Phosphorus acid

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White hygroscopic crystals that is usually marketed as a 20% aqueous solution. Phosphorus acid is a good reducing agent and a stronger acid than phosphoric acid. Phosphorous acid is primarily used as a reagent and to make phosphate salts.

Synonyms and Related Terms

orthophosphorous acid; phosphonic acid; phosphorous phlogisticated

Chemical structure

Phosphorous acid.jpg


  • Corrosive on contact. Causes burns.
  • Fisher Scientific: MSDS

Physical and Chemical Properties

Soluble in water, ethanol.

Composition H3PO3
CAS 13598-36-2
Melting Point 73 C
Density 1.65 g/ml
Molecular Weight mol. wt. = 82.0
Boiling Point 200 C (dec)

Resources and Citations

  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 7502
  • Susan E. Schur, Conservation Terminology: A review of Past & Current Nomenclature of Materials, Technology and Conservation, Spring (p.34-39); Summer (p.35-38); Fall (p.25-36), 1985