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OXYACIDS OF PHOSPHORUS

*Hypophosphorous acid, H₃PO₂ , is made by the decomposition of barium Hypophosphite with the calculated quantity of sulphuric acid:

Ba(H₂PO₂)₂ + H₂SO₄ = BaSO₄ + H₃PO₂

 Barium Hypophosphite is obtained by the warming white phosphorus with barium hydroxide solution:

P₄ + Ba(OH)₂ = Ba(H₂PO₂)₂ + PH₃

Hypophosphorous acid is a solid , M.P. 27?. when  either the acid or its salt are heated phosphine( PH₃) is evolved.

Hypophosphorous acid and its salt are powerful reducing agents, precipitating silver from silver nitrate solution, the acid is monobasic.

*Orthophosphorous acid, H₃PO₃, may be obtained by treated phosphorous trichloride with water and evaporating the solution until the temperature rises to 180?.Crystals separate when the solution is cooled:

PCl₃ + 3 H₂O = H₃PO₃ + 3HCl

Orthophosphorous acid is a white , very soluble solid , M. P. 74?, when heated strongly it decomposes to phosphoric acid and phosphine:

4H₃PO₃ =PH₃ + H₃PO₄

Orthophosphorous acid is very strong reducing agent, it precipitate gold , silver , and copper from solutions of their salts, and also reduces mercuric chloride to mercurous chloride.

Orthophosphorous acid is dibasic acid, and its salts are NaH₂PO₃, Na₂HPO₃.

*Orthophosphoric acid, H₃PO₄, is prepared in the lab. by oxidation of red phosphorus with concentrated nitric acid diluted with its own volume of water:

P₄ + 10HNO₃ + H₂O = 4H₃PO₄ + 5NO +5NO₂

Towards the end of the reaction  a little concentrated nitric acid is added and the mixture is heated, to be sure that any phosphorous acid which may have been formed is oxidizes to Orthophosphoric acid, the resulting solution is diluted with an equal volume of water , filtered , and concentrated by evaporation until the temperature of the liquid rises to 180?. Deliquescent crystals of Orthophosphoric acid are deposited.

Orthophosphoric acid is made on the industrial scale by treating bone ash with dilute sulphuric acid . The insoluble ash is separated by filtration , and the solution of Orthophosphoric acid is concentrated by evalporation.

 Orthophosphoric acid is a colorless crystalline solid, M.P.42?, very soluble in water . It is a weak tribasic acid from which may be derived the three sodium salt, NaH₂PO₄ (rhombic prisms with 12 molecules of water of crystallization , and Na₃PO₄ which  crystallizes in hexagonal prisms with 12 molecules of water of crystallization).

If NaOH solution is added to a solution of Orthophosphoric acid containing litmus , the litmus will change from red to blue when sufficient naoh has been added to convert the acid Na₂ HPO₄. Phenolphthaline changes from colorless to pink at the same stage. If  NaOH solution is added to a solution of Orthophosphoric acid containing methyl orange , the methyl orange changes from orange to yellow when sufficient NaOH has been added to convert the acid NaH₂ PO₄.It follows therefore that a solution of  Na₃PO₄ is alkaline to litmus and a solution of Na₂HPO₄ is neutral.

*Pyrophosphoric acid, H₄P₂O₇, is made by heating Orthophosphoric acid to 220?:

2H₃PO₄ = H₄P₂O₇ + H₂O

Sodium Pyrophosphate is made by heating disodium hydrogen phosphate:

Na₂HPO₄ = Na₄P₂O₇ + H₂O

Pyrophosphoric acid is a colorless crystalline solid, m.p.61?. its salts are stable in aqueous solution , but the free acid slowly reverts to Orthophosphoric acid by combination with water:

H₄P₂O₄ + H₂O = 2 H₃PO₄

Metaphosphoric acid , H₃P₃O₉, is made by heating either* Orthophosphoric acid or Pyrophosphoric acid to 316?:

3H₃PO₄ = H₃P₃O₉ + 3H₂O

Also it may be prepared by heating sodium dihydrogen orthophosphate:

3NaH₂PO₄ = Na₃P₃O₉ + 3H₂O

Metaphosphoric acid is very soluble , translucent solid. If this acid heated with carbon a distillate of white phosphorus is obtained :

4 H₃P₃O₉ + 30C= 6H₂O + 30CO +3P₄

The molecule of Metaphosphoric acid has a ring structure.