CYANOGEN COMPOUNDS 157 



Complex Cyanides. 



The cyanides of sodium and potassium are converted into ferro- 

 cyanides on boiling their solutions with ferrous salts in alkaline solution. 

 This reaction is made use of in testing for nitrogen in organic com- 

 pounds. 



Potassium Ferrocyanide. K 4 Fe(CN) 6 . 



Preparation. 



Potassium ferrocyanide is prepared by fusing together protein re- 

 sidues, such as blood, or horn, or leather with scrap iron and potassium 

 carbonate. The fused mass is extracted with water and the yellow 

 solution which results is evaporated down until it crystallises. 



Potassium ferrocyanide is also prepared from the hydrogen cyanide,, 

 which is a bye-product in the manufacture of coal gas. The hydrogen 

 cyanide is absorbed by iron oxide in the "purifiers". By boiling this 

 material with lime, calcium ferrocyanide is formed ; potassium ferro- 

 cyanide is obtained by treating it with potassium carbonate. Some- 

 times the hydrogen cyanide is converted into sodium ferrocyanide by 

 passing it into an alkaline solution of ferrous salts. 



Properties and Reactions. 



Potassium ferrocyanide forms large yellow crystals (yellow 

 prussiate of potash) which are easily soluble in water. 



(1) On adding concentrated hydrochloric acid to a saturated solu- 

 tion of potassium ferrocyanide, hydroferrocyanic acid is thrown down 

 as a white precipitate. It turns blue on filtering owing to decomposi- 

 tion and oxidation. 



(2) On adding a drop of ferric chloride solution to a solution of 

 potassium ferrocyanide, a blue precipitate (Prussian blue) is formed : 



3 K 4 Fe(CN) 6 + 4 FeCl 3 = Fe 4 [Fe(CN) 6 ] 3 + I2KC1. 



(3) Other metals also form insoluble ferrocyanides if their solutions be 

 added to a solution of potassium ferrocyanide. 



Zinc ferrocyanide is white, copper ferrocyanide is reddish-brown, uranium 

 ferrocyanide is brown. 



Sodium Nitroprusside. Na 2 Fe(CN) 5 . NO. 



Potassium ferrocyanide is converted into nitroprusside by the action of 

 moderately concentrated nitric acid ; potassium nitrate crystallises out and 

 is removed. The solution on neutralisation with sodium carbonate yields 

 sodium nitroprusside on evaporation. 



Sodium nitroprusside, Na 2 Fe(CN) 5 . NO + 2H 2 O, forms beautiful red 

 rhombic prisms which are easily soluble in water ; the solution is a sensitive 

 reagent for sulphides, acetone, etc. 



