Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/530

Rh 518 Metaphosphoric acid in the solid state, as obtained by evaporating its solution and heating the residue to redness, is a colourless, glassy, uncrystallizable mass, which dis solves slowly though somewhat abundantly in water, forming a strongly acid liquid ; but the solution is very unstable, and is converted gradually at ordinary tempera tures, and rapidly on heating, into phosphoric acid. Meta phosphoric acid is an extremely stable body ; it volatilizes completely at a bright red heat, and apparently is only in part decomposed into water and phosphoric anhydride. The metaphosphates, or salts of metaphosphoric acid, which may be produced by lieating the monometallic salts of phosphoric acid, for example NaH 2 PO 4 = NaPO 3 + H 2 O, Sodium phosphate. Sodium nietaphosphate. and in various other ways, are remarkable for exhibiting very different properties according to the manner in which they are prepared. Thus, ordinary sodium metaphosphate, obtained by igniting sodium phosphate and sudden cooling, forms a vitreous mass, the aqueous solution of which gives gelatinous precipitates with the salts of the heavy metals. But when a considerable quantity of ordinary fused sodium metaphosphate is allowed to cool very slowly, a crystalline mass results, and on digesting this in a slight excess of warm water the liquid separates into two layers, one con taining a crystalline sodium metaphosphate, and the other the ordinary vitreous salt. By heating copper oxide and a slight excess of phosphoric acid together to 350 C., a crystalline powder is formed, insoluble in water ; and by treating this copper salt with sodium sulphide another crystalline sodium metaphosphate is produced. If oxide of lead be employed, and the resulting lead salt decom posed with sodium sulphide, a sodium metaphosphate is obtained which forms with water a gummy mass, which will not pass through a filter. Lastly, a fifth variety of metaphosphates, remarkable for their insolubility in water, are formed by adding phosphoric acid in excess to solutions of sulphates or nitrates, evaporating to dryness, and heat ing the residue to 316 C. or upwards. They are crystal line powders. These different metaphosphates are generally regarded as polymeric compounds, that is to say, as compounds having different molecular weights although of the same empirical composition ; and the attempt has been made to infer their formula from the relative number of atoms of the two metals contained in the mixed salts derived from them. Thus, the sodium in ordinary sodium metaphos phate may be partially displaced by another metal, and a mixed salt produced containing the two metals in the ratio of 5 atoms of the one monad metal to 1 atom of ths other ; hence it is concluded that ordinary sodium metaphosphate is a hexmetaphosphate, NagP^O^. The mixed salts de rived from the second variety of sodium metaphosphate above described contain the two metals in the ratio of 2 atoms of the one monad metal to 1 atom of another, and are there fore regarded as trimetaphosphates, the sodium salt being represented by the formula Na 3 P 3 O 9. The mixed salts formed from the third and fourth varieties contain equal numbers of atoms of the two metals, and it is therefore supposed that they are respectively di- and tetra-meta- phosphates, and that their sodium salts, for example, have the composition Na.,P 2 O,. and Na 4 P 4 O 10. The insoluble crystalline metaphosphates are regarded as monometa- phosphates. I yrophosphoric add, H 4 P 2 O 7. The normal salts of this acid, which is tetrabasic, may be produced by igniting tli? dimetallic phosphates; for example 2Xa a HP0 4 = Na 4 P 2 O 7 + H 2. A solution of the acid may be prepared by decomposing [COMPOUNDS OF lead pyrophosphate suspended in water by hydrogen sulphide. Apparently the acid has not yet been obtained in a pure state. Pyrophosphoric acid is converted into metaphosphoric acid when heated to redness, and into phosphoric acid when boiled with water ; the latter change, it is stated, takes place also at ordinary temperatures, but very slowly. A solution of pyrophosphoric acid does not precipitate albumen or silver nitrate, but after neutralization it gives a white precipitate with the latter. Metaphosphoric acid coagulates albumen, and gives a white precipitate with silver nitrate ; and phosphoric acid does not coagulate albumen, and when neutralized gives a yellow precipitate with silver nitrate. In addition to the normal pyrophosphates, acid salta may be obtained, formed by the displacement of only one, two, or three of the four atoms of hydrogen in the acid by metals. By fusing together sodium pyrophosphate and sodium metaphosphate the salts Na 6 P 4 O 13 and Na 12 P ]0 O 31 have been obtained ; these salts may be regarded as formed from acids derived respectively from four and ten molecules of phosphoric acid by the abstraction of the elements of three and nine molecules of water in the same way that pyrophosphoric acid is derived from two molecules of phos phoric acid, by the abstraction of the elements of a single molecule of water 2H 3 PO 1 -H 2 O = H 4 P 2 O 7 4H 3 PO 4 - 3H 2 O = H 6 P 4 O 13 10H 3 P0 4 -9H 2 = H 12 P 10 31 These acids are terms in a series, of which pyrophosphoric acid is the first member, formed by the withdrawal of the elements of n - 1 molecules of water from n molecules of phosphoric acid. A ery little attention has been paid as yet to the thermo- chemical investigation of the phosphorus compounds. The results obtained by Andrews and by Thomsen are collected in the following table ; the numbers all refer to phosphorus in its ordinary state : Reaction. Units of heat evolved or absorbed. Remarks. (P, C1 5 &amp;gt; 75,000 Andrews. 362,800 Abria. (P 2, 5 ) ] 367,800 Andrews. (P0 4 H 3 , Aq) (PO,H S , Aq) 2,690 -130 / Heat, of solution of the crystalline [ ai ids (P0 2 H 3 , Aq) -170 Heat of solution of the fused adds (P, 4 , H 3 ) &amp;lt;P,0 3 ,H 3 ) (P,0 S&amp;gt; H 3 ) 5,210 2,940 2,140 at the same temperature. The dilTeivnce between the heat of solution of the fused and crys talline acids is the heat of ( fusion. (P,0 4 ,H 3 ) (P, 3, H 3 ) 302,600 227,700 / Formation of the crystalline acids ( from their elements. (P. 0,, H 3 ) 139,970 ) (P, 4, H 3 ) (P, 3 ,H 3 ) (P,0,.H 3 ) 300,080 224,630 137,660 Formation of the fused acids from their elements. (P,0 4 , H 3 ,Aq) (P,0 3 ,H 3 ,Aq) (P,0. 2) H 3 ,Aq) 305,290 227,570 139,800 / Formation of the acids in aqueous ( solution from their elements. (P S ,0 5 ,Aq) (P a ,0 3, Aq) (P S , 0, Aq) 405,400 250,060 74,520 / Formation of the acids from phos phorus, oxj gen, and water. Sulphides and Sulpho-Acids of Phosphorus. The compounds P 4 S 3 , P 2 S 3 , and P 2 S 5 are readily pro duced by carefully heating together sulphur and red amor phous phosphorus in the required proportions. It is stated that the lower sulphides P 4 S and P 2 S may also be obtained by melting together ordinary phosphorus and sulphur under hot water in the proportions indicated by these formulae,