# Hypophosphoric acid

Hypophosphoric acid is a mineral acid with the formula H$_4$P$_2$O$_6$, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H$_4$P$_2$O$_6$·2H$_2$O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P 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.

## Preparation and reactions

Hypophosphoric acid can be prepared by the reaction of red phosphorus with sodium chlorite at room temperature.
    2 P + 2 NaClO$_2$ + 2 H$_2$O → Na$_2$H$_2$P$_2$O$_6$ + 2 HCl
A mixture of hypophosphoric acid, phosphorous acid (H$_3$PO$_3$) and phosphoric acid (H$_3$PO$_4$) is produced when white phosphorus oxidises in air when partially immersed in water.
The tetrasodium salt Na$_4$P$_2$O$_6$·10H$_2$O crystallises at pH 10 and the disodium salt, Na$_2$H$_2$PO$_6$·6H$_2$O at pH 5.2. The disodium salt can be passed through an ion exchange column to form the acid dihydrate, H$_4$P$_2$O$_6$·2H$_2$O.
The anhydrous acid can be formed by vacuum dehydration over P$_4$O$_{10}$ or by the reaction of H$_2$S on lead hypophosphate, Pb$_2$P$_2$O$_6$.
Hypophosphoric acid is tetraprotic with dissociation constants pK$_{a1}$ = 2.2, pK$_{a2}$ = 2.8, pK$_{a3}$ = 7.3 and pK$_{a4}$ = 10.0.
On standing the anhydrous acid undergoes rearrangement and disproportionation to form a mixture of isohypophosphoric acid, HPO(OH)-O-PO$_2$(OH); pyrophosphoric acid H$_2$P$_2$O$_7$ and pyrophosphorous acid.
Hypophosphoric acid is unstable in hot hydrochloric acid, in 4 M HCl it hydrolyses to give H$_3$PO$_3$ + H$_3$PO$_4$.

## Structure

Hypophosphorus acid contains oxonium ions and is best formulated [H$_3$O$^+$]$_2$ [H$_2$P$_2$O$_6$]$^{2−}$. The acid is isostructural with the diammonium salt which contains the [HOPO$_2$PO$_2$OH]$^{2−}$ anion with a P−P bond length of 219 pm.
The HOPO$_2$PO$_2$OH$^{2−}$ anion in Na$_2$H$_2$P$_2$O$_6$·6H$_2$O has a symmetric, staggered ethane-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, K$_4$P$_2$O$_6$·8H$_2$O, Ca$_2$P$_2$O$_6$·2H$_2$O, K$_3$HP$_2$O$_6$·3H$_2$O, K$_2$H$_2$P$_2$O$_6$·2H$_2$O, KH$_3$P$_2$O$_6$.
On standing in air, hypophosphates tend to oxidise to pyrophosphates containing the P $_2$O$^{4−}$ $_7$ 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 PO$^{3−}$ $_4$.

## Polyhypophosphates

Polyhypophosphates are known containing linear anions, for example Na$_5$P$_3$O$_8$ containing O(PO$_2$)$_3$O$^{5−}$ with a P−P−P chain and Na$_6$P$_4$O$_{10}$·2H$_2$O containing O(PO$_2$)$_4$O$^{6−}$, with a P−P−P−P chain. The cyclic anion (PO $_2$)$^{6−}$ $_6$, (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.