Polyphosphoric acid

On my rather sketchy quest to synthesize methaqualone, the preparation I am following calls for something called polyphosphoric acid. Upon reading further into it I learned that on heating regular old orthophosphoric acid eventually polymerizes into a viscous and sticky mess that is useful for many things such as dehydration, alkylation and acylation, as well as inducing ring closure.

There seemed to be very little information on the procedure and a discouraging amount of what was there seemed to imply that glassware of any type shouldn't be used. There is an old video on YouTube of someone transferring polyphosphoric acid. There is also a very useful thread here on Sciencemadness discussing the topic. Thank you especially to Woelen for the encouragement!

Due to the seeming scarcity of available information I decided to report on my success here in the hopes that my experience might be helpful to someone else.

Nothing special about the setup. Just an electric heating mantle and 400 grams of 75% orthophosphoric acid in a 1000mL RBF. At about 254 degrees celsius the acid stopped boiling but was still letting off fumes. At this point the liquid was still clear.



Later, at about 300 degrees, the contents turned opaque from what I assume is corrosion of the borosilicate. Sigma-Aldrich states 300 centigrade as the boiling point of polyphosphoric acid and that's where the temperature decided to stop climbing. So I kept the flask at 300 for an arbitrary 30 minutes and called it a day. Despite this not a lot of product at all was lost to evaporation.



Once cooled, the phosphoric acid had indeed acquired a very thick, sticky consistency. This was particularly evident when emptying the flask into an empty Nutella jar.



I inspected the flask after thoroughly rinsing it and initially it looked pretty rough. However, it looked a lot worse than it actually was.



I heated a mild solution of potassium hydroxide in the flask with stirring for a couple of hours and the white tumors and frostiness mostly chipped away. Although considerably less opaque the glass is still visibly etched. The flask is still perfectly usable, though the quality (or lack thereof) of any stir bar used is highlighted as the porous surface is prone to decoupling ones with a weak magnet.

I didn't take a proper picture after the KOH treatment, but the added clarity is evident in this picture that was taken during a subsequent bromine synthesis:



So yes, it can be done with relative ease. And yes, borosilicate is suitable as long as any subsequent chemistry done isn't sensitive to the phosphates from corroded borosilicate. I guess.




[Edited on 15-9-2020 by Benignium]

[Edited on 16-9-2020 by Benignium]

Quote: Originally posted by njl

Interesting. What prep are you following that requires PPA?

You're very welcome, SuperOxide!

Quote: Originally posted by Benignium

SuperOxide, phosphoric acid is perfectly OTC where I live. Check out brewing supply stores. Sorry for the mistake with the links, and thanks for pointing it out. This is the thread I was talking about.

Quote: Originally posted by Benignium

On my rather sketchy quest to synthesize methaqualone, the preparation I am following calls for something called polyphosphoric acid. Upon reading further into it I learned that on heating regular old orthophosphoric acid eventually polymerizes into a viscous and sticky mess that is useful for many things such as dehydration, alkylation and acylation, as well as inducing ring closure. There seemed to be very little information on the procedure and a discouraging amount of what was there seemed to imply that glassware of any type shouldn't be used. There is an old video on YouTube of someone transferring polyphosphoric acid. There is also a very useful thread here on Sciencemadness discussing the topic. Thank you especially to Woelen for the encouragement! Due to the seeming scarcity of available information I decided to report on my success here in the hopes that my experience might be helpful to someone else. Nothing special about the setup. Just an electric heating mantle and 400 grams of 75% orthophosphoric acid in a 1000mL RBF. At about 254 degrees celsius the acid stopped boiling but was still letting off fumes. At this point the liquid was still clear. Later, at about 300 degrees, the contents turned opaque from what I assume is corrosion of the borosilicate. Sigma-Aldrich states 300 centigrade as the boiling point of polyphosphoric acid and that's where the temperature decided to stop climbing. So I kept the flask at 300 for an arbitrary 30 minutes and called it a day. Despite this not a lot of product at all was lost to evaporation. Once cooled, the phosphoric acid had indeed acquired a very thick, sticky consistency. This was particularly evident when emptying the flask into an empty Nutella jar. I inspected the flask after thoroughly rinsing it and initially it looked pretty rough. However, it looked a lot worse than it actually was. I heated a mild solution of potassium hydroxide in the flask with stirring for a couple of hours and the white tumors and frostiness mostly chipped away. Although considerably less opaque the glass is still visibly etched. The flask is still perfectly usable, though the quality (or lack thereof) of any stir bar used is highlighted as the porous surface is prone to decoupling ones with a weak magnet. I didn't take a proper picture after the KOH treatment, but the added clarity is evident in this picture that was taken during a subsequent bromine synthesis: So yes, it can be done with relative ease. And yes, borosilicate is suitable as long as any subsequent chemistry done isn't sensitive to the phosphates from corroded borosilicate. I guess. [Edited on 15-9-2020 by Benignium] [Edited on 16-9-2020 by Benignium]

Quote: Originally posted by draculic acid69

Should you repeat your experiment please include this information as equipment degradation is also a factor in chemistry that is so often overlooked.

I cheated :-(

One of the goals of the project I plan on using this for was using only chemicals from the hardware store and are totally legal and easy to get locally.

Obviously this is cheating (in regards to the local/availability one, this is totally legal and not a monitored chemical). But I got this for just a little more than what I would spend replacing the flask that got etched away making this, and it's way more pure.



P.S. Don't let the size of the bottle fool you (as it did me)... 500g of PPA is actually only like 1/3rd of that bottle volume, lol.




[Edited on 1-2-2021 by SuperOxide]

Quote: Originally posted by rockyit98

if you have P2O5 aka P4O10 adding it to H3PO4 will turn it in to Polyphosphoric acid.

Quote: Originally posted by Benignium

Nice purchase, SuperOxide! I'd like to point out that one flask can definitely be used for multiple batches, so I think it is economically very viable. Granted, it is not as pure as industrially made material but the silicon phosphate particles do sink to the bottom over several months, increasing purity and leaving the acid less opaque.

aprox 125ml of 85% orthophosphoric acid was heated on the maximum setting with a heating mantle.
The acid started boiling around 130C and vigorously boiled untill it reached a temperature of around 250C (from room temperature to this took about 1h), after which the bubbling became calmer.
After hitting 300C the heating mantle was adjusted to keep around that temp (max it hit was 307C) for 30min.
I let it cool to 60C and poured it into a small jar for temporary storage.
I determined the warm fluids (40C) density to be 1.965 g/cm3. (because using a volumetric pipette with the room temperature fluid would be impossible since its too thick)
This density is equal to 109% H3PO4, where as commercial polyphosphoric is 115%. I may reheat this in order to achieve the desired density.


For me the etching of the flask was very very minimal. Its currently drying after being washed with hydroxide, but during the washing i could not see any damage to the flask at all. The thick fluid product was completely clear too, no haze as OP had.

Im unsure if the product i produced contains enough polyphosphoric acid to be used as a reagent, im planning to test it by reaction of phenol and benzoic acid. If any of you know of a better test, please let me know.










[Edited on 22-2-2021 by RustyShackleford]

Quote: Originally posted by karlos³

I liked the P2O5/85% H3PO4 reaction to make polyphosphoric acid. Seems like the best way to produce a reliably functioning PPA for whatever reaction it is intended for.

Quote: Originally posted by draculic acid69

Rustyshackleford is that a Friedel craft rxn U r talking about with phenol& benzoic acid and if so Is benzophenone the resulting product? Also does anyone think that anhydrous hbr& HI gas could be made from a room temperature Mixing of polyphos acid and alkali halides?