstands the pyrophosphoric acid, probably you need a silver crucible.

If you add water again to the cooled down (HPO3)n or H4P2O7, then there is not a quick formation of H3PO4. The reaction between these polymerized acids and water is very slow and it takes days or even weeks before you get H3PO4 again.


For the OP: If you have very dilute phosphoric acid then by boiling it down you certainly can concentrate it quite a lot, but you will not get pure H3PO4 or a low hydrate. I think that you can get something like 50%, maybe even 60% or 70% but I would not go further if you value your glassware. Just boil the liquid and have a thermometer in it. If the temperature of the boiling liquid rises a few degrees above 100 C, then stop heating. At that point less and less water will be simply boiled away and more and more water will come from polymerization of the acid and then you also start destroying your glassware.

[Edited on 8-3-12 by woelen]

weiming1998 - 8-3-2012 at 02:14

A silver can is probably not needed. An aluminum can will probably work because the oxide layer+ the passivation of the metal (AlPO4 is extremely insoluble). But to prevent the decomposition of the H3PO4, an oil bath (high-boiling paraffin oil)+ carefully controlled temperature is probably needed. Of course, if a brass or copper vessel is found, you can simply use that.

If the aluminum can will be able to boil the phosphoric acid, then it would be a better method than to evaporate, but I don't have any phosphoric acid around right now, so I can't try that.

bbartlog - 8-3-2012 at 07:59

Quote: Originally posted by weiming1998  
An aluminum can will probably work


The chemical compatibility charts I find online don't make me optimistic about that at all. Do you have any other ill-informed speculation you'd like to grace us with?

Nicodem - 8-3-2012 at 10:29

Cross-linking to the previous thread on this topic for all those who refuse to UTFSE.

weiming1998 - 9-3-2012 at 01:30

Quote: Originally posted by bbartlog  
Quote: Originally posted by weiming1998  
An aluminum can will probably work


The chemical compatibility charts I find online don't make me optimistic about that at all. Do you have any other ill-informed speculation you'd like to grace us with?


I said probably, not certainly. The reason that I said probably is because aluminum has a protective oxide layer that protects it from corrosion+a passivating aluminum phosphate layer that stops further corrosion.

If you looked at MSDS sheets and came to the conclusion, then the sheet also says that phosphoric acid is extremely corrosive to copper (http://www.sciencelab.com/msds.php?msdsId=9927393)
If phosphoric acid can really corrode copper, then it is an oxidizing acid. And oxidizing acids have a similar effect on aluminum as nitric acid on it. The acid simply cannot get past the oxide.

If phosphoric acid is not an oxidizing acid, then I seriously doubt the validity of this MSDS sheet.

Formatik - 9-3-2012 at 02:56

I've heated H3PO4 in a copper dish and attack was minimal to negligble. Similar experiences by other members have been seen here when dehydrating the acid (like in the thread linked to above). The older chemists used to evaporate dilute H3PO4 in lead pans. Sciencelab has a big hate thread on this forum (take a look at it), I would not expect anything better out of their MSDS.

weiming1998 - 9-3-2012 at 03:41

Yes, I do not think their MSDS is accurate. I have read a few more MSDS for phosphoric acid, and none of them mentioned compatibility with copper or aluminum. If I could get cheaper phosphoric acid (the pure, 85% one is sold as some sort of soil pH regulator for about $10/250ml), then I would buy it. As I don't have much money, I don't want to buy that. Perhaps I could try browsing bottles of rust remover, then heat the acid in an aluminum can and see how that works.

AJKOER - 14-3-2012 at 09:43

OK, so far the effort to concentrate Phosphoric acid have focused on removing water, which to a limited degree is apparently successful. However, one may want to consider Phosphorus pentoxide (actually with the molecular formula P4O10), as it is the acid anhydride of phosphoric acid and a powerful desiccant:

P4O10 + 6 H2O = 4 H3PO4

So, one mole of P4O10 (a white crystalline solid) consumes 6 moles of water forming 4 moles of Phosphoric acid.

violet sin - 26-9-2012 at 01:14

so, I searched first. and apologies if this is a repeat for some one.
that being said, I am also interested in phosphoric acid. as I have none in stock and I never really liked purchasing chems over the net. most of the OTC stuff I find isn't so clean.

1) so is there any way to start w/ TSP(Na3PO4) or bonemeal as my phosphates and end up with a decent clean acid? I had remembered something about bone meal or phosphate rock can be leached with con. H2SO4 yielding basically gypsum(CaSO4) and a mobile phase of concentrated H3PO4. I brought up the TSP b/c it looks like nice clean crystal, compared to crushed bones or rock. I could sacrifice some of my purified drain cleaner w/o feeling too bad.

2) also does a home based guy have any prospect for making P2O5 for dessication purposes? I am not trying to make WP with any of these chems so no worries. I read that it was good for dessication but once hydrated it wasn't easy to regenerate w/o decomp? guess having nice fresh H3PO4 outta the moisture might not be the worst thing in the world. just I like my dessicators to be regenerative. using CaCl2 mainly.


and realize I am not trying to reinvent the wheel here, just have limited reagents for sure. If it doesn't work I will wait till I can afford to buy some.

zed - 26-9-2012 at 12:13

Mono-Ammonium Phosphate may be converted to Phosphoric Acid (or PPA) via Microwaving under Vacuum.

Repeated dehydration Cycles produce PPA (PolyPhosphoricAcid).

I don't know if P2O5 can be achieved by ordinary methods.

Somewhere in the Science Madness Archives, there are detailed discussions on the matter. Search.

violet sin - 26-9-2012 at 19:47

Ya found a book to read,.. yay. and came across a quick easy one just like I had mentioned for the acid. though I am pretty sure I won't be making the P2O5 my self. mainly b/c it revolves around making P4 first, then burning it. no thanks.

