Hypophosphoric acid is a

mineral acid A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water. Cha ...

with the formula H₄P₂O₆, with phosphorus in a formal

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of +4. In the solid state it is present as the dihydrate, H₄P₂O₆·2H₂O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond. Isohypophosphoric acid is a

structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a chemical compound, compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct chemical bond, b ...

of hypophosphoric acid in which one phosphorus has a hydrogen directedly bonded to it and that phosphorus atom is linked to the other one by an oxygen bridge to give a

phosphorous acid Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

mixed

anhydride An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

. The two phosphorus atoms are in the +3 and +5 oxidation states, respectively.

Preparation and reactions

Hypophosphoric acid can be prepared by the reaction of red phosphorus with

sodium chlorite Sodium chlorite (NaClO2) is a chemical compound used in the manufacturing of paper and as a disinfectant. Use The main application of sodium chlorite is the generation of chlorine dioxide for bleaching and stripping of textiles, pulp, and pa ...

at room temperature.Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, :2 P + 2 NaClO₂ + 2 H₂O → Na₂H₂P₂O₆ + 2 HCl A mixture of hypophosphoric acid,

(H₃PO₃) and

(H₃PO₄) is produced when white phosphorus oxidises in air when partially immersed in water. The tetrasodium salt Na₄P₂O₆·10H₂O crystallises at pH 10 and the disodium salt, Na₂H₂PO₆·6H₂O at pH 5.2. The disodium salt can be passed through an ion exchange column to form the acid dihydrate, H₄P₂O₆·2H₂O. The anhydrous acid can be formed by vacuum dehydration over P₄O₁₀ or by the reaction of H₂S on lead hypophosphate, Pb₂P₂O₆. Hypophosphoric acid is tetraprotic with dissociation constants p''K''_a1 = 2.2, p''K''_a2 = 2.8, p''K''_a3 = 7.3 and p''K''_a4 = 10.0. On standing the anhydrous acid undergoes rearrangement and disproportionation to form a mixture of isohypophosphoric acid, HPO(OH)-O-PO₂(OH); pyrophosphoric acid H₂P₂O₇ and pyrophosphorous acid. Hypophosphoric acid is unstable in hot

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

, in 4 M HCl it hydrolyses to give H₃PO₃ + H₃PO₄.

Structure

Hypophosphorus acid contains oxonium ions and is best formulated ₃O⁺sub>2 ₂P₂O₆sup>2−. The acid is isostructural with the diammonium salt which contains the OPO₂PO₂OHsup>2− anion with a P−P bond length of 219 pm. The HOPO₂PO₂OH²⁻ anion in Na₂H₂P₂O₆·6H₂O has a symmetric, staggered

ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petr ...

-like structure with a P−P bond of length 219 pm. Each phosphorus atom has two P−O bonds with length 151 pm, and a P−OH bond length of 159 pm.

Hypophosphate salts

Many hypophosphate salts are known, for example, K₄P₂O₆·8H₂O, Ca₂P₂O₆·2H₂O, K₃HP₂O₆·3H₂O, K₂H₂P₂O₆·2H₂O, KH₃P₂O₆. On standing in air, hypophosphates tend to oxidise to

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among other ...

s containing the ion where P has a formal oxidation state of +5. Hypophosphates are stable to alkali hydroxides. In fused

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

they convert rapidly to the orthophosphate containing .

Polyhypophosphates

Polyhypophosphates are known containing linear anions, for example Na₅P₃O₈ containing )₃O⁵⁻ with a P−P−P chain and Na₆P₄O₁₀·2H₂O containing )₄O⁶⁻, with a P−P−P−P chain. The cyclic anion , (hypohexametaphosphate) where each phosphorus atom has an oxidation state of +3 is formed when a suspension of red phosphorus in KOH is oxidised with bromine.

References

{{Reflist Mineral acids Phosphorus oxoacids

Preparation and reactions

Structure

Hypophosphate salts

Polyhypophosphates

See also

References