1902 Encyclopedia > Phosphorus and Phosphates

Phosphorus and Phosphates




PHOSPHORUS AND PHOSPHATES. " Phosphorus " (_____, light-bringer) had currency in chemistry as a generic term for all substances which shine in the dark without burning, until the name came to be monopolized by a peculiar kind of "phosphorus" which was discovered, some time previous to 1678, by the German alchemist Brand of Hamburg. Brand, hoping to obtain thereby an essence for the "ennobling" of silver into gold, sub-jected urine-solids to dry distillation. In lieu of the hoped-for essence he obtained as part of the distillate a wax-like, easily fusible solid which, besides being phos-phorescent, readily caught fire, to burn with a dazzling light into a white solid acid. The new phosphorus natur-ally excited universal interest; but it was, and remained, only a rather costly chemical curiosity until Scheele, in 1771, starting from the discovery of Gahn that bone-ash is the lime-salt of a peculiar non-volatile acid, proved that this acid is identical with the one formed in the com-bustion of phosphorus, and that the latter, being only " phlogisticated" bone-ash acid, can be obtained from it by distillation with charcoal at a high temperature. This method of Scheele's is used to the present day for the manufacture of phosphorus, and even the theoretical notion on which it rests is recognized as correct as far as it goes, anhydrous bone-ash acid being a compound of phosphorus with oxygen the formation of which involves the liberation of part of the energy ("phlogiston") of each in the kinetic form of heat. That phosphorus is an elementary sub-stance was originally a surmise, which, however, has been confirmed by all subsequent experiences. In compara-tively recent times it was found that Brand's phosphorus is susceptible of passing (by mere loss of energy) into two allotropic modifications, known as "red" and "metallic" phosphorus respectively, so that the name " phosphorus " has again come to assume a generic meaning, being used for these three substances and the element as such conjointly.
Manufacture.—For the manufacture of ordinary phos-phorus any kind of phosphate of lime might be used, and in fact mineral phosphates are used occasionally, though as a rule the bones of domestic animals are employed as a raw material. Such bones (apart from a large percent-age of water and a small admixture of fats and other subsidiary organic components) consist essentially of two things, namely, (1) osseine—a nitrogenous organic com-pound, insoluble in water, but convertible by long treat-ment with hot water into a solution of "glue"—and (2) an infusible and incombustible part,—the two being united together (perhaps chemically) into a cellular tissue. The following analysis of the humerus of an ox gives an idea of the constitution of the second part and its ratio to the whole.
Phosphate of lime, P^SCaC- 61-4
Phosphate of magnesia, P2033MgO 17
Carbonate of lime 8'6 71 "7
Osseine 28'3
100'0

The percentages, however, in bones generally are sub-ject to great variation. When bones are heated to red-ness in the absence of air the organic part is destroyed, and there remains ultimately a cellular tissue of bone-phosphate impregnated, so to speak, with finely-divided charcoal. This black residue, known as "bone-black," is used largely for the decoloration of sugar-syrup, and, after having been exhausted in this direction, forms a cheap material for the manufacture of bone-ash and con-sequently of phosphorus; but, as a rule, the phosphorus-manufacturer makes his bone-ash direct from bones, by burning them in a furnace (constructed and wrought pretty much like a limekiln) between alternate layers of coal.

The burned bones (which retain their original shape) are ground up into granules of about the size of lentils, and these are then placed in a wooden tank coated inside with lead, to be decomposed by means of about their own weight of chamber-acid, i.e., sulphuric acid containing about 60 per cent, of real H2S04. To accelerate the action the bone-meal is mixed with boiling water previous to the addition of acid, and steam may be passed into the magma when its temperature threatens to fall too low. The acid readily decomposes the carbonate of the bone-ash, and then acts, more slowly, on the phosphate, the process being completed in about twenty-four hours; and the result, in regard to the latter, is that about two-thirds of the phosphate are decomposed into sulphate of lime (gypsum), which separates out as a precipitate, and phos-phoric acid, which unites with the residual one-third of the phosphate and the water into a solution of superphos-phate of lime—

To eliminate the gypsum the mass is diluted with water, allowed to settle, and the solution drawn off with lead syphons, then the residue is washed by decantation, and ultimately filtered off through a bed of straw contained in a cask with a perforated bottom. The spent heat of the distillation-furnace is utilized to concentrate the united liquors to about P45 specific gravity, when a remnant of gypsum separates out, which must be removed. The clari-fied liquor is then mixed with about one-tenth of its woigbt of granulated charcoal, and the whole evaporated in an iron basin until the mass is sufficiently dry to be passed through a copper sieve and granulated. The granules are heated cautiously over a fire, to be dehydrated as far as possible without loss of phosphorus (as phosphuretted hydrogen); and the dry mass is then transferred to fire-clay retorts—either pear-shaped with bent-down necks, or cylinders, about 18 inches long and 4 inches in diameter, with straight necks—arranged within a powerful furnace. The condensers are made of earthenware, and must be so arranged that loss of phosphorus by combustion is avoided as far as possible; its condensation takes care of itself. One construction is to give the condenser the form of a bell-jar resting in a saucer containing water; lateral orifices in the bell serve to couple every two bells into one, to unite each with its retort-neck, and to send the vapour (of phosphuretted hydrogen, carbonic oxide, and other poisonous gases) into a chimney, where they take fire spontaneously, and the products are carried away by the draught. While the condensers are being adjusted the fire is kindled and raised very slowly, but ultimately forced up to the highest temperature which the retorts can stand, and maintained at this pitch until the appearance of the flames of the escaping vapours proves the absence from them of phosphorus, free or combined. This takes from thirty-six to forty-eight hours. The reduction-process, though in reality very complex, is in its principal features easily understood. The acid-phosphate behaves as if it were a mere mixture of _§ x P205 + \ x P2063CaO (bone-phos-phate). The quasi-free acid (§P205) is reduced by the charcoal with formation of carbonic oxide and phosphorus-vapour, one-third of the phosphorus remaining in its original form of bone-phosphate.

