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PYROPHOSPHORIC ACID.

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(FePO, 2 H2O), which is also sometimes employed to estimate the quantity of phosphoric acid in a solution.

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(449) Pyrophosphoric Acid (HP2, or 2 HO,PO). —When rhombic phosphate of sodium (Na,HPO4, 12 H2O) is exposed to heat, it melts in its water of crystallization; and by continuing to apply to it a temperature not exceeding 300°, it may be reduced to a hard, white, saline mass, which may be redissolved in water with all its former properties. The dry mass consists of Na2HPO. If, however, it be heated to redness before redissolving, two atoms of the salt coalesce and a new salt is formed; I atom of water is expelled; 2 Na,HPO becoming Na P ̧Ð,+H2¤; on redissolving the residue in water and evaporating the solution, the liquid no longer yields rhombic crystals, but furnishes acicular crystals, composed of NaР,,, 10 H ̧¤; and the solution, instead of yielding a yellow precipitate with nitrate of silver, now gives a white one, consisting of Ag,P,,. In this case, the solution, if neutral to litmus before intermixture with the silver salt, remains neutral afterwards, because no free acid is liberated :

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With a solution of acetate of lead the pyrophosphate of sodium also occasions a white precipitate, the composition of which is represented by the formula Pb,P,,: and if the lead salt be suspended in water, and decomposed with sulphuretted hydrogen, it yields a solution of pyrophosphoric acid (HP2O). The excess of sulphuretted hydrogen must be got rid of by exposure to the air (not by heat, otherwise the tribasic acid is formed by the assimilation of water; H20 + H ̧Ð1⁄2Ð, = 2 H ̧PO4), and the acid may be obtained in crystals by evaporation in vacuo over sulphuric acid. This phosphate of sodium, from the mode in which it is obtained, is often termed (from Tup, fire) the pyrophosphate of sodium, and the corresponding salts of the acid, pyrophosphates. No solid pyrophosphate of potassium or of ammonium can be obtained; these salts are stable while in solution, but on evaporation they become converted into tribasic phosphates by the assimilation of water (Graham). Two classes of pyrophosphates may be procured; one with 4 atoms of a fixed basyl, with the formula M,P,, like the ordinary pyrophosphate of sodium (NaP, 10 H ̧‡); the other containing two atoms of hydrogen and two of uniequivalent metal (M,H,P,,), corresponding in composition to the acid pyrophosphate of sodium (Na2H¿P207).

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PHOSPHATES-METAPHOSPHORIC ACID.

Neutral solutions of the pyrophosphates also give in solutions of salts of calcium and barium white precipitates, which are the pyrophosphates of these metals; salts of nickel and copper give with pyrophosphate of sodium double salts, containing 2 atoms of sodium to 3 of the other metal; 2 Nа P2, + 3 ¤u′′ 2 NO, Єu", Na2 2 P2, + 6 NaNO3.

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The following table furnishes a synoptic view of some of the principal phosphates, metaphosphates, and pyrophosphates :

I. ORTHOPHOSPHATES.

Na,Pe,. 12 H2e
Na,HP. 12 H

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3 NaO,PO, 24 HO
2 NaO,HO,PO,. 24 HO
NaH,Pe,. He NaO, 2 HO,PO,. 2 HO

Ꮇ ᏢᎾ, ; M2HPе ; ᎷᎻ ᏢᎾ, .
Subphosphate of sodium
Rhombic phospht. of sodium
Acid phosphate of sodium
Microcosmic salt
Basic phosphate of calcium
Superphosphate of calcium
Phosphate of magne-
sium and ammonium
Ferric phosphate

NaH NHPе,.4 He NaO,H,NO,HO,PO,. 8 HO
Đa”, 2 Pě | 3 CaO,PO.

