Molar mass 161.98 g/mol | Appearance White solid (dihydrate) | |
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Hypophosphoric acid meaning
Hypophosphoric acid is a mineral acid with the formula H4P2O6, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the P atoms are identical and joined directly with a P-P bond. There is an isomeric form isohypophosphoric acid which has a different structure with non-identical P atoms, one of which has a directly bonded H atom and an oxidation state of +3, which is joined by a P-O-P bridge to the second P atom which has an oxidation state of +5.
Contents
Preparation and reactions
Hypophosphoric acid can be prepared by the reaction of red phosphorus with sodium chlorite at room temperature.
2P + NaClO2 +8H2O → Na2H2P2O6 + 2HClA mixture of hypophosphoric acid, phosphorous acid (H3PO3) and phosphoric acid (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 pKa1 = 2.2, pKa2 = 2.8, pKa3 = 7.3 and pKa4 = 10.0.
On standing the anydrous 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, in 4M HCl it hydrolyses to give H3PO3 + H3PO4.
Structure
Hypophosphorus acid contains oxonium ions and is best formulated [H3O+]2 [H2P2O6]2−. The acid is isostructural with the diammonium salt which contains the [HOPO2PO2OH]2− anion with a P-P bond length of 219 pm.
The HOPO2PO2OH2− anion in Na2H2P2O6.6H2O has a symmetric,staggered ethane-like structure with a P-P bond of length 219 pm. Each P 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 e.g. K4P2O6.8H2O, Ca2P2O6.2H2O, K3HP2O6.3H2O, K2H2P2O6.2H2O, KH3P2O6.
On standing in air, hypophosphates tend to oxidise to pyrophosphates containing the P2O74− 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 PO43−.
Polyhypophosphates
Polyhypophosphates are known containing linear anions, for example Na5P3O8 containing O(PO2)3O5− with a P-P-P chain and Na6P4O10.2H2O containing O(PO2)4O6−, with a P-P-P-P chain. The cyclic cation (PO2)66−, (hypohexametaphosphate ) where each P atom has an oxidation state of +2 is formed when a suspension of red phosphorus in KOH is oxidised with bromine.