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Author: Subject: Phosphoric Acid dry distillation
Chemgineer
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[*] posted on 1-2-2023 at 11:14
Phosphoric Acid dry distillation


Recently I carried out the below using a dry distillation.

100g of ammonium dihydrogen phosphate (MAP fertilizer) + 200g of sodium bisulphate and recovered around 35ml of phosphoric acid.

I actually mixed up my sums as that is quite and excess of bisulphate, however what surprised me is once the reaction flask had slowly cooled down and left overnight I had a single solid mas in the flask but completely crystal clear like glass!

Could this be a double salt of ammonium sulphate and sodium sulphate or is it possible to get a really clear mix of the two salts?

Anyway I think my acid yield was poor (although quite concentrated I expect) so I will probably try a wet method next time I think.

[Edited on 1-2-2023 by Chemgineer]
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1-2-2023 at 11:41
BromicAcid
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[*] posted on 1-2-2023 at 15:22


There's a thread kicking around somewhere on the forum on pyro/poly phosphoric acids. In another life I was trying to make polyphosphoric acids by heating ammonium dihydrogen phosphate dry and ended up with massively corrosive solids. Generally phosphoric acid doesn't boil, it just keeps losing more and more water. But I suppose there has to be some point where something volatile comes over as it never gets to the oxide by heat alone.



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chornedsnorkack
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[*] posted on 2-2-2023 at 02:02


Quote: Originally posted by BromicAcid  
Generally phosphoric acid doesn't boil, it just keeps losing more and more water. But I suppose there has to be some point where something volatile comes over as it never gets to the oxide by heat alone.


The azeotropic phosphoric acid boils at around 850 C, and has a composition slightly towards P2O5 from HPO3.
A link:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&...
quotes another source (an 1950 one) as giving the azeotrope around 870 C and 92,1% P2O5 (HPO3 would be 88,8%)

[Edited on 2-2-2023 by chornedsnorkack]
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[*] posted on 2-2-2023 at 03:07


I was getting a red indication for low PH to begin with, i've just checked my distillate bottle again and the final solution actually indicates a PH of around 9 and smells of ammonia! I guess this is a failed attempt.
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chornedsnorkack
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[*] posted on 2-2-2023 at 03:26


Phosphoric acids form no crystal forms between H4P2O7 and the various forms of P2O5. Just glasses. So the residue must have contained all of your phosphorus, plus freeze into the glass some water, all Na and whatever else does not distil over. What is likely is that NH3 did go over in your distillate. If your melt got hot enough, some SO3 too.
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[*] posted on 2-2-2023 at 13:09


If phosphoric acid does not boil why does literature online state it's boiling point at 158-200 deg C?
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chornedsnorkack
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[*] posted on 3-2-2023 at 00:24


Quote: Originally posted by Chemgineer  
If phosphoric acid does not boil why does literature online state it's boiling point at 158-200 deg C?


Because compositions that are not azeotropes do not boil at constant composition. Composition H3PO4 does reach total vapour pressure 1 bar and boil with bubbling, at a reproducible temperature. The matter is that the vapour at that condition is pure water.
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RustyShackleford
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[*] posted on 3-2-2023 at 06:29


Phosphoric acid polymerizes on heating, this forms the actually pretty useful polyphosphoric acid, powerful dehydrating agent useful for many organic reactions.
The reason it has a stated boiling point (or range) is because that is the temperature at which 100% phosphoric acid will start polymerizing with the loss of water.
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[*] posted on 3-2-2023 at 14:49


Some details

https://isolab.ess.washington.edu/resources/H3PO4.pdf

Page 12 (16 of pdf) Figure 2 has boiling point curve. Cuts off above 160 C short of 90%. Boiling point at 100% is elsewhere reported around 260 C

Page 19 (23 of pdf) Figure 11 has vapour pressure of acid at various temperatures and concentrations. Note the 100% acid. Even at 100 C, the pressure 3,65 mm Hg is just 15% of the 24 mm Hg vapour pressure of 0% acid (pure water) at 25 C. Since the pressure of water vapour tends to be the same in furnace and lab, then unless your lab has humidity under 15%, the H3PO4 will still absorb water from air at 100 C.
Page 20 (24 of pdf) discusses vapour composition. Small amounts of acid vapour are shown from around 300 C, but the composition is already H3PO4-H4P2O7 mix. Figure 12 reaches 800 C and 55% P2O5 in vapour, but this is short of azeotrope (870 C and 92% P2O5 in vapour)
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