222 OXIDES OF PHOSPHORUS-PHOSPHORIC ANHYDRIDE. composition with which they are tipped takes fire when rubbed upon the phosphorized surface, but not by ordinary friction upon any other substance, as the match itself contains no phosphorus. Although vitreous phosphorus acts as a powerful irritant poison upon animals when taken internally, the amorphous variety may be swallowed with impunity; the vitreous phosphorus forms the active ingredient in the phosphorus paste frequently used to destroy cockroaches and other kinds of vermin. Owing to its strong attraction for oxygen, phosphorus reduces some of the oxidized compounds of the metals to the metallic state : a stick of phosphorus placed in a solution of chloride of gold or of nitrate of silver, becomes speedily incased in reduced gold or silver. Salts of palladium, platinum, and copper, are also reduced gradually when a stick of phosphorus is immersed in their solutions. (445) OXIDES OF PHOSPHORUS.-Phosphorus is usually stated to furnish four compounds with oxygen, but only two of them are known in the anhydrous condition, viz. :— Phosphorus forms three oxidized acids, which are respectively monobasic, dibasic, and tribasic, in proportion as the quantity of oxygen increases: these acids are the following: 2 2 Hypophosphorous acid (monobasic) HPH2O2 НОРНО3 (446) PHOSPHORIC ANHYDRIDE: P,O=142.-The most important of the oxides of phosphorus is that which when acted on by water forms phosphoric acid; it occurs native in considerable quantity in the form of phosphate of calcium. The anhydride of this acid is the sole product of the rapid combustion of phosphorus in dry oxygen or in atmospheric air. By means of the apparatus shown in fig. 312, a large quantity of phosphoric anhydride may be readily obtained in a few hours: E is a three-necked globe, in the centre of which is suspended a porcelain dish, c; this dish is attached by means of platinum wire to the wide tube a, b, which is closed at a with a cork; the bottle, f, is connected by the tube g, with an aspirator, or other convenient means of maintaining a continuous current of air through the apparatus: the air as it enters is thoroughly dried by passing over pumice moistened with sulphuric acid, in the tube, d. HYDRATES OF PHOSPHORIC ACID. 223 A fragment of well dried phosphorus is placed in the dish, c, and a snow-white,flocculent, non-crystalline, anhydrous, but extremely deliquescent, powder, which fuses at an elevated temperature, and by a still stronger heat, approaching to whiteness, may be sublimed. When dropped into water it combines with it, emitting a hissing noise; the greater part is instantly dissolved, leaving a few gelatinous flocculi, which slowly disappear. After it has once been dissolved, it cannot again be converted into the anhydride by mere elevation of temperature, as the whole com pound is gradually dissipated in vapour. It does not emit vapour at ordinary temperatures, and owing to its powerful attraction for water, this anhydride is often used as a desiccating and dehydrating agent; and for this purpose it surpasses in efficacy almost every known substance. The pure acid is generally 1 part of phosphorus in The phosphorus becomes (447) Hydrates of Phosphoric Acid. procured in a hydrated state, by boiling 13 parts of nitric acid of sp. gr. 1'20. oxidized by the nitric acid, which is decomposed with escape of nitric oxide, and the phosphoric acid is dissolved as it is formed. When the phosphorus has all disappeared, the excess of nitric acid is expelled by evaporating the liquid in a platinum vessel until dense white fumes begin to arise; on cooling, the acid solidifies to a transparent glassy mass, frequently termed glacial phosphoric acid. This glacial acid is extremely deliquescent, producing a solution which, when saturated, has a sp. gr. of 2'0. It is intensely acid, but not caustic. The oxidation of phosphorus by nitric acid furnishes an easy 224 HYDRATES OF PHOSPHORIC ACID. means of ascertaining the composition of phosphoric anhydride. For this purpose, 31 grains of phosphorus are boiled in a glass retort with pure diluted nitric acid. The greater part of the excess of water and nitric acid having been distilled off, the acid solution is added to 350 grains of oxide of lead, in a weighed platinum dish: the liquid is slowly evaporated and the residue ignited; by a red heat the whole of the nitric acid is expelled, and the phosphoric anhydride alone remains in combination with the oxide of lead. The oxide and anhydride together will be found to weigh 421 grains, showing an increase in weight upon the phosphorus and oxide of lead of 40 grains: 31 parts of phosphorus therefore require 40 parts of oxygen for conversion into phosphoric anhydride. A less pure acid is procured by adding to a solution of superphosphate of lime (prepared from bones by the process already described as a preliminary step towards procuring phosphorus) carbonate of ammonium till effervescence ceases; tribasic phosphate of calcium is precipitated, leaving phosphate of ammonium in solution. The precipitated phosphate of calcium is separated by filtration, the liquid evaporated to dryness, and the residue ignited. Ammonia is expelled, and phosphoric acid (contaminated with all the soluble salts which the bones contained) remains behind. There are three different hydrates of phosphoric acid, each of which possesses the properties of a distinct acid: viz.Metaphosphoric acid HP, or HO,PO Orthophosphoric or ordinary phosphoric acid H,PO, or 3 HO,PO. Pyrophosphoric acid 3 4 2 HP2, or 2 HO,PO. These different hydrates of the acid retain their peculiar characteristics when dissolved in water, and combine with 1, with 3, or with 4 equivalents of basyls