The distillation of phosphorus is rather a dangerous operation, because the connecting pipes at the condensers are apt to get blocked up with frozen phosphorus, and consequently must be cleared from lime by copper or iron wires being pushed through them (at a certain risk to the operator). Another difficulty is that, although a retort may be quite whole in the ordinary sense, it may, and as a rule does, admit of the perspiration of phosphorus-vapour. To render retorts as nearly as possible impermeable to the vapour they are being provided vith two or three coats of some kind of cement, such as a mixture of slaked lime and borax, or a magma of clay, horse-dung, and water. In the collecting and further manipulation of the phosphorus the dangerous inflammability of the substance demands that all operations be conducted under water.

As soon as the retorts have cooled down sufficiently the condensers are detached and their tubuli bunged up to prevent access of air to the inside. The necks of the retorts are knocked off and thrown into water to save the phosphorus which has condensed within them and to unite it with that of the condensers. From the analysis of the ox-bone quoted we calculate that its ash contains 17 '6 per cent, of phosphorus, of which two-thirds (= IT7 per cent.) should be recoverable as free phosphorus; according to Fleck, the yield of phosphorus is 8 per cent., while Payen puts it down at 8 to 10 per cent. But this crude phos-phorus is largely contaminated with blown-over bone-ash and charcoal and with "red" phosphorus. Its purifica-tion used to be effected everywhere by melting it under water of about 60° C, and pressing it through chamois leather by means of a force-pump. In certain French works porous fireclay serves as a filtering medium, while superheated steam supplies at the same time the necessary heat and pressure. By the addition of coarsely-powdered charcoal to the phosphorus the clogging-up of the pores of the fireclay septum is precluded. A more effectual method of purification is to re-distil the crude (or perhaps the previously filtered) phosphorus from out of cast-iron retorts, the necks of which dip half an inch deep into water contained in a bucket. A chemical method of purification is that of Böttcher, who fuses the crude phos-phorus (100 parts) under water, with addition of 3"5 parts of oil of vitriol and 3-5 parts of bichromate of potash. The phosphorus passes, with a feeble gas-evolution, into an almost colourless liquid, with a loss of only 4 per cent, of its weight, as against the 10 to 15 per cent, unavoid-ably involved in the distillation process. To bring the purified phosphorus into the traditional form of sticks it is fused under water and sucked up into slightly conical glass tubes about two-fifths of an inch wide and a foot long; the tubes are closed below with the finger and immersed in cold water to cause the contents to freeze. The solid stick is then pushed out by means of a rod, and cut into pieces with a pair of scissors. For emission into commerce the sticks are put into cylindrical wide-necked glass bottles, or into tin canisters, full of water, which latter had better be mixed with a sufficiency of alcohol or glycerin to prevent freezing (and bursting) in winter time.

Seubert, about 1844, invented an ingenious apparatus for the continuous casting of phosphorus-sticks, consisting of a funnel-shaped vessel of copper, terminating below in a long horizontal copper tube, the outer end of which lies within a tank full of cold water. The phosphorus is placed in the funnel, covered with water, and the whole up to the cold-water tank raised (by means of a water-bath and steam-pipes) to a suitable temperature, matters being ar-ranged so that the phosphorus freezes just on arriving at the exit end of the tube. The workman then catches the protruding button of phosphorus and pulls out an endless stick, which is cut up into pieces of the desired length. This ingenious apparatus, however, has not been found to work satisfactorily, and has been given up again in favour of some form of the old method. The loss of one-third of the phosphorus contained in the bone-ash, which is unavoid-ably involved in the ordinary method of phosphorus-making, can be avoided, according to Wöhler, by adding finely-powdered quartz to the mixture which goes into the retorts. The superphosphate is then completely decom-posed with formation of a residue of silicate, instead of phosphate, of lime. An improvement by Fleck aims at the utilization of the organic part of the bones. He pro-poses to recover the fat from the bones by boiling them with water and then the gelatin by digesting them in hydrochloric acid of P05 specific gravity. The gelatin remains in a coherent form; the phosphate passes into solution as mono-calcic salt, which is recovered by evapora-tion in crystals and then reduced by distillation with charcoal. None of these (and other) proposals have been much heeded; the manufacture of phosphorus at present, in fact, is almost a monopoly, the bulk of what occurs in commerce being produced by two firms, viz., Albright and Wilson of Oldbury, near Birmingham, and Coignet and Son in Lyons. According to E. Kopp, the production in 1874 amounted to 1200 tons.

