Acids.—Hypophosphorous and hypophosphoric acids, H3P02 and (or have no known anhydrides; phosphorous acid, is derived from ; meta-, pyro- and ortho phosphoric acids are derived from and are described under PHOSPHATES ; perphosphoric acids, and H4P208 are also known. Hypophosphorous acid, is prepared as its barium salt by boiling phosphorus with baryta water, removing excess of baryta by passing in carbon dioxide, and crystallizing the filtrate; if this salt is decomposed by the calculated amount of dilute sulphuric acid, the pure acid can be obtained, by careful evaporation of the filtrate under reduced pressure, as white crystals melting at 26.5° C. It is a monobasic acid and is slowly oxidized in air; on heating it gives phosphine and phosphoric acid. It is usually sold as a solution of sp.gr. 1.137 (about 32%) which is fairly stable. When warmed with copper sulphate solu tion it produces a red precipitate of the hydride, thus demonstrating its exceptionally strong reducing properties ; its salts are also effective reducing agents. Phosphorous acid, is formed from its anhydride, P406, or by passing chlorine into melted phosphorus covered with water, the phos phorus trichloride being decomposed in situ. The crystals melt at 7o°, are very deliquescent, and oxidize in the air; on heating they give phosphine and phosphoric acid. It is an energetic re ducing agent and, although normally dibasic, it gives rise to organic esters of the type Hypophosphoric acid, H4P206 or H2P03, occurs among the products of oxidation of phosphorus in moist air or in water; the solution is neutralized by sodium carbonate, and the sparingly soluble lead salt is made, and then decomposed by hydrogen sulphide; sticks of phosphorus immersed in copper nitrate solu tion are also oxidized to this acid by means of nitric acid. The aqueous solution of the free acid is stable, but it decomposes on concentration unless this be done in a vacuum, in which case the hydrate (H4P206, 2H20) separates and can be dried below so° to give the acid; this melts at 7o° and decomposes. The solution does not readily act as a reducing agent. Owing to uncertainty as to the constitution, formulae such as and have been proposed; an ester (CH3)2P03 is known (Rosenheim, 191o), so the salts are probably best regarded as derived from the polymerized acid. Permono- and perdi-phos phoric acids are obtained from phosphoric anhydride or meta- or pyro-phosphoric acids and hydrogen peroxide at o° C, large excess of peroxide favouring the production of the di-acid. The salts are also obtained by electrolysis of dipotassium hydrogen phosphate, a high anode current density increasing the proportion of the permono-salt ; forms good crystals. The two acids differ in their action on iodides, the permono-acid liberating iodine at once, and the other slowly (compare persulphates, see SULPHUR).
colourless fuming gas of b.p. — 75° and m.p. —83°, and is dis sociated by large induction sparks but not by heat (compare and ; it is readily decomposed by water and combines with ammonia to give Phosphoryl fluoride, POF is best obtained by heating powdered cryolite with II times its weight of phosphoric anhydride; it is a colourless, fuming gas of m.p. -68° and b.p. Phosphorus trichloride, obtained by passing a slow stream of chlorine over heated red phosphorus or through a solution of yellow phosphorus in carbon disulphide and fractionally dis tilling the product, is a colourless, mobile liquid of sp.gr. 1.613 (at o° C) and b.p. 76°, and freezes at - I I 2°. As prepared above, it may be contaminated with from which it is freed by dis tillation from excess of phosphorus. Cold water gives hydro chloric and phosphorous acids, and hot water gives red phosphorus and other products. When diluted with hydrogen and led into liquid ammonia, yields NH which gives on heating; a similarly diluted mixture, when submitted to the electric discharge, forms the dichloride, P2C14, an unstable fuming liquid, boiling and decomposing at 180°. Phosphorus penta chloride, obtained by the action of excess of chlorine on the element or its trichloride, is a pale straw-coloured solid, which sublimes and dissociates on heating, but may be melted at 148° under pressure; the dissociation is repressed by excess of chlorine: Water effects decomposition rapidly in two stages : The pentachloride is extensively used in organic chemistry, e.g., for replacing hydroxyl groups by chlorine: Phosphoryl chloride, or phosphorus oxychloride, may be produced as shown above, or by the interaction of phosphoric oxide on the pentachloride or on hydrogen chloride: It is a colourless liquid of b.p. 107° and m.p. I° ; with water it gives hydrochloric and phosphoric acids, with dilute alcohol ethylphosphoric acid, C2H5.H2PO4, and with absolute alcohol triethyl phosphate, Pyrophosphoryl chloride, P 203C14, is also known. Thiophosphoryl chloride, is formed by direct combustion of sulphur and by the action of sulphuretted hydrogen, antimony trisulphide or phosphorus penta sulphide upon or by dissolving phosphorus in sulphur chloride and distilling : 2P+3S2C12=4S+2PSC13. It is a colourless liquid (b.p. 125°), is decomposed by water to acids corresponding to each of the three elements, and gives thiophosphates with alkalis, e.g., PS(OK)3.