IRON, COBALT, AND NICKEL 



341 



Fe 2 (N0 3 ) 6 ; it is obtained by dissolving iron in an excess of nitric acid, 



much more stable in the form of double salts, like all the ferric salts and also the 

 salts of many other feeble bases. Potassium or ammonium chloride forms with it very 

 beautiful red crystals of a double salt, having the composition Fe 2 Cl 6 ,4KCl,2H 2 O. 

 When a solution of this salt is evaporated it decomposes, with separation of potassium 

 chloride. 



B. Eoozeboom (1892) studied in detail (as for CaCl 2 , Chapter XIV., Note 50) the 

 separation of different hydrates from saturated solutions of Fe 2 Cl 6 at various concen- 

 trations and temperatures ; he found that there are 4 crystallohydrates with 12, 7, 5, and 

 4 molecules of water. An orange yellow only slightly hygroscopic hydrate, Fe 2 Cl 6 ,12H 2 0, 

 is most easily and usually obtained, which melts at 37 ; its solubility at different tempera- 

 tures is represented by the curve BCD in the accompanying figure, where the point B 



0' 50' 



FIG. 95. Diagram of the solubility of Fe a Cl. 



100' 



corresponds to the formation, at -55, of a cryohydrate containing about Fe 2 Cl,j + S6H 2 0, 

 the point C corresponds' to the melting-point ( + 37) of the hydrate Fe 2 Cl 6 ,12H 2 O, and 

 the curve CD to the fall in the temperature of crystallisation with an increase in the 

 amount of salt, or decrease in the amount of water (in the figure the temperatures are 

 taken along the axis of abscissae, and the amount of n in the formula nEe^Clg -4- 100H O 

 along the axis of ordinates). When anhydrous Fe 2 Cl 6 is added to the above hydrate 

 (12H 2 0), or some of the water is evaporated from the latter, very hygroscopic 

 crystals of Fe.,Cl 6) 5H 2 O (Fritsche) are formed ; they melt at 56, their solubility is 

 expressed by the curve HJ, which also presents a small branch at the end J This 

 again gives the fall in the temperature of crystallisation with an increase in the amount 

 of Fe. t C\ 6 . Besides these curves and the solubility of the anhydrous salt expressed by 

 the line KL (up to 100, beyond which chlorine is liberated), Roozeboom also gives the 

 two curves, EFG and JK, corresponding to the crystallohydrates, Fe 2 Cl 6 ,7H 2 O (melts at 

 + S2-5, that is lower than any of the others) and Fe.jCla^HjO (melts at 73 0< 5), which 

 he discovered by a systematic research on the solutions of ferric chloride. The curve 

 AB represents the separation of ice from dilute solutions of the salt. 



The researches of the same Dutch chemist upon the conditions of the formation of 

 Crystals from the double salt (NH 4 Cl) 4 Fe 2 Cl c J2H 2 O are even more perfect. This salt 

 was obtained in 1889 by Fritsche, and is easily formed from a strong solution of Fe 4 Cl<, 

 by adding sal-ammoniac, when it separates in crimson rhombic crystal.?, which, after 

 dissolving in water, only deposit again on evaporation, together with the sal-ammoniac. 



Roozeboom (1892) found that when the solution contains b molecules of Fe u Cl c , and 



