Hypophosphorous acid (H3PO2)

Structure and chemical properties

The molecule contains phosphorus in a relatively low positive oxidation state (+1). Structurally it features P–H bonds as well as P–O functionality, which underlies both its single acidic proton and its reducing character. Typical chemical properties include the ability to form hypophosphite salts upon neutralization and to act as a mild to moderate reducing agent in aqueous and organic reactions. For basic chemical data and physical constants see chemical data.

Preparation and reactions

Industrial and laboratory preparations generally derive from the controlled oxidation or hydrolysis of elemental phosphorus, or from the acidification of hypophosphite salts. H3PO2 can be oxidized further to higher phosphorus oxoacids, and it can reduce many inorganic and organic substrates. Many reactions exploit the P–H bond, and the hypophosphite ion is commonly used where gentle reducing conditions are required. Additional preparative details are available at preparation notes.

Uses and applications

Hypophosphorous acid and its salts have practical roles in several industries. The hypophosphite salts (for example, sodium hypophosphite) are central to electroless nickel plating processes, acting as the reducing agent that deposits metal without an external current. H3PO2 is also a reagent and intermediate in certain organic syntheses and in the production of other phosphorus compounds. Typical applications and methods are summarized in industry and technical references: applications overview.

Safety, handling and distinctions

Solutions of hypophosphorous acid are corrosive and must be handled with appropriate protective equipment. On decomposition or in reactions with strong reducing agents or active metals, phosphorus-containing gases such as phosphine may be generated; such gases are toxic and flammable, so good ventilation and controls are necessary. H3PO2 is distinct from phosphorous acid (H3PO3, diprotic) and phosphoric acid (H3PO4, triprotic) by its single acidic proton and its characteristic P–H bonds. For safety guidance consult material safety resources: safety information.

• Key anion: hypophosphite (H2PO2-)
• Main traits: monoprotic, reducing agent
• Common use: electroless metal deposition