text: Two or more orthophosphoric acid molecules can be joined by condensation into larger molecules by elimination of water. Condensation of a few units yields the oligophosphoric acids, while larger molecules are called polyphosphoric acids. (However, the distinction between the two terms is not well defined.)
  For example, pyrophosphoric, triphosphoric and tetraphosphoric acids can be obtained by the reactions
  The "backbone" of a polyphosphoric acid molecule is a chain of alternating P and O atoms.  Each extra orthophosphoric unit that is condensed adds 1 extra H (hydrogen) atom, 1 extra P (phosphorus) atom, and 3 extra O (oxygen) atoms. The general formula of a polyphosphoric acid is  or .
  Polyphosphoric acids are used in organic synthesis for cyclizations and acylations; an alternative is Eaton's reagent.