Recently purified phosphorus is a slightly yellowish or colourless solid of about the consistence of beeswax. At low temperatures it is brittle; specific gravity = P83 at 10° C. It fuses at 44°"3 C. into a strongly light-refracting liquid of 1" 743 (Kopp) specific gravity. Neither in the soh'd nor in the liquid state does it conduct electricity. When heated further (in an inert atmosphere such as hydrogen or carbonic-acid gas) it boils at 290° C, and assumes the form of a colourless vapour which at 1040° C. is 4'5 times as heavy as air or 654 times as heavy as hydrogen, whence it follows that its molecular weight is 2 x 65T = 130'2 = very nearly four times the atomic weight of phosphorus (31'0). Phosphorus is insoluble in water, more or less sparingly soluble in alcohol, ether, fatty oils, and oil of turpentine, and very abundantly soluble in bisul-phide of carbon. When exposed to the air, and especially to moist air, it suffers gradual oxidation into phosphorous and phosphoric acids with evolution of a feeble light. Phos-phorus does not phosphoresce in the absence of oxygen. Singularly, it does not phosphoresce in pure oxygen either, unless the tension of the gas be reduced to some point considerably below one atmosphere (Graham). Phosphorus is a most dangerous poison; doses of as little as 0T gramme (= 1 • 5 grains) are known to have been fatal to adults. The heads of a few lucifer matches may suffice to kill a child. Phosphorus is used chiefly for the manu-facture of lucifer matches (see MATCHES, vol. xv. pp. 625, 626) and also in the manufacture of iodide of methyl and other organic preparations used as auxiliary agents in the tar-colour industry. Phosphorus-paste, made by working up a small proportion of phosphorus melted under water in a hot mortar with flour, is used as poison for vermin.





Red Phosphorus.—A red infusible solid which is always produced when ordinary phosphorus is made to burn in an insufficient supply of air, and also by the long-continued action of sunlight on phosphorus-sticks kept under water, used to be taken for a lower oxide of the element, until A. v. Schrotter of Vienna showed, in 1845, that it is nothing but an allotropic modification of the elementary substance. A given mass of ordinary phosphorus can be converted almost completely into the red modification by keeping it at 240" to 250° C. in the absence of air for a sufficient time. The addition of a trace of iodine to phosphorus at 200° C. brings about the conversion suddenly with large evolution of heat (Brodie). Red phosphorus is now an article of chemical manufacture. The phos-phorus is simply heated, and kept at the requisite temperature, within a large iron pot which communicates with the atmosphere by only a narrow pipe. At a very slight expense of the material the air within the apparatus is quickly deoxygenated and con-verted into (inert) nitrogen. The requisite steady temperature is maintained by means of ? bath of molten solder. By the mere effect of the heat the phosphorus becomes more and more viscid and darker and darker in colour, and is at last completely con-verted into a dark-red opaque infusible solid. This, however, always includes a small proportion of the ordinary modification, which is most readily extracted by powdering the crude product and exhausting it with bisulphide of carbon, which does not affect the red kind. A less expensive method is to boil the powdered raw product with successive quantities of caustic-soda ley, when the ordinary phosphorus only is dissolved as hypophosphite with evolution of phosphuretted hydrogen. The residue is washed and dried and then sent out in bottles or canisters like any ordinary chemical preparation. It is not at all affected by even moist air, nor by aerated water, hence it is neither phosphorescent nor poisonous. When heated in air to about 260° C. it begins to pass into the ordinary modification and consequently burns, readily enough, into the same phosphoric acid P206 as ordinary phosphorus does. But its combustion-heat amounts to only 5070 Centigrade-units per unit-weight of fuel as against the 5953 units produced in the combustion of ordinary phosphorus. The balance of 883 units is the equivalent of the surplus of energy contained in the yellow as compared with the red modification. This accounts for the relative chemical inertness of the latter. The specific gravity •of red phosphorus is 2'089 to 2'106 at 17° C.; its electric conduct-ive power is about '000,000,1 of that of silver wire (Matthiesen). It is used in making safety-matches.

Metallic Phosphorus.—This, discovered by Hittorf, is obtained by heating ordinary phosphorus with lead in sealed-up tubes to redness for forty hours. After removal of the lead by nitric acid metallic phosphorus remains, partly in the shape of dark re-splendent plates, partly in the form of microscopic rhombohedra. It requires a temperature of 358° to be converted into ordinary phosphorus-vapour. The specific gravity is 2 "34 at 15° C.

