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Author: Subject: Make Phosphoric acid
reactofurnace
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[*] posted on 3-4-2020 at 22:46
Make Phosphoric acid


Is it feasible to prepare phosphoric acid (H3PO4) by a double displacement reaction of potassium dihydrogen phosphate (KH2PO4) and Conc. Hydrochloric acid? I think distilling over the water (from research it doesn't seem to have an azeotrope), and decanting the crude phosphoric acid.

KH2PO4 + HCl ---> H3PO4 + KCl
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artemov
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[*] posted on 3-4-2020 at 23:16


I was thinking about this too, use sodium/potassium dihydrogen phosphate instead of phosphate since the former is just 1 hydrogen away ...

If I use sulfuric acid instead of hydrochloric acid, boil some water away and
crystallise out the sodium/potassium sulfate at low temp, I might be able get fairly concentrated phosphoric acid with some impurity.
Haven't try it out yet tho.

Or would monocalcium phosphate be better since calcium sulfate is practically insoluble?

Phosphoric acid can't be distilled I think.

[Edited on 4-4-2020 by artemov]
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[*] posted on 4-4-2020 at 02:04


IIRC phosphoric acid decomposes before boiling and that is quite high temp over 300C.
I have made phosphoric acid once via a similar method. I can't remember whether I used calcium phosphate or iron phosphate. I put a layer in the bottom of a plastic container along with a beaker of concentrated HCl. Then I sealed the container and left it for several months. No idea of its purity but I ended up with a puddle of some 40mL of pale yellow, concentrated phosphoric acid. It was one of my forst successes in my lab before I had anything much in the way of equipment.
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[*] posted on 4-4-2020 at 03:30


It is almost certainly possible to make HCl by heating KCl with H3PO4
(by analogy with one of the common ways to produce HI).

The fact that the reaction goes in that direction tends to suggest that it won't easily go in the other.
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reactofurnace
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smile.gif posted on 4-4-2020 at 05:18
Update


(test run)
Adding an arbitrary quantity of KH2PO4 to conc. HCL gave this white precipitate which I presume is KCl. I Will update further on how it goes an make a detailed post on the process.




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reactofurnace
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[*] posted on 4-4-2020 at 05:22


Quote: Originally posted by j_sum1  
IIRC phosphoric acid decomposes before boiling and that is quite high temp over 300C.
I have made phosphoric acid once via a similar method. I can't remember whether I used calcium phosphate or iron phosphate. I put a layer in the bottom of a plastic container along with a beaker of concentrated HCl. Then I sealed the container and left it for several months. No idea of its purity but I ended up with a puddle of some 40mL of pale yellow, concentrated phosphoric acid. It was one of my forst successes in my lab before I had anything much in the way of equipment.

The yellow may have been caused by iron(II) perhaps. Don't have a vacuum set up so I might need to use the crude acid formed. Hopefully evaporating the water would precipitates all the KCl.
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S.C. Wack
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[*] posted on 4-4-2020 at 10:51


There were glass etching issues for me from trisodium phosphate which I think may involve fluoride contamination instead of H3PO4. Excess conc. HCl was essential to make NaCl insoluble.



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j_sum1
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[*] posted on 4-4-2020 at 11:14


Quote: Originally posted by reactofurnace  
Quote: Originally posted by j_sum1  
IIRC phosphoric acid decomposes before boiling and that is quite high temp over 300C.
I have made phosphoric acid once via a similar method. I can't remember whether I used calcium phosphate or iron phosphate. I put a layer in the bottom of a plastic container along with a beaker of concentrated HCl. Then I sealed the container and left it for several months. No idea of its purity but I ended up with a puddle of some 40mL of pale yellow, concentrated phosphoric acid. It was one of my forst successes in my lab before I had anything much in the way of equipment.

The yellow may have been caused by iron(II) perhaps. Don't have a vacuum set up so I might need to use the crude acid formed. Hopefully evaporating the water would precipitates all the KCl.

I can't remember which phosphate I ended up using. I do remember taking a lot of time to make the decision.
There is potential for all kinds of contaminants from the rragents used and so yellow is not surprising. I still chalk it up as a success. Only actually used it once and that was for some molybdenum blue. So, only a miniscule amount needed.
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clearly_not_atara
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[*] posted on 4-4-2020 at 11:48


Ammonium phosphates decompose to release ammonia, leaving phosphoric acid, at around 200 C. This seems like a particularly easy prep yielding phosphoric acid free of metal ions, which may otherwise be difficult to remove. I don't share the optimism that KCl would be nearly insoluble in concentrated phosphoric acid.

If you must start with potassium phosphates, I think potassium can be mostly removed by adding a concentrated solution of ammonium sulfate, which will precipitate relatively insoluble potassium sulfate. With K2SO4 removed, the most insoluble component of the solution is now NH4PO4H2, which should produce crystals upon evaporation.

Monoammonium phosphate exhibits a strong temperature dependence of water solubility so it should not be too difficult to recrystallize.




[Edited on 04-20-1969 by clearly_not_atara]
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