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Author: Subject: Concentrating phosphoric acid
Harmless

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Concentrating phosphoric acid

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
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Evaporate it.

The single most important condition for a successful synthesis is good mixing - Nicodem
Harmless

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

For eg, at 70% concentration sulphuric acid produces heavy fumes on boiling.
zoombafu
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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
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Use vacuum distillation:

The single most important condition for a successful synthesis is good mixing - Nicodem
weiming1998
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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
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If you can find a desiccant that works with water but does not react with phosphoric acid that that might work.

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Harmless

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

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Nicodem
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 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
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 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.

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Magpie
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 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.

The single most important condition for a successful synthesis is good mixing - Nicodem
Lambda-Eyde
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 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.

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Magpie
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 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.

The single most important condition for a successful synthesis is good mixing - Nicodem
Formatik
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Phosphoric acid can also be concentrated by heating it in copper vessels.
weiming1998
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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

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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]

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weiming1998
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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
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 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?

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Nicodem
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Cross-linking to the previous thread on this topic for all those who refuse to UTFSE.
weiming1998
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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
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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
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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.
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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.

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 Sciencemadness Discussion Board » Fundamentals » Chemistry in General » Concentrating phosphoric acid Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues