Dear Jet-Sing,

Maybe my answer is not satisfactory. In general, the polyphosporic acid can be prepared from H3PO4 by its thermal dehydratation followed with condensation.  From the above text and character of your question one may guess, that it would be simpler to melt the.S. pure and cristalline phosphoric acid in a mixture with P2O5, this might be possibly advantage also with regards of yield and purity of the product. 

Using 85 % acid may bring a disadvantage of need to evapoate lager amount of water from the reaction mixture, not to speak about a strong exothermal reaction between P2O5 and H2O in the  acid.

You may also prepare another kind of polyphosporic acid by heating NaH2PO4 up to 600 oC followed by an instant cooling of the melted salt. The product is amorphous and it is a sodium salt of polyphosporic acid.

Regards, R. S.

Thank you for your answer Rastislav.

Do you know if there is a particular temperature required if using P2O5 and crystalline H3PO4 as I have read 'high temperatures' when looking up online but no particular temperature. Or do you suggest only using P2O3 rather than P2O5? Thanks.

Dear Jet-Sing,

1/ In my previous answer I made a mistake, instead of P2O5 I wrote an incorrect formula "P2O3".

2/ In my opinion, the working temperature in your case might be 400 to 600 oC . More info about synthesis you are interested in you might obtain from any good inorganic chemistry.

Good luck,

R. S.

85%by weight solution  is good for dehydration of alcohols especially cyclodehydration.

Instead of preparing polyphosphoric acid you can buy it in high quality:

Example

http://www.sigmaaldrich.com/catalog/product/sial/208213?lang=de®ion=DE

Kind regards

Peter

I am following a prep of a similar reaction to mine which uses polyphosphoric acid of specific phosphorus content so I am more interested in preparing this myself at this time.

Thank you for your answer though.

See: http://www.innophos.com/en/products-and-markets/browse-product-categories/polyphosphoric-acid/polyphosphoric-acid/ppa-117c-ppa-118c

Another supplier:

http://www.icl-pp.com/en-us/Solutions/Industrial/Construction/Pages/Asphalt_Paving.aspx

I read somewhere where they advised to heat 85% Phosphoric acid and Phosphorus pentoxide at 200 ''C for 30 minutes:

http://reag.paperplane.io/00002328.htm.

Maybe you want to check this related RG discussion: https://www.researchgate.net/post/How_i_can_make_85_Polyphosphoric_acid_solution_from_solid_Polyphosphoric_acid