1911 Encyclopædia Britannica/Phosphates
PHOSPHATES, in chemistry, the name given to salts of phosphoric acid. As stated under Phosphorus, phosphoric oxide, P2O5, combines with water in three proportions to form H2O·P2O5 or HPO3, metaphosphoric acid; 2H2O·P2O5 or H4P2O7, pyrophosphoric acid; and 3H2O·P2O5 or H3PO4, orthophosphoric or ordinary phosphoric acid. These acids each give origin to several series of salts, those of ordinary phosphoric acid being the most important, and, in addition, are widely distributed in the mineral kingdom (see below under Mineral Phosphates).
Orthophosphoric acid, H3PO4, a tribasic acid, is obtained by boiling a solution of the pentoxide in water; by oxidizing red phosphorus with nitric acid, or yellow phosphorus under the surface of water by bromine or iodine, and also by decomposing a mineral phosphate with sulphuric acid. It usually forms a thin syrup which on concentration in a vacuum over sulphuric acid deposits hard, transparent, rhombic prisms which melt at 41·7°. On long heating the syrup is partially converted into pyrophosphoric and metaphosphoric acids, but on adding water and boiling the ortho-acid is re-formed. It gives origin to three classes of salts: M′H2PO4 or M″H4P2O8; M′2HPO4 or M″HPO4, M′3PO4, M″3P2O8 or M‴PO4, wherein M′, M″, M‴ denote a mono, di- and tri-valent metal. The first set may be called monometallic, the second dimetallic, and the third trimetallic salts. Per-acid salts of the alkalis, e.g. (K,Na,NH4)H5(PO4)2, are also known; these may be regarded as composed of a monometallic phosphate with phosphoric acid, thus M′H2PO4 H3PO4. The three principal groups differ remarkably in their behaviour towards indicators. The monometallic salts are strongly acid, the dimetallic are neutral or faintly alkaline, whilst the soluble trimetallic salts are strongly alkaline. The monometallic salts of the alkalis and alkaline earths may be obtained in crystal form, but those of the heavy metals are only stable when in solution. The soluble trimetallic salts are decomposed by carbonic acid into a dimetallic salt and an acid carbonate. All soluble orthophosphates give with silver nitrate a characteristic yellow precipitate of silver phosphate, Ag3PO4, soluble in ammonia and in nitric acid Since the reaction with the acid salts is attended by liberation of nitric acid: NaH2PO4+3AgNO3＝Ag3PO4+NaNO3 +2HNO3, Na2HPO4+3AgNO3＝Ag3PO4+2N3NO3+HNO3, it is necessary to neutralize the nitric acid if the complete precipitation of the phosphoric acid be desired. The three series also differ when heated: the trimetallic salts, containing fixed bases are unaltered, whilst the mono- and dimetallic salts yield meta- and pyrophosphates respectively If the heating be with charcoal, the trimetallic salts of the alkalis and alkaline earths are unaltered, whilst the mono- and di-salts give free phosphorus and a trimetallic salt. Other precipitants of phosphoric acid or its salts in solution are: ammonium molybdate in nitric acid, which gives on heating a canary-yellow precipitate of ammonium phosphomolybdate, 12[MoO3] (NH4)3PO4, insoluble in acids but readily soluble in ammonia; magnesium chloride, ammonium chloride and ammonia, which give on standing in a warm place a white crystalline precipitate of magnesium ammonium phosphate, Mg(NH4)PO4·6H2O, which is soluble in acids but highly insoluble in ammonia solutions, and on heating to redness gives magnesium pyrophosphate, Mg2P2O7; uranic nitrate and ferric chloride, which give a yellowish-white precipitate, soluble in hydrochloric acid and ammonia, but insoluble in acetic acid, mercurous nitrate which gives a white precipitate, soluble in nitric acid, and bismuth nitrate which gives a white precipitate, insoluble in nitric acid.
Pyrophosphoric acid, H4P2O7, is a tetrabasic acid which may be regarded as derived by eliminating a molecule of water between two molecules of ordinary phosphoric acid, its constitution may therefore be written (HO)2OP·O·PO(OH)2. It may be obtained as a glassy mass, indistinguishable from metaphosphoric acid, by heating phosphoric acid to 215°. When boiled with water it forms the ortho-acid, and when heated to redness the meta-acid. After neutralization, it gives a white precipitate with silver nitrate. Being a tetrabasic acid it can form four classes of salts; for example, the four solium salts Na4P2O7, Na3HP2O7, Na2H2P, O7, NaH3P2O7 are known. The most important is the normal salt, Na4P2O7, which is readily obtained by heating disodium orthophosphate, Na2HPO4. It forms monoclinic prisms (with 10H2O) which are permanent in air. All soluble pyrophosphates when boiled with water for a long time are converted into orthophosphates.