Detection of Phosphorus.—The detection of (ord.) phosphorus in medico-legal cases offers no difficulty as long as the phosphorus has not disappeared by oxidation. In the case of a mass of food or the contents of a stomach the first step is to spread out the mass on a plate and view it in the dark. A very small admixture of phosphorus becomes visible by its phosphorescence. Failing this, the mass is distilled with water from out of a glass flask connected with a glass Liebig's condenser in a dark room. The minutest trace of phosphorus suffices to impart phosphorescence to the vapours at some stage of the distillation. Should this second test fail we must search for phosphorous acid, which may be there as a product of the oxidation of phosphorus originally present as such. To test for phosphoric acid would be of no use, as salts of this acid are present in all animal and vegetable juices and tissues. Phosphorous acid, if present, can be detected by treating the mass, in a properly constructed gas-evolution appa-ratus, with pure hydrochloric acid and zinc. The hydrogen gas evolved must be purified by passing it over pieces of solid caustic potash, and made to stream out of a narrow platinum nozzle. If the reagents are pure and phosphorous acid is absent the gas burns with a colourless flame, which remains so even when depressed by means of a porcelain plate ; in the presence of phos-phorous acid the gas contains phosphuretted hydrogen, which causes the flame of the gas to exhibit a green core, at least when depressed by means of a porcelain plate. The test is very _delicate, but in interpreting a positive result it must be remembered that it applies likewise to hypophosphorous acid, and that certain salts of this acid are recognized medicinal agents.

Of all phosphorus compounds ortho-phosphates are the com-monest, and they can be detected by the tests given below under "Phosphates." All other phosphorus compounds, when fused with carbonate of alkali and nitre, or heated in sealed-up tubes with strong nitric acid to a sufficient temperature, are changed so that the phosphorus assumes the form of ortho-phosphoric acid, which is easily detected. Either of the two operations named (by the mere action of the alkali or of the acid qua acid) converts what may be present of meta- phosphoric or pyro-phosphoric into ortho-phosphoric acid.

Phosphor-Bronze.—This name has been given to a class of useful metallic substances produced by the chemical union of either pure copper or of copper alloys with phosphorus. Most commercial -copper is contaminated with a small proportion of its own sub-oxide, which, in the case of an otherwise pure metal, detracts from its tenacity and plasticity ; and all ordinary bronze is subject to a similar contamination, because, whatever kind of copper may have been used in making it, the tin is sure to suffer partial oxidation, •and some of this oxide, as Montefiori-Levi and Kiinzel found, remains diffused throughout the casting, and diminishes its homo-geneity and solidity. Experience shows that both in the case of copper and bronze the oxygen present as metallic oxide can be re-moved by introduction into the fused metal of a judiciously limited proportion of phosphorus, which takes out the oxygen (and itself) into the slag as phosphate, and thus produces a purely metallic and consequently superior metal. A small excess of phosphorus in either case effects further improvement. A phosphor-copper con-taining 0'1 to 0'5 pw cent, of the non-metallic element has all the plasticity of the pure metal coupled with higher degrees of hard-ness and solidity. An alloy of from 0 • 5 to 2 • 0 per cent, gives good castings, because, unlike the pure metal, it does not form blisters on solidifying. In the case of phosphorized bronze the presence of somewhat more than 0'5 per cent, of phosphorus (in the finished alloy) produces a warmer tone of colour (more gold-like than that of the plain alloy), a finer grain (similar to that of steel), a higher degree of elasticity, and a higher breaking-strain. The latter may be more than double that of the corresponding plain bronze. By increasing or diminishing the proportion of phosphorus the mechan-ical properties of a phosphor-bronze can be modified at will, wdthin wide limits. By its fine colour and its perfect fluidity when molten it lends itself particularly well for the casting of artistic or orna-mental articles. The introduction of phosphorus into the metal is best effected by fusing it with the proper proportion of a rich phosphor-copper. A phosphor-copper containing about 9 per cent, of phosphorus can be produced as follows. A kind of potential phosphorus ("phosphorus mass") is made by mixing superphos-phate of lime with 20 per cent, of charcoal, and dehydrating the mixture at a dull red heat. Six hundred parts of this mass are mixed with 975 of copper-turnings and 75 of charcoal, and kept at copper-fusion heat for sixteen hours within a graphite crucible. The phosphor-copper is obtained in the form of detached granules, which are picked out, re-fused, and cast out into cast-iron moulds. Phosphor-bronze has only come to be popularly known during the last decade or two ; but as early as 1848 A. & H. Parkes of Bir-mingham took out a patent for phosphoriferous metallic alloys.

