Phosphorus trichloride is a colorless, pungent liquid, boiling at 76° and retaining its liquidity at-115°. Its specific gravity at 0° is 1.613. It fumes in contact with moist air, and is decomposed by water into hydrochloric and phosphorous acids:

Phosphorus pentachloride, PCl5, is formed by the action of dry chlorine on phosphorus trichloride. This is best accomplished by passing the dry chlorine on the surface of the trichloride by a wide tube, occasionally stirring until the whole solidifies to a crystalline mass. As there is considerable heat developed, the vessel in which the reaction takes place must be kept well cooled.

Phosphorus pentachloride is a yellowish-white, crystalline powder, which fumes strongly on exposure to air, with a very irritating odor. It sublimes at 100° without previously melting; by higher temperatures it is decomposed into phosphorus trichloride and chlorine, the dissociation being complete at 336°. It is decomposed by a small quantity of water into phosphorus oxychloride and hydrochloric acid:

With an excess of water, phosphoric and hydrochloric acids are produced:

On account of this affinity for the elements of water, phosphorus pentachloride is a useful laboratory reagent for determining the presence of hydroxyl, especially in organic compounds. For example, the reaction with ethyl alcohol is as follows:

CH5OH + PC = C2HCl + POCl + HCl.

With acetic acid there is formed acetyl chloride, according to the following reactions:

C2H2O.OH + PCl5 = CH3O.Cl + POCl + HCl. With an aldehyde in which the oxygen is united by both its bonds to carbon the following reaction expresses the change:

CH,CO.H + PCl5 = CHCC. H + POCI.

Phosphorus tribromide, PBrg, is prepared like the trichloride, which it closely resembles. It has the specific gravity 2.925 at 0°, and boils at 175°.

Phosphorus pentabromide, PBr, resembles the corresponding compound of chlorine in its mode of formation and properties. It is a yellow crystalline solid, which melts below 100° and decomposes into the tribromide and bromine.

A compound of phosphorus with both chlorine and bromine may be formed by bringing together phosphorus trichloride and bromine in the proportion of their molecular weights. The resulting compound is called

phosphorus chlorobromide, PC,Br. It is a yellowish-red, crystalline solid, which decomposes at 35°.

Phosphorus di-iodide, P214, is prepared by dissolving 41 parts iodine and 5 parts phosphorus in carbon disulphide and cooling, or distilling off the disulphide, when yellow crystals of the di-iodide separate. The crystals melt at 110°, and are decomposed by water into amorphous phosphorus, phosphorous acid, and hydriodic acid:

3Pal+ 12H2O

=

P24H,POg + 12HI.

Phosphorus di-iodide is analogous in composition to liquid hydrogen phosphide, PH4, and to hydrazine, N2H4.

Phosphorus tri-iodide, Plg, is prepared like the preceding compound, using 12 parts iodine to 1 part phosphorus. It is a dark-red, crystalline solid, melting at 55°. It is decomposed by water, as follows:

Phosphorus pentafluoride, PF, is formed by adding arsenic trifluoride to phosphorus pentachloride :

It is a colorless gas, possessing a density of 63. It is decomposed by water into phosphoric and hydrofluoric acids. Its chief use is in the manufacture of a number of compounds of fluorine.

PHOSPHORUS AND OXYGEN.

Two oxides and six acids of phosphorus are known, as follows:

Phosphorus pentoxide, P2O. Pyrophosphoric acid, HÅP2O7. Orthophosphoric acid, H,PO.

No oxides have been discovered corresponding to hypophosphorous and hypophosphoric acids; the three acids of phosphoric oxide may be considered as derived by the addition of water, as follows:

+ H2O

=

=

2HPO,, Metaphosphoric acid. HP,O,, Pyrophosphoric acid. + 3H2O = 2H,PO4, Orthophosphoric acid.

This compound is prepared by gently heating phosphorus with incomplete access of air. An imperfect combustion takes place with the formation of the trioxide.

It is a white, amorphous powder, of a garlic-like odor. contact with air it inflames, forming the pentoxide.

In

Preparation.--This acid is formed when phosphorus is exposed to moist air; although in this case it is mixed with hypophosphoric and phosphoric acids. It is prepared pure and in large quantity by leading a stream of chlorine into melted phosphorus under water. Phosphorus trichloride is first produced, which is decomposed, as fast as formed, by the water present:

PC13 + 3H2O = H,PO, + 3HCI.

The solution is evaporated until it attains a temperature of 180°, when a thick fluid mass remains, which, on cooling, solidifies to a mass of crystals, having a melting point of 70.1°. On heating above 180° decomposition takes place, with formation of phosphoric acid and hydrogen phosphide, as follows:

Phosphorous acid is a powerful reducing agent, precipitating gold, silver, and mercury in the metallic state from solutions of their salts.