Metaphosphoric acid, HPO3, is a monobasic acid which may be regarded as derived from orthophosphoric acid by the abstraction of one molecule of water, thus H3PO4—H2O＝HPO3, its constitution is therefore (HO)PO2. The acid is formed by dissolving phosphorus pentoxide in cold water, or by strongly heating orthophosphoric acid It forms a colourless vitreous mass, hence its name “glacial phosphoric acid.” It is readily soluble in water, the solution being gradually transformed into the ortho-acid, a reaction which proceeds much more rapidly on boiling Although the acid is monobasic, salts of polymeric forms exist of the types (MPO3)n, where n may be 1, 2, 3, 4, 6. They may be obtained by heating a monometallic orthophosphate of a fixed base, or a dimetallic orthophosphate of one fixed and one volatile base, e.g. microcosmic salt: MH2PO4＝MPO3+H2O, (NH4) NaHPO4= NaPO3+NH3+H2O; they may also be obtained by acting with phosphorus pentoxide on trimetallic orthophosphates: Na3PO4+P2O5＝3NaPO3. The salts are usually non-crystalline and fusible. On boiling their solutions they yield orthophosphates, whilst those of the heavy metals on boiling with water give a trimetallic orthophosphate and orthophosphoric acid: 3AgPO3+3H2O＝Ag3PO4+2H3PO4. On heating with an oxide or carbonate they yield a trimetallic orthophosphate, carbon dioxide being evolved in the latter case. Metaphosphoric acid can be distinguished from the other two acids by its power of coagulating albumen, and by not being precipitated by magnesium and ammonium chlorides in the presence of ammonia.) (C. E.*)
Mineral Phosphates.—Those varieties of native calcium phosphate which are not distinctly crystallized, like apatite (q.v.), but occur in fibrous, compact or earthy masses, often nodular, and more or less impure, are included under the general term phosphorite. The name seems to have been given originally to the Spanish phosphorite, probably because it phosphoresced when heated. This mineral, known as Estremadura phosphate, occurs at Logrossan and Cáceres, where it forms an important deposit in clay-slate. It may contain from 55 to 62% of calcium phosphate, with about 7% of magnesium phosphate. A somewhat similar mineral, forming a fibrous incrustation, with a mammillary surface, and containing about 9% of calcium carbonate, is known as staffelite, a name given by A. Stein in 1866 from the locality Staffel, in the valley of the Lower Lahn, where (as also in the valley of its tributary the Dill) large deposits of phosphorite occur. Dahllite is a Norwegian phosphorite, containing calcium carbonate, named in 1888 by W. C. Brögger and H. Bäckström after the Norwegian geologists T. and J. Dahll. Osteolite is a white earthy phosphorite occurring in the clefts of basaltic rocks, named in 1851 by J. C. Bromeis from the Greek ὀστέον, bone.
Phosphorite, when occurring in large deposits, is a mineral of much economic value for conversion into the superphosphate largely used as a fertilizing agent. Many of the impure substances thus utilized are not strictly phosphorite, but pass under such names as “rock-phosphate,” or, when nodular, as “coprolite” (q.v.), even if not of true coprolitic origin. The ultimate source of these mineral phosphates may be referred in most cases to the apatite widely distributed in crystalline rocks. Being soluble in water containing carbonic acid or organic acids it may be readily removed in solution, and may thus furnish plants and animals with the phosphates required in their structures. On the decay of these structures the phosphates are returned to the inorganic world, thus completing the cycle.
There are three sources of phosphates which are of importance geologically. They occur (a) in crystalline igneous and metamorphic rocks as an original constituent, (b) in veins associated with igneous rocks, and (c) in sedimentary rocks either as organic fragments or in secondary concretionary forms.
The first mode of occurrence is of little significance practically, for the crystalline rocks generally contain too little phosphate to be valuable, though occasionally an igneous rock may contain enough apatite to form an inferior fertilizing agent, e.g. the trachyte of Cabo de Gata in south-east Spain, which contains 12–15% of phosphoric acid. In many deposits of iron ores found in connexion with igneous or metamorphic rocks small quantities of phosphate occur. The Swedish, Norwegian, Ontario and Michigan mines yield ores of this kind; and though none of them can be profitably worked as a source of phosphate, yet on reducing the ore it may be retained in the slags, and thus rendered available for agriculture.