Phosphuretted Hydrogens.—Of these three are known, namely, (1) phosphine, a gas of the composition and specific gravity PH3, (2) a volatile liquid of the composition and vapour-density P2H4, and (3) a yellow solid of the probable composition P4H2. The liqiud compound (No. 2) at once takes fire when it comes into contact with air, and a small admixture of its vapour to any inflam-mable gas, such as coal-gas, renders the latter self-inflammable. The most important and best known of the three hydrides is phosphine, PH3. This gas is formed when (syrupy) phosphorous acid is heated—thus, 4PH303 = 3PH304 + PH3; also when phos-phorus is being dissolved in hot solutions of caustic potash, soda, or baryta,
4P + 3(KHO + H„0) = 3PH2K0.2 + PH3;
Hypophos-phite.
also by the action of water on the phosphides of highly basilous metals. The gas evolved by any of these processes is impure ; that obtained by the second or third invariably includes vapour of P2H4, and consequently is self-inflammable. Pure phosphine can be obtained only by decomposing solid iodide of phosphonium with concentrated caustic potash - ley in a suitable gas - evolution bottle previously filled with hydrogen to avoid explosions. It is a colour-less gas, smelling intensely like putrid fish, and very poisonous. It is slightly soluble in water, and takes fire in air only beyond 100° C. It may be mixed with pure oxygen without change ; but when the mixture is suddenly expanded it explodes violently. Notwith-standing its analogy to ammonia (NH3), phosphine is only very feebly basic. It unites with gaseous hydriodic or hydrobromic acid into solid phosphonium salts PH4(I or Br) ; but these are both decomposed by water into the respective acids and phosphine. Pure phosphine is little known; chemists are more familiar with the (impure) gas which is evolved when "phosphide of calcium" is thrown into water, and which, containing vapour of P2H4, at once catches fire when it bubbles out of the water into the air, with formation of steam and a smoke of meta-phosphoric acid, which latter, in a still atmosphere, assumes the form of an exquisite vortex-ring. During the last decade or so this reaction has come to be pretty extensively utilized in navigation for producing a light on the surface of the sea at night, in ease of accidents, and for other purposes. A British patent for this useful application of phosphide of calcium was granted (as No. 1828) to the agent of Silas and Pegot Ogier of Paris on the 8th of August 1859, but allowed to lapse in 1863, to be subsequently wrought by others. The manufacture of the phosphide is now (1884) being chiefly carried on by one firm (in Warrington, England), and through the courtesy of their chemist, Mr W. G. Johnston, the writer is enabled to give the following details. The preparation of the phosphide is effected within a crucible standing on a support within a furnace, and divided by a perforated false bottom into two compartments. The lower is charged with pieces of phos-phorus, the upper, up to the closely-fitting lid, with fragments of quicklime. The firing is conducted so that the lime is red hot before the phosphorus, through the radiation and conduction of the heat applied above, begins to volatilize. A charge yielding 20 lb of product is finished in from five to eight hours. The reaction is very complex, but it is easy to see through its general course ; part of the phosphorus deoxidizes lime with formation of P205, which unites with other lime into phosphate, and of calcium, which combines with other phosphorus into phosphides. Of the latter, PCa seems to predominate, and consequently the product, when thrown into water, should yield chiefly the hydride P„H4 ; but this latter very readily breaks up into phosphine and solid hydride P2H. The crude phosphide forms a brown stone-like mass, which must at once be secured in air-tight receptacles. But most of it is immediately worked up into "lights" of various kinds, of which the "life-buoy light" may be selected as an example. It consists of a cylindrical tinned-iron box, the upper half of which is taken up by an inverted hollow box, which serves as a float when the light is in the water. The lower half contains some 16 oz. of fragments of phosphide of calcium. Two small circular portions of the top and bottom respectively consist of soft metal (lead). These are pierced with an appended pricker before the apparatus goes overboard along with the buoy, to which it is attached by means of a cord. The water penetrates through the lower hole and the gas comes out through the upper and burns with a brilliant flame, which is from 9 to 18 inches high and lasts for about half an hour. A larger similar contrivance, intended to be accommodated within a bucket full of water on deck, serves as an inextinguishable night-signal to ships in distress. By the British Merchant Shipping Act, 1876, Vict. 21, every sea-going passenger-steamer and every emigrant-ship must be provided with arrangements for inextinguishable distress - lights and life-buoy lights. In the British navy a peculiar form of the phosphide of calcium light is used in connexion with torpedo-practice.





Phosphorus Bases.—This is a generic name for organic bases which are related to phosphine (PH3), as the "compound ammonias " are to NH3. See CHEMISTRY, vol. v. p. 516 s^.; also METHYL, vol. xvi. p. 197. Tri-ethyl phosphine P(C2H5)3, a colourless self-inflammable liquid, readily unites with bisulphide of carbon into a red crystal-line compound, and consequently is available as a delicate reagent for the detection of the vapour of this compound in coal-gas.

PHOSPHATES.

" Phosphates," in chemistry, is a generic term for the salts formed by the union of the acid-anhydride P205 with bases or water or both. As explained in CHEMISTRY (vol. v. pp. 517, 518) there are three classes of phosphates customarily distinguished by the prefixes ortho, pyro, and meta. The last two nowhere occur in nature, and are hardly known to the arts ; hence in this article only the ortho-compounds will be noticed, and their specific prefix will be dropped except where it is needed for definiteness. Combined phosphoric acid is universally diffused throughout the three kingdoms of nature, and (it is perhaps as well to add) to the practical, if not absolute, exclusion of all other phosphorus compounds. All organic tissues and juices contain it: of animal matters bones and blood-solids, of vegetable the seeds of cereals, may be referred to as being exceptionally rich in phosphates. Of mineral phosphates the follow-ing may be here referred to :—pyro-morphite, 3(P205. 3PbO) + PbCl,, where the chlorine may be replaced partially by fluorine; wavellite, 2(Al203.P205) + Al2033H>0 + 9Aq (this is a crystalline mineral; an amorphous or massive phosphate of alumina, known as " rotondo-mineral," occurs as a large deposit onaWest Indian island); vivianite, P2063FeO + 8H20. All these and any others that might be named are rare minerals compared with apatite and its derivatives.