Phosphites.-The alkali phosphites are all soluble in water, the others are difficultly soluble. The acid is dibasic, one of the three hydrogen atoms not being replaceable by metal. A number of organic salts of phosphorous acid have been prepared, as triethyl phosphite, (C,H5)3PO3. The phosphites are distinguished from the hypophosphites by the former causing precipitates with barium and calcium hydrates.

Preparation. It is prepared by first making calcium or barium hypophosphite. For pharmaceutical purposes the former of these is preferred, and it is made by boiling phosphorus with calcium hydrate :

3Ca(OH)2 + 2P4 + 6H2O
Calcium Phosphorus. Water.
Hydrate.

=

зCa(H2PO2)2 + 2PH3.

Calcium
Hypophosphite.

Hydrogen Phosphide.

The excess of calcium hydrate is converted into carbonate by carbon dioxide, the solution filtered from the calcium carbonate

and a small quantity of phosphate, which is formed at the same time, and the clear filtrate either evaporated for the calcium hypophosphite or decomposed with oxalic acid for the free acid:

When barium hypophosphite is used, sulphuric acid is employed to decompose it.

The calcium oxalate is filtered from the acid and the latter evaporated until it contains 50 per cent. of hypophosphorous acid; in which form it usually comes into the market. The acid may be obtained in the solid form, and nearly absolute, by evaporating carefully at or below 130°, and then placing in a freezing mixture. It melts at 17.4°.

Properties. When strongly heated hypophosphorous acid is decomposed into orthophosphoric acid and hydrogen phosphide. Solution of hypophosphorous acid decomposes salts of gold, silver, and mercury, with formation of the metal. It is oxidized by chlorine and potassium permanganate.

Acidum Hypophosphorosum Dilutum, U. S. P., contains 10 per cent. by weight of absolute HPH2O2.

This acid has a specific gravity of 1.046 at 15°, and the 50 per cent. acid at the same temperature has a specific gravity of 1.406. The latter is found in commerce.

Impurities and Tests.-The most probable impurities in this acid are hydrochloric, sulphuric, phosphoric, and oxalic acids, and calcium hypophosphite. The acids may all be readily detected by their appropriate tests, and the calcium by the addition. of oxalic acid.

Hypophosphites.-The salts of this acid are prepared from the calcium or barium salt by double decomposition. They are all soluble in water.

In the dry state they are quite stable, but in solution they readily undergo change. In pharmaceutical preparations they are usually protected by sugar.

Hypophosphoric Acid, H,PO,, is formed with phosphoric and phosphorous acids when phosphorus is slowly oxidized in moist air. By evaporating the solution to a syrupy consistence, and adding cold, saturated solution of sodium acetate, the sodium salt having the formula NaHPO,.3H2O separates out. The acid gives a white precipitate with silver nitrate which does not blacken on boiling. This serves to distin guish it from phosphorous and hypophosphorous acids.

Phosphorus pentoxide, or phosphoric oxide, is prepared by burning phosphorus in dry air or oxygen. It is a bulky, white, amorphous powder, which is capable of being sublimed. It has a powerful affinity for water, hissing when thrown into it. Unless it be rigidly excluded from moist air deliquescence rapidly takes place. On account of this affinity for moisture it is much used in the drying of gases.

History. The existence of this acid was first noticed by Marggraf, in 1746, on account of the peculiar yellow precipitate it produced with silver nitrate. Graham, in 1833, first pointed out the difference between the ortho-, pyro-, and meta- acids.

Occurrence. The free acid is not known in nature, but in combination with calcium it occurs abundantly in bones, in all vegetable and animal tissues, and in minerals. The deposits of impure calcium phosphate in Southeastern United States and the adjacent islands are the most abundant occurrences as well as sources of this acid in combination.

Preparation. The use for which the phosphoric acid is intended determines the method of manufacture. For many purposes the acid is made from boneash by treatment with sulphuric acid:

The clear liquid is decanted from the calcium sulphate and evaporated to a small bulk, when, after settling, the supernatant liquid is again decanted from the further deposit of calcium sulphate; the temperature is then raised to 300°, and maintained at that point until all sulphuric acid is removed. The acid made in this way is useful in the arts and in the manufacture of phosphates.

The official acid is, or should be, prepared directly from phosphorus, by the action of nitric acid. 16 parts of phosphorus are heated in a capacious retort with 100 parts of nitric acid (sp. gr. 1.420), diluted with an equal weight of distilled water. The retort is connected with a condenser, and heat applied until the reaction commences. The heat is then carefully regulated to prevent the action becoming too violent, and, if necessary,