COMPOUNDS CONTAINING NITROGEN. 55 



ink), and is decomposed by alkalies with separation of brown ferric 

 hydroxide and formation of potassium ferrocyanide. The addition 

 of an acid restores the blue precipitate. 



4. Soluble ferrocyanides give with cupric solutions a brownish-red 

 precipitate of cupric ferrocyanide. (Plate III., 5.) 



5. Soluble ferrocyanides produce, with solutions of silver, lead, and 

 zinc, white precipitates of the respective ferrocyanides. 



6. Ferrocyanides give with ferrous salts a white precipitate of 

 ferrous ferrocyanide, soon turning blue by absorption of oxygen. 

 (Plate I., 4.) 



Potassium ferricyanide, K 3 Pe(CN) G (Red prussiate of potash). Ob- 

 tained by passing chlorine through solution of potassium ferrocyanide: 



K 4 Fe(CN) 6 -f Cl = KC1 + K 3 Fe(CN) 6 . 



Potassium Chlorine. Potassium Potassium 



ferrocyanide. chloride. ferricyanide. 



While apparently this decomposition consists merely in a removal 

 of one atom of potassium from one molecule of potassium ferro- 

 cyanide, the change is actually more complete, as the atoms arrange 

 themselves differently, the iron passing also from the ferrous to the 

 ferric state. 



Potassium ferricyanide crystallizes in red prisms, soluble in water. 

 It forms, with ferrous solutions, a blue precipitate of ferrous ferricy- 

 anide, or TurnbuWs blue : 



2K 3 Fe(CN) 6 + 3FeSO 4 = 3K 2 SO 4 + Fe 3 Fe 2 (CN) 12 . 



With ferric solutions no precipitate is produced by potassium ferri- 

 cyanide, but the color is changed to a deep brown. 



Sodium nitroferricyanide, Na 2 Fe(CN) 5 N0.2H 2 O. (Sodium nitroprusside.) 

 This is a salt of nitroferricyanic acid, which is obtained by the action of nitric 

 acid on potassium ferrocyanide. Potassium nitrate is crystallized out by con- 

 centrating and cooling the solution, which is then neutralized by sodium car- 

 bonate, and the sodium salt crystallized. Addition of alcohol increases the 

 separation of potassium nitrate. The salt forms large ruby-red crystals, solu- 

 ble in 2.5 parts of water and in alcohol. The aqueous solution decomposes on 

 standing. It serves as a delicate test for soluble sulphides (but not free H 2 S), 

 giving a purple color which quickly passes into violet. It is also used as a test 

 for acetone (Legal's test). 



Cyanides and isocyanides of organic radicals. Only one form of hydro- 

 cyanic acid and of metallic cyanides is known, but among organic cyanide* 

 two isomeric forms exist, known respectively as cyanides or nitrites, and isocyan- 

 ides or carbylamines. Experiments show that the constitution of these com- 

 pounds is represented by the formulas : 



R-C=N, 



Cyanide. Isocyanide. 