Apatite.—This exists in a variety of forms, but, as long as unde-composed, always answers the formula 3(P205.3CaO) + (CaX2). In the fluor-apatites the X2 is wholly F2 (fluorine); in the chlor-apatites it stands for (Clj, F)2, i.e., chlorine and fluorine coming up conjointly to two equivalents. See vol. xvi. p. 407.

Phosphorites.—Phosphorite is the name given to many impure forms of amorphous or massive apatite, modified more or less by disintegration. It occurs (a) in massive, irregular, corroded-looking nodules embedded in limestone or other kinds of soft rock near Amberg (Bavaria), in Baden, Wiirtemberg, the Weser hills, and in the Teutoburger Wald, and contains from 40 to 80 per cent, of phosphate and up to 3 per cent, of fluoride of calcium ; the phos-phorite nodules in the sandstone of Kursk and Voronezh, the "South Carolina phosphate," and the "Lot phosphate" belong to the same category. It is met with (b) in more or less extensive beds, as "kidneys," as stalactites, or as a connective cement in breccias ; such phosphorite, of which large quantities are found in the Lahn valley, generally contains only from 25 to 60 per cent, of phosphate of lime, and includes large percentages of clay or marl, and more or less of the phosphates of iron and alumina. Another variety is (c) black phosphorite slate. A deposit contain-ing 20 per cent, of P«05 occurs in the Coal-measures of Horde (Westphalia), also in Wales; an earthy deposit is found in the " braunkohle " of Pilgramsreuth in the Fichtelgebirge. Phosphorite is also found (d) in veins, as a stone of very varying structure, generally intermixed with quartz,—for instance at Logrosan in Estremadura (65 to 80 per cent, of phosphate and up to 14 per cent, of fluoride of calcium), also in the Silurian slate of the Dniester.

Goprolites.—According to Buckland, coprolites are derived from the excrements of extinct animals. They consist of highly impure phosphate of lime. All native phosphate of calcium being fluor-iferous, we need not wonder at the constant occurrence of traces of fluorides in the bones of vertebrate animals ; the wonder is that the fluorine in these amounts to only '005 per cent.

Preparation. —For the preparation of phosphates the oxide P206 affords a natural starting-point. This substance is produced when phosphorus burns in an abundant supply of oxygen or air. Appa-ratus for the convenient execution of the process on a preparative scale are described in the handbooks of chemistry. Phosphoric anhydride forms a snow-white, loose, inodorous powder, which, when heated in a hard glass tube to redness, sublimes slowly. It is extremely hygroscopic. When thrown into water it hisses like a red-hot iron and passes into the meta-acid, most of which, in spite-of its abundant solubility, separates out as a sticky precipitate, which is rather slow in dissolving. It is the most energetic of all dehydrating agents ; even sulphuric acid, when distilled with an excess of it, suffers dehydration, and passes into S03. The pre-paration is liable to be contaminated with red phosphorus and phosphorous anhydride (P203), also with "white arsenic," because most commercial phosphorus, being made by means of pyrites- vitriol, is arseniferous. A freshly-prepared solution of the anhydride in water, being one of the meta-acid, coagulates albumen (as HN03 does) and gives a wdiite precipitate with nitrate of silver. But, when the solution is allowed to stand, the dissolved meta-acid gradually passes into pyro-acid (P,062H20), and this latter again gradually passes into ortho-acid (P2053H20), the highest hydrate. At a boiling heat, especially if a little nitric acid be added, the whole of the dissolved P205 is converted into ortho-acid in the course of one or two hours. The solution then does not coagulate albumen ; it gives no precipitate with nitrate of silver unless the mixture be neutralized with an alkali, when a yellow precipitate of the salt P205.3Ag20 comes down. The aqueous ortho-acid, when evaporated at temperatures not exceeding 160° C, and ultimately dried at this temperature, leaves its substance P205.3H20 as a thick syrup, which, when left to itself in a dry atmosphere, slowly freezes into crystals. At 215° C. the ortho-acid loses one-third of its water and becomes pyro-acid ; at a red-heat it is reduced to a "glass" of meta-acid, P206H20, which retains its water even at the highest temperatures. The substance known in pharmacy as "acidum phosphoricum glaciale " is very impure meta-acid.

