with phosphoric acid, thus M′H2PO4 H3PO4. The three principal
groups differ remarkably in their behaviour towards indicators.
The mono metallic salts are strongly acid, the bimetallic are
neutral or faintly alkaline, whilst the soluble trimetallic salts
are strongly alkaline. The mono metallic 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 bimetallic 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 tetra basic 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 meta phosphoric acid, by heating phosphoric acid to 215°. When boiled with water it forms the ortho-acid, and when heated to redness the metaacid. 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 mono basic 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 pent oxide 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 mono basic, 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 mono metallic orthophosphate of a fixed base, or a bimetallic 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 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 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. Brogger and H. Bäckström after the Norwegian geologists T. and T. 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 super phosphate 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 witlx 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