I appreciate the info zed, though I am not sure I would know where to get the (NH4)H2PO4 in the first place. the decomp is low temp and vacuum isn't hard to manage, which is nice. my 12th merck said it's for a leavening agent. though I already have ~10#'s bonemeal for my garden. only $1.25/# here and I have acid. think I will take that route.

ya I'm new to posting here. been reading for over a month, almost every day. and as it turns out I had not noticed the "next page" button in lower right on the search engine so I didn't even get to look at all the threads that had pertaining info.. sorry for the quick draw on that last post.

azeotropic distillation

TheVoid - 5-1-2018 at 23:52

sorry for reviving such an old thread but has anyone tried azeotropic distillation? why wouldnt P-xylene work?

Magpie - 6-1-2018 at 17:20

See "Phosphoric acid from calcium phosphate" in Prepublication.

Duff - 11-5-2020 at 19:12

Obligatory apology for necro.

The attached patent claims that distillation of a mixture of phosphoric and acetic acid will result in a phosphoric acid residue with less than 1% by weight of water. Can anyone comment on the veracity of these claims? It might work since the low boiling point of acetic acid allows it to vaporize and exit the mixture when heated to 100-110 C, pulling water molecules along with it.

On a side note it is very difficult to find primary source data on the temperatures at which phosphoric acid forms various concentrations of metaphosphoric acid. I've seen claims ranging from 100-300 C for when phosphoric acid begins to form metaphosphoric acid when heated. I would love to see some primary source data.

Attachment: US3284171.pdf (346kB)
This file has been downloaded 332 times

[Edited on 12-5-2020 by Duff]

Mateo_swe - 24-5-2020 at 10:33

Earlier in this thread is was said very concentrated phosphoric acid dissolves glass.
I find it strange that this isnt mentioned in the patent where they claim to get 99% phosphoric acid.
Or is this only a problem when one have long time contact between glass and very high concentrations of phosphoric acid?

Fery - 24-5-2020 at 23:47

Mateo_swe is right, boroglass 3.3 is not resistant against hot conc H3PO4
that's why Magpie suggested vacuum distillation (to avoid hot acid)
sorry this is in my language, you need a translator https://www.verkon.cz/vlastnosti-skla/
here something in eng:
https://www.duran-group.com/en/ueber-duran/duran-eigenschaft...
DURAN® properties
Only hydrofluoric acid, concentrated phosphoric acid and strong alkali cause appreciable surface removal of the glass (glass corrosion) at elevated temperatures (>100 °C).

Master Triangle - 11-2-2023 at 02:19

I just found a paper that gives some nice relevant details.
It's got charts of:
>Molecular composition of phosphoric acids over concentration
>H2O vapor pressure over concentration with isotherms
>Bubble point temperature isobars over concentration.
(Also conductivity and viscosity)
Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers:
https://link.springer.com/chapter/10.1007/978-3-319-17082-4_...

Also here's a paper that details the freezing of phosphoric acids into hydrated and pure crystalline solids:
https://pubs.acs.org/doi/pdf/10.1021/ie50190a031


Concentrations up to 95% will not equilibrate to have any appreciable amount of pyrophosphoric acid and the freezing point is depressed to a local minima of 24C.
This seems to be a good concentration to stop at for most purposes, it's about the limit before you start getting some pyrophosphoric acid, and it is easy to keep it all liquid (the freezing point increases rapidly past here).
The best way to measure concentration as you boil it down would be with a hydrometer, but you could probably just work out your starting concentration and then work out water removed by weighing occasionally.

The hydrated solid (2:1 H3PO4:H2O) equates to a 91.5% w/w solution and begins to crystallise out at 29C with a seed crystal.
It seems that many concentrations will easily supercool by a fair bit, putting a little bit in the freezer to get seed crystals should work, and apparently the crystallisation is very rapid when supercooled and seeded. "The fused anhydrous acid and a solution corresponding to the hydrate were frequently cooled to almost 0 C. before spontaneous crystallization occurred and both may be kept in a closed vessel indefinitely without crystallization."

Edit: How do I get an account for the SM wiki? I want to add some of this to the phosphoric acid page.

[Edited on 11-2-2023 by Master Triangle]

H3PO4 sulubility.jpg - 766kB

chornedsnorkack - 11-2-2023 at 03:46

Quote: Originally posted by Master Triangle  
I just found a paper that gives some nice relevant details.
It's got charts of:
>Molecular composition of phosphoric acids over concentration
>H2O vapor pressure over concentration with isotherms
>Bubble point temperature isobars over concentration.
(Also conductivity and viscosity)
Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers:
https://link.springer.com/chapter/10.1007/978-3-319-17082-4_...

Unfortunately it is paywalled.
Some links that did open:
https://cdnsciencepub.com/doi/pdf/10.1139/v56-102
quotes the endpoint of concentrating, the azeotrope, as 92,1% P2O5 (HPO3 is 88,8%) and boiling at 869 C.
https://isolab.ess.washington.edu/resources/H3PO4.pdf
Vapour composition vs. temperature, in Figure 12 on Page 20 (24 of pdf).
Only up to 800 C, not all the way to azeotrope, and without accompanying liquid concentration.
Figure 14 (page 22, 26 of pdf) gives viscosity up to 118 % phosphoric acid. Unfortunately it gives only the liquid temperatures and does not mark gel point and glass transition temperatures, nor extend to azeotrope (127%)

Amos - 11-2-2023 at 06:46

Quote: Originally posted by chornedsnorkack  

Unfortunately it is paywalled.


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