Ortho-Phosphoric Acid, H3P04.—The synthetical method de-scribed in the last paragraph is not so easy in practice as it appears on paper ; hence it is generally preferred to prepare this substance by the oxidation of ordinary phosphorus with nitric acid. An acid of l-2 specific gravity works best; weaker acid acts too slowly ; if stronger it may act wdth dangerous violence. One part of phos-phorus is placed in a large tubulated retort, connected wdth an ordinary globular receiver, and treated therein, at a carefully regu-lated heat, with ten or twelve parts of the acid. When about half the acid has distilled over, it is poured back and the opera-tion resumed and kept on until all the phosphorus is dissolved. The excess of nitric acid is then distilled over as far as conveniently possible and thus recovered. Towards the end of the distillation a fresh gas-evolution sets in through the conversion of previously produced phosphorous acid (H3P03) into phosphoric. The residual liquid in the retort is now poured out into a Berlin porcelain (or, what is better, a platinum) basin, and, if it still contains phosphor-ous acid, fully oxidized by evaporation with occasional addition of strong nitric acid. Phosphorous acid, if present, is easily detected by the following tests: (1) its solution, when mixed with nitrate of silver and excess of ammonia, gives a black precipitate of metallic silver; (2) when heated wdth a solution of corrosive sublimate, HgCl„ it produces a white precipitate of calomel, HgCl; (3) when, heated to boiling with excess of aqueous sulphurous acid it gives a precipitate of sulphur, or, if arsenious acid is present, of sulphide, of arsenic. Wher. the final oxidation is accomplished the acid needs only be freed of the remnant of nitric acid by repeated evaporation with water to be ready for use if arsenic be absent. As a rule, however, this impurity is present and must be removed by diluting the acid, passing in sulphuretted hydrogen first at 70° O, and then in the cold, and allowing to stand for twenty-four hours, when all the arsenic is converted into sulphide, which, after elimination of the excess of sulphuretted hydrogen by continued exposure to air at a gentle heat, is filtered off. In practice, as a rule, the filtrate is being concentrated to some predetermined specific gravity and preserved as aqueous phosphoric acid, which preparation is official, and used besides for the cleansing of metallic surfaces, in lithography, and for other purposes. The British pharmacopoeia prescribes for the official acid a strength corresponding to 10 per cent, of P205.

Hager has published a complete table showing the dependence-of the specific gravity, taken at 17°'5 O, on the strength of the acid. From it the following is extracted.

== TABLE ==

Aqueous phosphoric acid has all the properties of a decided acid, but, for a mineral acid, the exceptional qualities of an agreeably sour taste and of rao«-poisonousness. Phosphoric is the only mineral acid which might be used as a condiment in place of vinegar or citric acid ; but the writer is far from recommending the substi-tution. Professor Gamgee has made the very surprising discovery that meta-phosphoric and pyro-phosphoric, although so closely allied to ortho-phosphoric acid, are poisons, as phosphorous acid is.
Phosphoric acid readily combines with and neutralizes alkalis, even when these are given as carbonates. The concentrated acid, when heated in porcelain or glass, strongly attacks either material; hence its concentration ought always to be effected in platinum. In former times, when phosphorus was expensive, the acid, or rather an apology for the same, used to be prepared from bone-ash.

Alkaline Phosphates.—Of these the di-sodic salt is of the greatest practical importance. It is prepared by somewhat more than neutralizing the hot aqueous acid with carbonate of soda. A cheaper (manufacturing) process is to prepare a solution of "super-phos-phate " from bone-ash by the action of vitriol, and, after elimination of the gypsum, to supersaturate the liquid with carbonate of soda and filter off the phosphate of lime produced (see p. 815 supra, where the process is explained indirectly). The salt, from sufficiently strong hot solutions, separates out in large transparent crystals of the composition P04HNa2 +12H20, which lose their crystal-water on exposure to dry air, even at ordinary temperatures, and very quickly at 100° C. The residue, P04HNa2= ,(P205.2Na2O.H20), when heated to redness, loses its remnant of water and becomes pyro-phosphate, which latter retains its specific character on being dissolved in water. A solution of the (original) salt in water has a mild taste (hence its preferential application as a pleasant purga-tive) ; it colours red litmus-paper intensely blue, and does not act upon alkaline carbonate. But, wdien evaporated with the calculated proportion of carbonate of soda (Na2C03 per P203) to dryness at, ultimately, a red heat, it yields a residue of tri-sodic salt (P04Na3) as a white mass, infusible at the highest temperature producible within a platinum crucible over a glass blowpipe. The solution of this salt in water has all the properties of a mixed solution of P04lSra2H + NaOH ; yet it is capable of depositing crystals of the composition P04Na3 + 12H,0. The mono-sodic salt (P04H2Na), producible by mixing together solutions containing the quantities H3P04 and Na2HP04, is of no importance. Of the three potash salts, the mono-metallic salt (P04ICH2) is the most readily produced. It forms beautiful anhydrous quadratic crystals which at a red heat lose their H20 and become meta-phosphate, P03K.

Ammonia Salts.—A strong solution of the acid, when super-saturated with ammonia, deposits on cooling crystals of the di-arnmonic salt P04(NH4)2H, liable to be contaminated with the mono-ammonic salt. The tri-ammonic salt is very unstable, and hardly known.

The double salt P04(NH4)NaH + 4H20 was known to the al-chemists as "sal mieroeosmicum urinse," and is interesting his-torically as having served Brand as a raw material for the mak-ing of phosphorus. It is easily prepared, either by mixing the . solution of the two quantities P04Na2HP04 and P04(NH4)2HP04 together and allowing to crystallize, or by dissolving the foimer along with NH4C1 parts of sal-ammoniac in water, and removing the chloride of sodium produced by crystallization in the heat. Microcosmic salt, when heated to redness, leaves a viscid glass of meta-phosphate of soda, which dissolves all basic metallic oxides pretty much as fused borax does, with formation of glasses which often exhibit colours characteristic of the dissolved oxides. Hence its application in blowpipe analysis.

