#polyphosphoric_acid  a series of oxyacids of phosphorus
  supertype:  #oxyacid__oxygen_acid  any acid that contains oxygen
  subtype:  #metaphosphoric_acid  a glassy solid acid ([HPO3]n) often used as a dehydrating agent
  subtype:  #pyrophosphoric_acid  a solid acid formed by reactions of orthophosphoric acid

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