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ea"H, 2 Pe ČaO, 2 HO,PO,

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Mg"H,N,Pe,. 6 H,02 MgO,H,NO,PO ̧ . 12 HO

Fe""Pe 2 H2e Fe,O,,PO. 4 HO

Phosphate of lead

Pb", 2 Pe

Phosphate of bismuth

Bi""Pe

3 PbO,PO
BiO,PO

Phosphate of silver (yellow)

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2 PbO,PO

2 AgO,PO,

III. PYROPHOSPHATES. M,P,O,; MM',P,0,

Pyrophosphate of sodium
Pyrophosphate of lead
Pyrophosphate of copper &
Pyrophosphate of silver

Pb"Pe

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sodium eu",Na,2P,0, 3 CuO,NaŎ, 2 PO,
Ag Pe

(450) Metaphosphoric Acid (HPO1).—If, in preparing the rhombic phosphate of sodium (448), two equal portions of phosphoric acid be taken, and after neutralizing one portion with carbonate of sodium, as above directed, the second quantity of acid be added to the neutralized solution, a tribasic phosphate of sodium, consisting of NaH,PO, will be obtained on evaporating the liquid to dryness; but on igniting the residue, the 2 atoms of hydrogen will be expelled in the form of water, and a fusible monobasic phosphate, or metaphosphate, of sodium (NaPO) will remain in the form of a transparent glass. This, if dissolved in water, gives with nitrate of silver a gelatinous white precipitate (AgP‡ ̧) different in appearance and composition from either of the former phosphates of silver; it is soluble in excess of the sodium salt. With acetate of lead a white precipitate also is formed (Pb 2 PO3): it is fusible in boiling water, and when decomposed with sulphuretted hydrogen it yields the corresponding acid (HP),

MODIFICATIONS OF THE METAPHOSPHATES.

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which is distinguished from the other hydrates by its power of coagulating the albumen of white of egg. It also gives white precipitates with chloride of barium and nitrate of silver. Acetic acid does not coagulate albumen, neither does a solution of metaphosphate of sodium; but if the two solutions be mixed, the acetic acid liberates metaphosphoric acid, and the albumen becomes coagulated. Tribasic phosphoric acid by prolonged heating to redness loses water, the glassy residue being converted almost entirely into metaphosphoric acid. Metaphosphate of sodium is capable of combining with water of crystallization, and retains 1 atom if dried at 212°: this water is not basic, for on again dissolving the salt, it gives the usual reactions of the metaphosphates. If, however, the salt be heated to 300°, it does not lose weight, but becomes converted into the acid pyrophosphate of sodium, the water by the application of heat having changed its function in the salt, its hydrogen having now become basic (Graham) :—

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2 NaPO,H2 becomes Na,H,P,7.

This change of properties in the salt, without any change in the proportions of its components, here admits of a satisfactory explanation; and it is a striking and instructive illustration of the facility with which chemical compounds, by a change in molecular constitution, may sometimes give rise to substances the properties of which may be very different, though the results of their analysis in 100 parts may numerically coincide.

Metaphosphate of sodium forms with salts of barium a white insoluble metaphosphate of barium (Ba 2 PO2); but this precipitate when boiled becomes gradually dissolved, assimilating 2 atoms of water, and becoming converted into the acid tribasic phosphate of barium (BaH 2 PO). The compounds of this modification of phosphoric acid are monobasic. Their solutions redden litmus feebly.

The aqueous solution of metaphosphoric acid, when boiled, becomes converted into the ordinary tribasic acid, consequently it cannot be concentrated by the action of heat, HP+H2→ becoming H,PO: but the solution may be preserved at common temperatures without change.

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Fleitmann and Henneberg (Liebig's Ann. lxv. 324) have described two classes of salts which are probably anhydro-phosphates. By melting pyrophosphate and metaphosphate of sodium together, in the proportion of 1 atom of pyrophosphate and 2 of meta

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MODIFICATIONS OF METAPHOSPHORIC ACID.

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phosphate, they obtained a salt, a, consisting of 2 Na,POP ̧ ̧; and by fusing 8 atoms of the metaphosphate of sodium with 1 atom of the pyrophosphate, a definite sodium salt, b, was obtained, which consisted of 4 Na,PO,, 3 P,,; both these salts are very unstable, and in solution pass quickly into a mixture of pyrophosphate and metaphosphate. Definite salts of silver and of magnesium corresponding to these compounds were obtained.

