Polyphosphoric Acid (H3PO4): Properties And Uses


Polyphosphoric acid (H3PO4) is a colorless, odorless and highly viscous liquid compound with strong hydroscopic properties. It is a mixture of linear phosphoric acids and orthophosphoric acid.

Phosphoric acids with less than 95% H3PO4 (68% P2O5) contain the simple orthophosphoric acid. However, at higher concentrations, the acid consists of a mixture of ortho, pyro, tri, tetra and highly condensed phosphoric acids.  In this regard, acids with a concentration more than 68% P2O5 are often referred to as Polyphosphoric acids.

Polyphosphoric acids are miscible with water, hydrolyzing to orthophosphoric acid with the generation of heat. They are insoluble in hydrocarbons and halogenated hydrocarbons. Polyphosphoric acid is mainly used in organic synthesis for cyclization, acylation and to produce phosphate esters that are being used in various industries.

Physical And Chemical Properties

  • Polyphosphoric acid is a mixture of orthophosphoric acid and linear phosphoric acids.
  • Polyphosphoric acids are odorless, high viscousity liquids with strong hygroscopic properties.
  • Polyphosphoric acids are miscible with water, hydrolyzing to orthophosphoric acid with the generation of heat.
  • It is a strong mineral acid.
  • They are insoluble in hydrocarbons and halogenated hydrocarbons.
  • Polyphosphoric acid melts at 16oC.
  • The acid has a boiling point of 300oC at 760 mmHg
  • The acid has a density of 2.0g g/ml at 25oC
  • Molecular weight of polyphosphoric acid is 98.0
  • It is synthesized by reacting phosphoric acid with phosphorous (V) oxide.
  • Polyphosphoric acid is weaker than sulphuric acid and nitric acid and has no oxidizing power.

Reactivity Profile Of Polyphosphoric Acid

  • Polyphosphoric acid reacts with water to generate heat and phosphoric acid (orthophosphoric acid). The reaction is generally not violent/vigorous.
  • Polyphosphoric acid does not burn but may decompose upon heating to produce corrosive and/or toxic fumes.
  • Polyphosphoric acid reacts exothermically with chemical bases (amines, amides, inorganic hydroxides).
  • May generate flammable or toxic gases when it comes in contact with dithiocarbamates, isocyanates, nitriles, nitride, sulfides and strong reducing agents.
  • It reacts with or corrodes active metals such as aluminum and iron to release hydrogen gas.
  • It can initiate polymerization of certain classes of organic compounds
  • The concentration of polyphosphoric acid is represented by two methods; either it can be expressed as the concentration of P2O5% or in term of conc. Of H3PO4% i.e the content of H3PO4 in polyphosphoric acid.


Polyphosphoric acid is obtained by heating the wet process phosphoric acid. During this process, phosphoric acid forms chains and eliminate the water molecules. It contains around 72% phosphorous (as P2O5) and represents a series of phosphoric acids containing more than one atom of phosphorous.

Polyphosphoric acid can be produced by adding phosphorus pentoxide to water and then heating and condensing the mixture to yield finish product.

Uses Of Polyphosphoric Acid (H3PO4)

  • Used widely as an analytical reagent in chemical industry.
  • Powerful dehydrating agent for organic synthesis.
  • It is used in production of acid phosphates and pharmaceuticals.
  • Intermediate for production of phosphate esters with high mono-ester content.
  • Polyphosphoric acid can be used as a catalyst during synthesis of dimethyl carbonate from urea and methanol.
  • It is widely employed as acylation and alkylation reagent in various reactions
  • It can be used as a catalyst in the synthesis of various aromatic ketones.
  • It may be used to prepare silica-supported polyphosphoric acid, an easy to handle, reusable heterogeneous catalyst.
  • Polyphosphoric acid may be used in the synthesis of aromatic sulfones, N-substituted amides and 4-aminobenzophenones.
  • Used as a catalyst and absorbent of ammonia during the synthesis of dimethyl carbonate (DMC).
  • Used as a solvent in the synthesis of hyperbranched polybenzoxazoles.


Chemical NamePolyphosphoric Acid
CAS Number8017-16-1
Molecular FormulaH3PO4
Molecular Weight98.0
AppearanceClear, colorless, oily liquid, odorless, hygroscopic
Physical State At  STPViscous Liquid
Melting Point16oC
Density2.06g/ml at 25oC
Boiling Point300oC at 760 mmHg
StorageStore at room temperature (Below 30oC)
Vapor Pressure2 hPa (20oC)

Health Hazards, Storage And Handling

Health Hazards

  • May cause severe irritation of the respiratory tract with sore throat, coughing, shortness of breath and delayed lung edema. Causes chemical burns to the respiratory tract.
  • May cause severe and permanent damage to the digestive tract if ingested.


  • Store at room temperature
  • Keep container closed when not in use
  • Keep away from water and metal surfaces
  • Keep away from reach by children
  • Avoid storing in metal containers


  • Keep the container tightly closed
  • Do not get on skin or eyes
  • Do not ingest or inhale
  • Use with adequate ventilation
  • Do not allow contact with metal or water.