Another group of phosphatic deposits connected with igneous rocks comprises the apatite veins of south Norway, Ottawa and other districts in Canada. These are of pneumatolytic origin (see Pneumatolysis), and have been formed by the action of vapours emanating from cooling bodies of basic eruptive rock. Veins of this type occur at Oedegarden in Norway and Dundret in Lapland. From 1500 to 3500 tons of apatite are obtained yearly in Norway from these veins. In Ontario apatite has been worked for a long time in deposits of similar nature. The total output of Canada in 1907 was only 680 tons.
The phosphatic rocks which occur among the sedimentary strata are the principal sources of phosphates for commerce and agriculture. They are found in formations of all ages from the Cambrian to those which are accumulating at the present day. Of the latter the best known is guano (see Manures and Manuring). Where guano-beds are exposed to rain their soluble constituents are removed and the insoluble matters left behind. The soluble phosphates washed out of the guano may become fixed by entering into combination with the elements of the rock beneath. Many of the oceanic islets are composed of coral limestone, which in this way becomes phosphatized; others are igneous, consisting of trachyte or basalt, and these rocks are also phosphatized on their surfaces but are not so valuable, inasmuch as the presence of iron or alumina in any quantity renders them unsuited for the preparation of artificial manures.
The leached guanos and phosphatized rocks, which are grouped with them for commercial purposes, have been obtained in great quantities in many islands of the Pacific Ocean (such as Baker, Howland, Jarvis and McKean Islands) between long. 150° to 180° W. and lat. 10° N. to 10° S. In the West Indies from Venezuela to the Bahamas and in the Caribbean Sea many islands yield supplies of leached guanos; the following are important in this respect Sombrero, Navassa, Aves, Aruba, Curaçoa. Christmas Island has been a great source of phosphates of this type; also Jaluit Island in the Maldive Archipelago, Banaba or Ocean Island, and Nauru or Pleasant Island. On Christmas Island the phosphate has been quarried to depths of 100 ft. To these leached guanos and phosphatized limestones the name sombrerite has been given. It has been estimated that 500,000 tons of phosphate were obtained in Aruba, 1,000,000 tons from Curaçoa since the deposits were discovered in 1870, and Christmas Island in 1907 yielded 290,000 tons.
In the older formations the phosphates tend to become more and more mineralized by chemical processes. In whatever form they were originally deposited they often suffer complete or partial solution and are redeposited as concretionary lumps and nodules, often called coprolites. The “Challenger” and other oceanographic expeditions have shown that on the bottom of the deep sea concretions of phosphate are now gathering around the dead bodies of fishes lying in the oozes, consequently the formation of the concretions may have been carried on simultaneously with the deposition of the strata in which they occur.
Important deposits of mineral phosphates are now worked on a large scale in the United States, the annual yield far surpassing that of any other part of the world. The most active operations are carried on in Florida, where the phosphate was first worked in 1887 in the form of pebbles in the gravels of Peace river. Then followed the discovery of “hard rock-phosphate,” a massive mineral, often having cavities lined with nearly pure phosphorite. Other kinds not distinctly hard and consisting of less rich phosphatic limestone, are known as “soft phosphate”: those found as smooth pebbles of variable colour are called “land pebble-phosphate,” whilst the pebbles of the river-beds and old river-valleys, usually of dark colour, are distinguished as “river pebble-phosphate.” The land pebble is worked in central South Florida; the hard rock chiefly between Albion and Bay City. In South Carolina, where there are important deposits of phosphate, formerly more productive than at present, the “land rock” is worked near Charleston, and the “river rock” in the Coosaw river and other streams near Beaufort. The phosphate beds contain Eocene fossils derived from the underlying strata and many fragments of Pleistocene vertebrata such as mastodon, elephant, stag, horse, pig, &c. The phosphate occurs as lumps varying greatly in size, scattered through a sand or clay; they often contain phosphatized Eocene fossils (Mollusca, &c.). Sometimes the phosphate is found at the surface, but generally it is covered by alluvial sands and clays. Phosphate mining began in South Carolina in 1868, and for twenty years that state was the principal producer. Then the Florida deposits began to be worked. In 1892 the phosphates of Tennessee, derived from Ordovician limestones, came into the market. From North Carolina, Alabama and Pennsylvania, also, phosphates have been obtained but only in comparatively small quantities. In 1900 mining for phosphates was commenced in Arkansas. In 1908 Florida produced 1,673,651 tons of phosphate valued at 11 million dollars. All the other states together produce less phosphate than Florida, and among them Tennessee takes the first place with an output of 403,180 tons.