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Odling proposes to represent these various classes of salts as follows, comparing quantities of each which contain equal amounts of metallic basyl, the quantity of phosphoric anhydride successively increasing in each series:

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and he represents the pyrophosphates, as well as the salts discovered by Fleitmann and Henneberg, as compounds of the orthophosphates, with different proportions of a metaphosphate. A pyrophosphate, for example, may be represented thus:Na,PO,NaPO, Na,P,O,; Fleitmann and Henneberg's salt a being Na,PO, 3 NaPO, NaPO, and the salt b being Na,PO 9 NaPO, NaP10031

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Modifications of Metaphosphoric Acid. If the ordinary or glassy metaphosphate of sodium be fused and allowed to cool very slowly it furnishes a beautiful crystalline mass, which, when dissolved in a small quantity of hot water, forms a liquid which divides into two strata; the smaller of these contains unchanged metaphosphate of sodium; but the bulk of the liquid is a solution of the crystalline salt, which may be obtained on evaporation in oblique rhombic prisms (6 NaPO,,12 H2O): the solution of this salt is neutral and has a cooling saline taste, whilst that of the ordinary or vitreous metaphosphate is insipid. The crystalline salt, by boiling, is rapidly converted into the acid orthophosphate (NaH,PO); a beautifully crystallized silver salt, consisting of 6 AgPО,,2 H2O, may be obtained from the crystalline sodium salt by precipitation, and a similar lead salt (3 Pb 2 PO3,3 H2O) may also be procured.

Maddrell (Proceed. Chem. Soc., 1847, p. 273) has described a series of monobasic metaphosphates which are anhydrous, crystalline, and insoluble in water, but soluble in oil of vitriol. They were formed by heating a solution of the sulphate or nitrate of

PHOSPHOROUS ACID-PHOSPHITES.

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the metal with an excess of phosphoric acid, until the sulphuric or other acid of the salt was expelled. Salts of potassium, sodium, aluminum, copper, nickel, and others were thus procured. The sodium salt, if prepared with phosphoric acid which contains magnesium, or any metal isomorphous with magnesium, forms an insoluble double metaphosphate: the magnesium double salt is crystalline, and consists of (3 Mg 2 PO3, 2 NaPO). These different varieties of metaphosphates are supposed to be due to the existence of several polymeric varieties of metaphosphoric acid.

(451) PHOSPHOROUS ANHYDRIDE (P ̧Ð ̧) may be procured by burning phosphorus in a limited current of dry air. A white, volatile, deliquescent, inflammable powder, destitute of crystalline structure, is thus obtained, often mixed with small quantities of oxide of phosphorus and phosphoric anhydride.

Phosphorous acid (H,PHO) may be obtained in solution by transmitting a stream of chlorine very slowly through a deep layer of phosphorus melted under water, so that each bubble of gas shall be completely absorbed by the phosphorus; terchloride of phosphorus (PCI) is formed, and is immediately decomposed by the water into hydrochloric and phosphorous acids; PC1, +3 H2→ =H,PHO2+3 HCl. If the acid liquid be concentrated by a heat not exceeding 400°, hydrochloric acid is expelled, and the acid is obtained in deliquescent rectangular prisms. When exposed to the air it gradually absorbs oxygen; and by a high temperature it is decomposed into phosphoric acid and phosphuretted hydrogen (4 H,PHO2=3 H,PO+H,P). It is also furnished in a less pure form, by the slow combustion which occurs when phosphorus is left exposed to the action of the atmosphere; this may be safely effected by placing sticks of phosphorus separately in tubes open at both ends, the lower aperture of the tube being a little contracted so as to prevent the phosphorus from falling out; a number of these tubes are then placed in a funnel, and the dense acid liquid which is gradually formed drains into a vessel placed for its reception. In this process phosphorous acid is first produced; the acid being deliquescent attracts moisture from the air, and then, by gradually absorbing oxygen, it forms phosphoric acid. The oxidation never proceeds so far as to convert the whole into phosphoric acid; the liquid therefore contains a mixture of phosphorous and phosphoric acids, which was at one time supposed to be a peculiar oxide of phosphorus, and was termed phosphatic acid. Phosphites.-Phosphorous acid is dibasic, and forms two classes of salts, the general formula of its normal salts being M,PHO,, and that of the acid phosphites is MHPHO. Neutral phosphite of sodium,

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