Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/529

Rh PHOSPHORUS.] CHEMISTRY 517 in an excess of dry air, but it j s almost impossible to obtain the former free from the latter. The best mode of prepar ing the trioxide is to burn phosphorus in a very slow cur rent of dry air; it then condenses as a bulky white amorphous sublimate. Phosphorous anhydride is readily soluble ; it absorbs moisture with avidity, and dissolves in water, producing phosphorous acid : P 2 O 3 + 3H 2 O = 2H 3 PO 3. Phosphoric anhydride is a snow-white, flocculent, amor phous substance. It sublimes below a red heat. It has an attraction for water which probably is only exceeded by that of sulphuric anhydride, deliquescing quickly in moist air, and dissolving in water with a hissing noise and great development of heat ; the solution contains metaphos- phoric acid : P 2 5 + H 2 = 2HPO 3 ; but this acid gradually combines with a further quantity of water, forming phos phoric acid : HPO 3 + H 2 = H 3 PO 4 By virtue of its affinity to water, phosphoric anhydride causes the separa tion of the elements of water from many compounds ; sulphuric acid, for example, when heated with phosphoric anhydride is converted into sulphuric anhydride H 2 S0 4 ft.lphnHc.cld. PA - SO 3 + 2HP0 3. Sulphuric Metaphosphoric anhydride. acid. Oxyacids of Phosphorus. The following oxyacids of phosphorus are known : Hypophosphorous acid ..................... HPH 2 2 Phosphorous acid ............................. H 2 PH0 3 Phosphoric acid ............................... H 3 P0 4 Metaphosphoric acid ......................... HP0 3 Pyrophosphoric acid ......................... H 4 P 2 7 Hijpophosphorous acid, HPH 2 2. To prepare this acid a solution of barium hypophosphite is treated with exactly sufficient sulphuric acid to precipitate the barium, and concentrated until its boiling point rises to 130 C. ; on the liquid cooling, a mass of crystals of the acid is obtained. The solution of the barium salt is obtained by warming (not boiling) phosphorus with an excess of a solution of barium hydroxide, and afterwards removing the excess of hydroxide by passing carbon dioxide through the liquid : 3Ba(OH) 2 + 8P + GOH 2 = 3Ba(PH 2 2 ) 2 + 2PH 3. Hypophosphorous acid melts at 17 C. ; it is perfectly stable at ordinary temperatures, but its solution is slowly oxidized to phosphorous and phosphoric acids when ex posed to the air. It is a powerful reducing agent, pre cipitating silver and gold from solutions of their respective salts, and when heated to about 60 C. with a solution of copper sulphate it forms a precipitate of copper hydride, Cu.,H 9. When zinc is dissolved in the acid, the hydrogen produced reduces a portion of the acid to phosphine, which is given off as gas. Hypophosphorous acid is a monobasic acid. Its salts are crystallizable and soluble in water. They are per manent when dry, but their solutions are gradually oxidized on exposure to the air, especially if heated. When boiled with alkaline hydroxides they are decomposed into phosphate and hydrogen, thus KPH 2 O 2 + 2KHO = K 3 PO 4 + 2H,. The dry salts are decomposed by heat, and in most cases furnish a residue of pyrophosphate and metaphos- phate, phosphine and hydrogen being evolved, and some times water produced ; the proportions in which these pro ducts are formed is not constant, and depends on the nature of the salt, thus 5XaPH 2 O 2 = Na 4 P 2 7 +NaPO 3 + 2PH 3 +2H 2 7Sr(PH 3 2 ), =3Sr,P,0 7 + Sr(PO.), + 6PH 3 + 4H S + H 2 y + Ba(P0 3 ) 8 +12PH 3 The behaviour of the nickel and cobalt, and uranyl salts is peculiar, a phosphide being also formed 6Co(PH 9 ) 9 = 4Co(P0 3 ) 2 + 2CoP +2PH 3 + 9H 2 9(U0 2 )(PH 2 6 2 ) 2 = 6UP 2 O 7 +U(PO 3 ) 4 + 2UP+18H 2 Phosphorous acid, H 2 PH0 3. This acid is most readily prepared by decomposing phosphorus trichloride with water. By concentrating the solution by boiling until the temperature rises to 180 C., and then cooling, it is obtained in the crystalline state. The crystals melt at 70 C. Like hypophosphorous acid, it is resolved into phosphine and phosphoric acid when heated. Its solution is oxidized on exposure to the air. It is a powerful re ducing agent, precipitating silver, gold, and mercury from their salts, but it does not form copper hydride with copper sulphate. Iron and zinc dissolve in a solution of phosphorous acid with evolution of phosphine, a portion of the acid being reduced by the hydrogen resulting from the conversion of another portion into the metallic salt. Phosphorous acid is a dibasic acid, the salts produced on neutralizing its solution with alkaline hydroxide being formed by the displacement of at most two atoms of hydrogen by metals ; thermochemical investigation also indicates that it is a dibasic acid. It is possible, how ever, to displace a third atom of hydrogen by metals, and to produce such a compound as Na 3 PO 8, for example, but this cannot exist in presence of water. Many of the phosphites apparently must be regarded not as derived from the acid H 2 PH0 3, but as derivatives of a distinct- acid of the composition H 4 P 2 H 4 O 7, or 2H 2 PH0 3 + H 2 0; barium phosphite dried at 200-250 C., for instance, has the composition Ba 2 P 2 H 4 O 7. The phosphites are much more stable than the hypo- phosphites, but are all decomposed by heat. Salts, such as the barium salt Ba 2 P 2 H 4 O 7, furnish hydrogen and pyro phosphate on ignition but the normal phosphites, such as zinc phosphite ZnPHO 3, furnish hydrogen, a pyrophosphate, and a phosphide 1 4ZnPH0 3 = 7H 2 + GZn 2 P 2 7 + Zn 2 P 2. Phosphoric acid, H 3 PO 4. This acid may be produced by oxidizing phosphorus with nitric acid, by the oxidation of hypophosphorous and phosphorous acids, by the action of water on phosphoric anhydride and on phosphorus pentachloride, and by decomposing its salts with acids, tricalcium phosphate or bone earth, for example, with sulphuric acid. It may be obtained in crystals by concentrating an aqueous solution by boiling until the temperature rises to 215 C., and when the liquid is cold adding a few crystals, which cause it to solidify. The crystals melt at about 38 C. They are readily soluble in water, and furnish a strongly acid solution, which at a boiling heat decomposes the salts of most volatile acids. Phosphoric acid is a tribasic acid, and furnishes three classes of salts, of which the three sodium salts NaH 2 PO 4, Na 2 HP0 4 , Na 3 P0 4 Sodium dihydrogen Disodium hydrogen Trf.^^ phosphate. phosphate. phosphate. may serve as examples. It has a great tendency, however, to furnish dimetallic salts, such as Na 2 HPO 4, which are always produced when a solution of phosphoric acid is neutralized with a metallic carbonate (see p. 488). Metapliosphoric acid, HPO 3. This acid is the product of the action of water on phosphoric anhydride, and is also obtained by heating phosphoric acid to redness, and by decomposing the metaphosphates with another acid.