Phosphates

PHOSPHATES, salts of phosphoric acid. Phosphoric oxide, P205 or P4010 (Tilden and Barnett, v. inf .), combines with water in three proportions to form or metaphosphoric acid; or pyrophosphoric acid; and or orthophosphoric or ordinary phosphoric acid. These acids each give origin to several series of salts, those of ordinary phosphoric acid being the most important.

Orthophosphoric acid, a tribasic acid, is obtained by boiling a solution of the pentoxide in water; by oxidizing red phosphorus with nitric acid, or white phosphorus under the sur face 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° C. After long heating the syrup is partially converted into pyro phosphoric and metaphosphoric acids, but on addition of water and boiling the ortho-acid is re-formed. It gives origin to three classes of salts: or M'2HPO4 or M"HPO4; and M"3(PO4)2 or wherein M', M", M"' denote a uni-, bi-, and trivalent metal. These correspond to H3PO4 in which one, two or three atoms of hydrogen, respectively, have been replaced by a metal. The three principal groups differ re markably in their behaviour towards indicators. Those of the first type are strongly acid, the second are neutral or faintly alkaline, whilst the third are strongly alkaline and are decomposed by car bon dioxide : The salts of the types NaH2PO4 and may be obtained in crystal form, but those of the heavy metals are only stable when in solution. 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 = + + Na2HPO4+3AgNO3= it is necessary to neutralize the nitric acid if the complete precipitation of the phosphoric acid be de sired. The three series also differ when heated : the trimetallic salts containing fixed bases are unaltered, whilst the mono- and di-metallic salts yield meta- and pyrophosphates respectively. Other common 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 phosphomolyb date, [M003] insoluble in acids but readily soluble in ammonia; magnesium chloride, ammonium chloride and am monia, which give on standing in a warm place a white crystalline precipitate of magnesium ammonium phosphate, Mg(NH4)PO4 which is soluble in acids but highly insoluble in ammonia solutions, and on heating to redness gives magnesium pyrophos phate, Mg2P207.

Of the organic phosphates, triphenyl phosphate is important, acting as a filler or "dope," and as a substitute for camphor. It is a colourless solid of m.p. C and b.p. C.

Pyrophosphoric acid,

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 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 ex ample, the four sodium salts Na4P207, Na2H2P207, are known. The most important is the normal salt, which is readily obtained by heating disodium ortho phosphate, Na2HPO4. It forms monoclinic prisms (with which are permanent in air. All soluble pyrophosphates when boiled with water are converted into orthophosphates.

Metaphosphoric acid,

or more correctly H2P206 as determined by vapour density (Tilden and Barnett), is a mono basic acid which may be regarded as derived from orthophos phoric acid by the abstraction of one molecule of water, thus its constitution is therefore (HO)P02. 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 where n may be r, 2, 3, 4 or 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 : The salts are usually non crystalline and fusible. Their solutions on boiling yield ortho phosphates, whilst those of heavy metals give a trimetallic ortho phosphate and orthophosphoric acid : Ag3PO4+ 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 pres ence of ammonia. For perphosphoric acids, see PHOSPHORUS.