342 



PRINCIPLES OF CHEMISTRY 



b-i- 



taking care as far as possible to prevent any rise of temperature. 1 * Tht 

 normal salt separates from the brown solution when it is concentrated 



a molecules of NH 4 C1, per 100 molecules H 2 O, then at 15 one of the following separa- 

 tions takes place : (1) crystals, Fe 2 Cl 6 ,12H 2 O, when a varies between and 11, and 6 

 between 4'65 and 4'8, or (2) a mixture of these crystals and the double salt, when a, 

 -1-86, and 6 = 4-47, or (8) the double salt, Fejd^NEUCl^iHjO, when a varies 

 between 2 and 11'8, and 6 between 8'1 and 4'56, or (4) a mixture of sal-ammoniac with 

 the iron salt (it crystallises in separate cubes, Retgers, Lehmann), when a varies 

 between 7'7 and 10'9, and 6 is less than 8'88, or (5) sal-ammoniac, when a = ll'88. And 

 as in the double salt, a ' 6-4 : 1 it is evident that the double salt only separates out 

 when the ratio a ' b is less than 4 . 1 (i.e. when Fe 2 Cl 6 predominates). The above is 



seen more clearly in the accom- 

 panying figure, where a, or the 

 number of molecules of NH 4 C1 

 per 100H 2 O, is taken along the 

 axis of abscissae, and 6, or the 

 number of -molecules of Fe a Cl 6 , 

 along the ordinates. The curves 

 ABCD correspond to saturation 

 and present an iso- therm of 15. 

 The portion AB corresponds to 

 the separation of chloride of iron 

 (the ascending nature of this 

 curve shows that the solubility of 

 Fe 2 Cle is increased by the pre- 

 sence of NH 4 C1, while that, of 

 NH 4 C1 decreases in the presence 

 of Fe 2 Cl 6 ), the portion EC to the 

 double salt, and the portion CD 

 to a mixture of sal-ammoniac- and 

 ferric chloride, while the straight 



line OF corresponds to the ratio Fe 2 Cl 6 ,4NH 4 Cl, or a : &"4 : 1. The portion CE shows 

 that more double salt may be introduced into the solution without decomposition, but- 

 then the solution deposits a mixture of sal-ammoniac and ferric chloride (see Chapter 

 XXIV. Note 9 bb ). If there were more such well-investigated cases of solutions, our 

 knowledge of double salts, solutions, the influence of water, equilibria, isomorphous 

 mixtures, and such-like provinces of chemical relations might be considerably advanced. 

 24 The normal ferric salts are decomposed by heat and even by water, forming basic 

 salts, which may be prepared in various ways. Generally ferric hydroxide is dissolved 

 in solutions of ferric nitrate ; if it contains a double quantity of iron the basic salt is 

 formed which contains Fe 2 O 3 (in the form of hydroxide) + 2Fe 2 (NO 3 ) 6 =3Fe./)(NO3) 4 , 

 a salt of the type Fe 2 OX 4 . Probably water enters into its composition. With con- 

 siderable quantities of ferric oxide, insoluble basic salts are obtained containing various 

 amounts of ferric hydroxide. Thus when a solution of the above-mentioned basic acid 

 is boiled, a precipitate is formed containing 4(Fe 2 O 3 )8,2(N 2 O 5 ),8H 2 O, which probably 

 Contains 2Fe 2 O s (NO 3 )2 + 2Fe 8 O 3 ,8H 2 O If a solution of basic nitrate be sealed in a 

 tube and then immersed in boiling water, the colour of the solution changes just in 

 the same way as if a solution of ferric acetate had been employed (Note 22). The 

 solution obtained smells strongly of nitric acid, and on adding a drop of sulphuric or 

 Hydrochloric acid the insoluble variety of hydrated ferric oxide is precipitated. 



Normal ferric orthopjiosphaf is soluble in sulphuric, hydrochloric, and nitric acids, 

 but insoluble in others, such as, for instance, acetic acid. The composition of this salt 

 in the anhydrous state is FePO 4 , because in orthophosphoric acid there are three atoms 

 of hydrogen, and iron, in the ferric state, replaces the three atoms of hydrogen. This 

 .Bait is obtained from ferric acetate, which, with disodium phosphate, forms a white pre- 



II IS 



FIG. 96. Diagram of the formation, at 15, of the double 

 salt Fe a Cl e 4NH 4 C12H a O or FeCNH^Cl.E^O, (After 

 Roozeboom.) 