to form salts, according as the metaphosphoric, the orthophosphoric, or the pyrophosphoric acid is employed. Owing to the important influence which the study of these combinations has exercised upon the theory of saline combinations in general, it will be necessary to examine them somewhat in detail. (448) Orthophosphoric or Tribasic Phosphoric Acid (H ̧PÐ ̧ or 3 HO,PO). If the liquid formed by dissolving the glacial acid in water be boiled for some time, and carbonate of sodium be then added until the solution becomes slightly alkaline, a tribasic phosphate of sodium and hydrogen is obtained, which on evaporation crystallizes in large transparent rhombic prisms (Na,HPO4, 12 H2O). If this solution be mixed with a neutral solution of HYDRATES OF PHOSPHORIC ACID. 225 nitrate of silver, a canary-yellow precipitate of tribasic phosphate of silver (Ag,PO) is formed. Although this solution was neutral or slightly alkaline before admixture with nitrate of silver, it will be found afterwards to have a decidedly acid reaction upon litmus, nitric acid having been liberated : : Na2HPO+3 AgN¤ ̧=Ag¿PÐ ̧+2 NaNO3+ HNO3. 4 Acetate of lead may be used as a precipitant instead of nitrate of silver, and in this case a white tribasic phosphate of lead (Pb,2 PO) subsides. If this phosphate of lead be well washed, suspended in water, and exposed to the action of a current of sulphuretted hydrogen, pure orthophosphoric acid is liberated and becomes dissolved in the liquid, whilst the black insoluble sulphide of lead is formed; Pb, 2 PO1+3 H2S=2 H ̧Pе+3 PbS. The sulphide of lead may be removed by filtration, and the acid obtained in deliquescent hard, brittle, prismatic, transparent crystals, by evaporation in vacuo over sulphuric acid. It requires 3 atoms of a uniequivalent basyl for saturation. The salts of this hydrate form the orthophosphates or the orthophosphates or common tribasic phosphates. 4 There are three varieties of these salts, which may be indicated by general formulæ as follows: It is not necessary that the 3 equivalents of basyl should consist of the same metal in these salts: two or even three different basyls may coexist in the salt; as, for example, in microcosmic salt, or phosphate of sodium, ammonium and hydrogen, (NaH NHPO 4 H2O). In the first class, the 3 atoms of hydrogen in the acid have been displaced by 3 equivalents of a metal, as for example, in the tribasic phosphate of sodium (NaPO4, 12 H2O); these salts when soluble have a strongly alkaline reaction: in the second class 2 atoms of the basic hydrogen have been displaced by 2 equivalents of a metal; they are like the ordinary rhombic phosphate of sodium (Na2HP, 12 H2O); the soluble salts of this class are neutral, or have a feebly alkaline reaction: whilst the third class contains only 1 equivalent of metal with 2 atoms of basic hydrogen; they are of the form of the salt (NaH,PO, H2O) frequently called the biphosphate of soda; these salts have a strongly acid reaction, and are often spoken of as the superphosphates. 2 226 ORTHOPHOSPHORIC OR TRIBASIC PHOSPHORIC ACID. 2 The soluble orthophosphates are characterized by the yellow phosphate of silver which their neutral solutions form with nitrate of silver; this precipitate is freely soluble both in nitric acid and in ammonia. They also yield a crystalline precipitate when a clear solution of sulphate of magnesium, rendered alkaline by ammonia, is briskly stirred with them; this precipitate is insoluble in water which contains free ammonia; it consists of (Mg"H,NPO, 6H,O): when ignited, the water and ammonia are expelled, and it becomes converted into pyrophosphate of magnesium (Mg,P,,), a compound frequently employed as a means of estimating the amount of phosphates in solutions which contain them; 100 parts of the ignited residue containing 640 of P.* * Neutral solutions of the orthophosphates give precipitates with salts of barium and calcium; the phosphates of barium and calcium are readily soluble in acetic acid; but free phosphoric acid gives no precipitate in solutions of the nitrates of calcium, barium, silver, or sesquichloride of iron; when molybdate of ammonium is added to a solution of a phosphate acidulated with nitric acid, a characteristic yellow precipitate of molybdophosphate of ammonium is formed. The quantity of phosphoric acid in a solution may also be ascertained, if neither sulphuric nor hydrochloric acid be present, by means of acetate of lead; the solution, before this salt is added to it, should be neutralized by ammonia, and then acidulated freely with acetic acid; the precipitate (PbHPO) should be well washed and ignited, by which means it is rendered anhydrous; ICO parts of the ignited residue represent 23.65 of (PO). Chancel has lately shown that the acid solution of nitrate of bismuth furnishes an admirable method of separating phosphoric acid from many metals, such as iron, calcium, and aluminum, which form phosphates soluble only in acidulated liquids. Care is requisite to remove any chlorine, or sulphuric acid from the liquid, before adding the solution of bismuth, which is prepared by dissolving 1 part of the crystallized nitrate in 4 parts of nitric acid of sp. gr. 136, adding 30 parts of water, then boiling, and filtering if necessary. In separating the orthophosphate of bismuth by this reagent, the liquid must be boiled, and the precipitate well washed with boiling water and carefully dried 100 parts of the phosphate of bismuth (BiPO) correspond to 23.28 of phosphoric anhydride (PO). With ferric salts phosphoric acid forms an insoluble buff-coloured precipitate *The arseniates give precipitates both with ammoniacal salts of magnesium and molybdic acid, similar to those furnished by the phosphates. |