[Nobodys]

Hello guys and girls,
I want to synthesize poly-phosphoric acid and use it later on for a reaction(cyclization). I didn't found much on articles at the internet. But at the book " Encyclopedia of reagents for organic synthesis" i found a synthesis that says the following :"Preparative Methods: by mixing x mL of Phosphoric Acid (85%, d 1.7 g mL-1) with 2.2 x g of
Phosphorus(V) Oxide (P2O5) followed by heating to 200 °C for 30 min." Has anyone tried this before? If so, how did it turn out?
(Also, i don't want to buy the reagent online)

Thank you in advance.

[pgk]

It is better to buy the reagent because:
1). Heating up to 200oC and maintaining that temperature until the end of the reaction, is highly energy consumptive and costly.
2). There are safety issues because the reaction of water (H3PO4 solvent and liberated) with the excess of P2O5 is highly exothermic and may dramatically increase the already high temperature. Besides, P2O5 tends to sublime at such temperatures.
3). Then, you have to purify the prepared poly-phosphoric and acid and remove the unreacted excess of P2O5.

[Nobodys]

I need poly-phosphoric acid for an undergraduated lab course, in which we cannot buy the reagents and use only what's in lab. What if i mix P2O5 and phosphoric acid, wait some time to let P2O5 react with water and then raise the temperature to 200 C?

[pgk]

1). How will you heat to 200oC?
Heating up to such temperatures demands specific equipment (e.g. sand bath).
2). How will you protect yourself from P2O5 (sublimed) vapors?

[Nobodys]

For the heating i can use oil bath and for the sublime i can use crystalline phosphoric acid, so the P2O5 will not react with water exothermically. 

[pgk]

1). Ordinary oil bath is silicon oil, which thermally degrades above 160-170oC forming irritant and toxic vapors.
2). Water is liberated during the polymerization of H3PO4 and continues to react with the excess of P2O5.

[Nobodys]

I understand your points, and i saw oil baths getting ruined after a high temperature heat. But using fume hood(which is a must in every reaction) can solve those safety problems right?

[pgk]

There are silicon oils that are thermally resistant up to 300-350oC but they are expensive and offered for industrial use, only.
Not 100%. Anyway, remember to wear safety glasses and dust mask, when you open the protective screen of the fume hood.

[Nobodys]

Okay,
thank you very much for your time.

[OrganicDan96]

are you sure they haven't got polyphosphoric acid or is there another way to do the reaction? seems a bit of an odd task to get you to make such a chemical. how do you plan to purify it?

[Nobodys]

Polyphosphoric acid is used as an alternative for PCL5 (PCL5 when used as a condensation agent, it produces HCL). Also poly-phosphoric acid is created with the right analogue of phosphoric acid and P2O5 in the solution. So i believe there will be no excess P2O5 in the reaction, cause in reality they are not exactly reacting with each other.