Quote: Originally posted by SWIM  |
When phosphoric acid decomposes from heat, it is giving off water and becoming an even more powerful drying agent. It could be that the decomposition
of the phosphoric acid is vital, or at least helpful, to the process.
Idle speculation below...
Might even be that the decomposition of the phosphoric acid is more important than the high temperature itself, in which case making polyphosphoric
acid first to use in the reaction instead of manufacturing it in situ might allow somewhat lower reaction temperatures.
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@ SWIM........... Thank you for that, I didn't even realize that decomposition could be a good thing.......... Duh..........!!!
So upon looking deeper.............
"Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a mineral (inorganic) acid,
having the chemical formula H3PO4.
Orthophosphoric acid refers to phosphoric acid, which is the IUPAC name for this compound.
The prefix ortho- is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids.
Orthophosphoric acid is a non-toxic acid, which, when pure, is a solid at room temperature and pressure.
Orthophosphoric acid molecules can combine with themselves to form a variety of compounds, which are also referred
to as phosphoric acids, but in a more general way.
Anhydrous phosphoric acid, a white low-melting solid, is obtained by dehydration of 85% phosphoric acid by heating under
a vacuum.[Ref. 8]"
("Orthophosphoric Acid" in Handbook of Preparative Inorganic Chemistry, 2nd ed., G. Brauer (ed.) - Check the forum library
"Upon heating orthophosphoric acid, condensation of the phosphoric units can be induced by driving off the water formed
from condensation.
When one molecule of water has been removed for each two molecules of phosphoric acid, the result is pyrophosphoric acid
(H4P2O7). When an average of one molecule of water per phosphoric unit has been driven off, the resulting substance is a
glassy solid having an empirical formula of HPO3 and is called metaphosphoric acid.[10]
Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or
moisture-absorbing reagent.
Further dehydrating is very difficult and can be accomplished only by means of an extremely strong desiccant
(and not by heating alone). It produces phosphoric anhydride (phosphorus pentoxide), which has an empirical formula P2O5,
although an actual molecule has a chemical formula of P4O10.
Phosphoric anhydride is a solid, which is very strongly moisture-absorbing and is used as a desiccant."
Taken from - https://en.wikipedia.org/wiki/Phosphoric_acid
"Polyphosphoric acids
Polyphosphoric acid
When two orthophosphoric acid molecules are condensed into one molecule, pyrophosphoric acid (H4P2O7) is obtained as
follows:
2 H3PO4 ? H4P2O7 + H2O
The chemical structure of pyrophosphoric acid is also shown in the illustration.
Three orthophosphoric acid molecules can condense in a row to obtain tripolyphosphoric acid (H5P3O10), which is also
shown in the illustration. This condensation process can continue with additional orthophosphoric acid units to obtain
tetrapolyphosphoric acid (H6P4O13, pictured) and so on.
Polyphosphoric acids are used in organic synthesis for cyclizations and acylations."
Taken from - https://en.wikipedia.org/wiki/Phosphoric_acids_and_phosphate...
Excuse the wikipedia copy and paste but it gives plenty of back ground.
I am finding conflicting information on the thermal decomposition of H3PO4........... The general consensus seems that this happens at 213degrees C, -
"Decomposition products: converted to pyrophosphoric acid (H4P2O7) when heated to 213 deg C"
Source - https://pubchem.ncbi.nlm.nih.gov/compound/Phosphoric_acid#se...
but I am finding figures ranging from 150degrees C - https://chemiday.com/en/reaction/3-1-0-9596 - up to 225degress C - https://web.archive.org/web/20070403074509/http://www.bartle...
Back to the books.......... Over and out
/CJ
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