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Encyclopedia Britannica - Main :: PER-PIG |
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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 H2OP2O5 or HPO3, metaphosphoric acid; 2H2OP205 or H4P2O7, pyrophosphoric acid; and 3H2OP206 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 41.70. On long heating the syrup is partially converted into pyro - phosphoric 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"H4P203; M'2HPO4 or M"HPO4, M'3PO4, M"3P208 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 3AgP03+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 mag- nesium 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 Caceres, 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 some-what 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
the Greek oarEov, 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 sub-stances 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
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 1215% 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 68o 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 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+2NaNO3+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
standing
Pyrophosphoric acid, H4P207, 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)20P.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, Na2H2P2O7, 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 IoH2O) which are permanent in air. All soluble pyrophosphates when boiled with water for a long time are converted into orthophosphates. Meta phosphoric acid, HPO3, is a monobasic acid which may be regarded as derived from orthophosphoric acid by the abstraction of one molecule of water, thus H3PO4H2O=HP'03; 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 orthoacid, a reaction which proceeds much more rapidly on boiling. Although the acid is monobasic, salts of polymeric forms exist of the types (MPO3),,, 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+H20, (NH4) NaHPO4= NaPO3+NH3+HDO; they may also be obtained by acting with phosphorus pentoxide on trimetallic orthophosphates: Na3PO4+P203=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: 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 18o W. and lat. 10 N. to to 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, Curacoa. 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 too 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, t,000,000 tons from Curacoa since the deposits were discovered in 187o, 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
Algeria contains important deposits of phosphorite, especially near Tebessa
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
In the Lahn
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
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.(J. S. F.; F. W. R.) End of Article: PHOSPHATES If you wish, you can link directly to this article.
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