Hypophosphoric acid is a mineral acid with the formula H<sub>4</sub>P<sub>2</sub>O<sub>6</sub>, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H<sub>4</sub>P<sub>2</sub>O<sub>6</sub>÷2H<sub>2</sub>O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a PâÂÂP bond. Isohypophosphoric acid is a structural isomer 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/phosphoric acid mixed anhydride. The two phosphorus atoms are in the +3 and +5 oxidation states, respectively.
Hypophosphoric acid can be prepared by the reaction of red phosphorus with sodium chlorite at room temperature.
A mixture of hypophosphoric acid, phosphorous acid (H<sub>3</sub>PO<sub>3</sub>) and phosphoric acid (H<sub>3</sub>PO<sub>4</sub>) is produced when white phosphorus oxidises in air when partially immersed in water.
The tetrasodium salt Na<sub>4</sub>P<sub>2</sub>O<sub>6</sub>÷10H<sub>2</sub>O crystallises at pH 10 and the disodium salt, Na<sub>2</sub>H<sub>2</sub>PO<sub>6</sub>÷6H<sub>2</sub>O at pH 5.2. The disodium salt can be passed through an ion exchange column to form the acid dihydrate, H<sub>4</sub>P<sub>2</sub>O<sub>6</sub>÷2H<sub>2</sub>O.
The anhydrous acid can be formed by vacuum dehydration over P<sub>4</sub>O<sub>10</sub> or by the reaction of H<sub>2</sub>S on lead hypophosphate, Pb<sub>2</sub>P<sub>2</sub>O<sub>6</sub>.
Hypophosphoric acid is tetraprotic with dissociation constants pK<sub>a1</sub> = 2.2, pK<sub>a2</sub> = 2.8, pK<sub>a3</sub> = 7.3 and pK<sub>a4</sub> = 10.0.
On standing the anhydrous acid undergoes rearrangement and disproportionation to form a mixture of isohypophosphoric acid, HPO(OH)-O-PO(OH)<sub>2</sub>; pyrophosphoric acid H<sub>4</sub>P<sub>2</sub>O<sub>7</sub> and pyrophosphorous acid.
Hypophosphoric acid is unstable in hot hydrochloric acid, in 4 M HCl it hydrolyses to give H<sub>3</sub>PO<sub>3</sub> + H<sub>3</sub>PO<sub>4</sub>.
Hypophosphorus acid contains oxonium ions and is best formulated [H<sub>3</sub>O<sup>+</sup>]<sub>2</sub> [H<sub>2</sub>P<sub>2</sub>O<sub>6</sub>]<sup>2âÂÂ</sup>. The acid is isostructural with the diammonium salt which contains the [HOPO<sub>2</sub>PO<sub>2</sub>OH]<sup>2âÂÂ</sup> anion with a PâÂÂP bond length of 219 pm.
The HOPO<sub>2</sub>PO<sub>2</sub>OH<sup>2âÂÂ</sup> anion in Na<sub>2</sub>H<sub>2</sub>P<sub>2</sub>O<sub>6</sub>÷6H<sub>2</sub>O has a symmetric, staggered ethane-like structure with a PâÂÂP bond of length 219 pm. Each phosphorus atom has two PâÂÂO bonds with length 151 pm, and a PâÂÂOH bond length of 159 pm.
Many hypophosphate salts are known, for example, K<sub>4</sub>P<sub>2</sub>O<sub>6</sub>÷8H<sub>2</sub>O, Ca<sub>2</sub>P<sub>2</sub>O<sub>6</sub>÷2H<sub>2</sub>O, K<sub>3</sub>HP<sub>2</sub>O<sub>6</sub>÷3H<sub>2</sub>O, K<sub>2</sub>H<sub>2</sub>P<sub>2</sub>O<sub>6</sub>÷2H<sub>2</sub>O, KH<sub>3</sub>P<sub>2</sub>O<sub>6</sub>.
On standing in air, hypophosphates tend to oxidise to pyrophosphates containing the ion where P has a formal oxidation state of +5. Hypophosphates are stable to alkali hydroxides. In fused sodium hydroxide they convert rapidly to the orthophosphate containing .
Polyhypophosphates are known containing linear anions, for example Na<sub>5</sub>P<sub>3</sub>O<sub>8</sub> containing )<sub>3</sub>O<sup>5âÂÂ</sup> with a PâÂÂPâÂÂP chain and Na<sub>6</sub>P<sub>4</sub>O<sub>10</sub>÷2H<sub>2</sub>O containing )<sub>4</sub>O<sup>6âÂÂ</sup>, with a PâÂÂPâÂÂPâÂÂP 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.