Algeria contains important deposits of phosphorite, especially near Tebessa and at Tocqueville in the province of Constantine. Near Jebel Kouif, on the frontier between Algeria and Tunis, there are phosphate workings, as also in Tunis, at Gafsa. The deposits belong to the Lower Eocene, where it rests unconformably upon the Cretaceous. The joint production of Tunis and Algeria in 1907 was not less than a million tons. Phosphates occur also in Egypt, in the desert east of Keneh and in the Dakla oasis in the Libyan desert.
France is rich in mineral phosphates, the chief deposits being the departments of the Pas-de-Calais, Somme, Aisne, Oise in and Meuse, in the north-east, and another group in the departments of Lot, Tarn-et-Garonne and Aveyron, in the south-west: phosphates occur also in the Pyrenees. The deposits near Caylus and in Quercy occupy fissures and pockets in Jurassic limestone, and have yielded a remarkable assemblage of the relics of Tertiary mammals and other fossils. Phosphates occur in Belgium, especially near Mons, and these, like those of north-east France, are principally in the Upper Chalk. Two varieties of phosphate rock are recognized in these districts, viz. the phosphatic chalk and the phosphate sand, the latter resulting from the decomposition of the former. Large and valuable deposits of the sand have been obtained in sinks and depressions on the surface of the chalk. The production is on the whole diminishing in Belgium (180,000 tons in 1907), but in France it is still large (375,000 tons in 1907).
In the Lahn district of Nassau (Germany) there are phosphate beds in Devonian rocks. The deposits were rich but irregular and local, and were much worked from 1866 to 1884, but are no longer of economic importance. In northern Estremadura in Spain and Alemtezo in Portugal there are vein deposits of phosphate of lime. As much as 200,000 tons of phosphate have been raised in these provinces, but in 1906 the total production of Spain was only 1300 tons. Large deposits of phosphate occur in Russia, and those in the neighbourhood of Kertch have attracted some attention, it is said that the Cretaceous rocks between the rivers Dniester and Volga contain very large supplies of phosphate, though probably of low grade.
Phosphatic nodules and concretions, with phosphatized fossils and their casts, occur at various geological horizons in Great Britain. Bands of black nodules, highly phosphatic, are found at the top of the Bala limestone in North Wales; beds of concretions occur in the Jurassic series; and important deposits are known in the Cretaceous strata, especially in the Lower Greensand and at the base of the Gault. The Lower Greensand phosphates have been worked, under the name of “coprolites,” at Potton in Bedfordshire and at Upware and Wicken in Cambridgeshire. The Cambridge Greensand, rich in phosphatic nodules, occurs at the base of the Chalk Marl. The chalk occasionally becomes phosphatized, as at Taplow (Bucks) and Lewes (Sussex). At the base of the Red Crag in East Anglia, and occasionally at the base of the other Pliocene Crags, there is a “nodule bed,” consisting of phosphatic nodules, with rolled teeth and bones, which were formerly worked as “coprolites” for the preparation of artificial manure Professor R. J. Strutt has found that phosphatized nodules and bones are rich in radioactive constituents, and has brought this into relation with their geological age.
Bibliography.—For American phosphates see The Phosphates of America, by Francis Wyatt (5th ed., New York and London, 1894); the Annual Reports on Mineral Resources of the U.S. (U.S. Geol. Survey), including some valuable reports by C. W. Hayes, also those in Rothwell’s Mineral Industry; “Nature and Origin of Deposits of Phosphate of Lime,” by R. A. F. Penrose, Jun., Bull. U.S. Geol. Survey, No. 46 (1888); Florida, South Carolina and Canadian Phosphates, by C. C. Hoyer Miller (London, 1892); and The Non-metallic Minerals, by G. P. Merrill (1904). Many of the above include descriptions of mineral phosphates in other parts of the world. For a general discussion of the origin of the phosphates, see “The Natural History of Phosphate Deposits,” by, J. J. H. Teall, Proc. Geol. Assoc. xvi. 369 (1900). Consult also Étude complète sur les phosphates, by A. Deckers (Liége, 1894). (J. S. F.; F. W. R.*)