Phosphates of Lime.—The normal salt P205.3Ca0 or P04ca3, where ca = JCa=one equivalent of calcium, or perhaps a compound of it and carbonate of lime, forms the predominating component of bone-ash. A hydrate of the salt is produced by precipitating chloride of calcium solution with excess of ordinary phosphate of soda, mixed with enough of ammonia to produce (virtually) tri-alkaline salt, as a gelatinous precipitate similar in appearance and behaviour on filtration to precipitated alumina. A suspension of this precipitate in water, when mixed wdth a carefully adjusted quantity of hydrochloric acid, gradually passes into a mass of microscopic crystals of di-calcic salt, P04ca2H + a;Aq, wdiich latter is used medicinally. A solution of the di-calcic or tri-calcic salt, in the proper proportion of hot aqueous hydrochloric acid, deposits on cooling crusts of crystals of the mono-calcic salt P04H2ca, wdiich is soluble in about 700 parts of cold water, but is decomposed, by hot water or by prolonged contact with a proportion of cold water insufficient to dissolve it, into free acid and a precipitate of di-calcic salt, 2P04caH.2=P04H3 + P04ca2H. A very impure form of this salt, known as "superphosphate," enters into the composition of many artificial manures. Such superphosphate is made in-dustrially by treating broken-up bones, or powdered bone-ash, or powdered phosphorite, or coprolite, or occasionally apatite with chamber-acid, meaning vitriol of about 60 per cent., as it comes out of the chamber. The phosphate is mixed with the acid in a lead-lined trough by means of machinery, when a rather lively reaction sets in, involving the evolution of vapour of water mixed with hydrofluoric acid, and fluoride of silicon if mineral phosphate is used, possibly also with traces of fluoride or chloride of arsenic, and, in any case, with stinking volatile organic substances. The vapour, therefore, must bo removed by means of suitable draught arrangements. The mass passes from the trough into a (ventilated) chamber, wdiere the reaction gradually accomplishes itself with ulti-mate formation of a porous friable mass, dry to the touch. This is superphosphate as it goes out into commerce or is used as an ingredi-ent in making more complex manures. Its value is determined chiefly by its percentage of'' soluble phosphoric acid," meaning the percent-age of P205, extractable as P04fi3 or P04caH2 by a certain large proportion of cold water. This percentage is liable to decrease on long-continued storing, especially in the case of mineral superphos-phate, through a gradual formation of (or regeneration of origin-ally present) phosphate of iron and alumina, partly, perhaps, also through the spontaneous decomposition of some of the mono-calcic salt into insoluble di-calcic salt and free acid. The portion of the P205 which has thus become insoluble is designated "reduced" phosphoric acid. In regard to other phosphates than those named reference may be made to the handbooks of chemistry.

Analysis.—Phosphoric acid, when given in any form, soluble in solution of ammonia, can be detected and determined by " magnesia mixture " (a solution of chloride of magnesium and sal-ammoniac, MgCl2. 2NH4C1, strongly alkalinized by addition of aqueous am- monia). The phosphoric acid is very gradually, but at last completely, precipitated in microscopic crystals of the salt P04MgNH4 + 6H20, wdiich, though slightly soluble in water, can be washed pure, with- out loss, with dilute ammonia. All other acids except arsenic acid (As205)—which behaves like phosphoric, and, if present, must be removed by sulphuretted hydrogen—remain dissolved. The precipi- tate, when kept at a red heat, assumes the composition P2052MgO, and from the weight of the ignited precipitate that of the phos- phoric acid present is easily calculated. Phosphates soluble in acids, and reprecipitated from their solutions as such by ammonia—as phosphate of lime or alumina, or ferric oxide—used to give great difficulties to the analysts until Sonnenschein founded an excellent quantitative method for their analysis upon a reaction discovered by Swanberg and Struve, which is explained under MOLYBDENUM (vol. xvi. p. 697). The phosphate is dissolved in nitric acid (hydrochloric is less to be recommended) and the solution mixed, and kept for some hours at 40° O, with a large excess of a solution of molybdate of ammonia in excess of nitric acid. The phosphoric acid (along with any arsenic acid that may be present) comes down as yellow crystalline phospho-molybdate of ammonia, soluble in phos- phoric acid and slightly in water, but insoluble in dilute nitric acid in the presence of a sufficiency of nitrate of ammonia. The precipitate is soluble in aqueous ammcnia, and from the solution its P205 can be precipitated by magnesia mixture as above explained. Neither of the two methods applies directly to meta-phosphates or pyro- phosphates. Regarding these, see the last paragraph of the section "phosphorus" above. (W. D.)

Footnote

818-1 Some books (Nickles) quote as high percentages as 1 or 1-5, but these are based on erroneous analyses.



The above article was written by: Prof. W. Dittmar.



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