AMMONIUM CYANIDE. 317 



disengages much less heat (+ 3*0) than the formation of the 

 chloride, bromide, and iodide of potassium under the same 

 conditions (which disengages 13 "7). 



This difference would be increased by 17 Cal., if the hydro- 

 chloric and hydrocyanic acids were considered in the gaseous state. 



Hydrocyanic acid is therefore much less powerful than the 

 hydracids derived from halogen elements ; it is even displaced 

 in potassium cyanide in solution by most acids. 1 This inert- 

 ness of hydrocyanic acid itself contrasts with the greater energy 

 of the complex acids which it forms when associated with 

 metallic cyanides ; hydroferrocyanic acid, for example ; this will 

 be referred to later on. 



5. The conversion of potassium cyanide into formate 



KCN (in solution) + 2H 2 = KCH0 2 (in solution) 

 + NH 3 (in solution), gives off + 9*5 Cal. 



This reaction does take place in solutions of the cyanide, 

 although slowly. 



The same reaction, effected on the dry salt by means of 

 aqueous vapour, produces formate and ammonia gas ; it is much 

 more rapid, but gives off double the amount of heat : 17'7. 



If the temperature is raised, the reaction becomes complicated, 

 owing to the ultimate destruction of the formate by the heat or 

 excess of alkali ; this reaction, which takes place at about 300 

 and finally transforms the potassium cyanide into potassium 

 carbonate 



KCN (solid) + KHO (solid) + 2H 2 (gas) = K 2 C0 3 (solid) 

 + NH 3 (gas), gives off + 374. 



This point is important, because it is one of the most 

 effective causes of the destruction of potassium cyanide during 

 its industrial preparation; in this case the melted salts are 

 operated upon, and this fact causes a slight modification of the 

 above values ; without, however, altering their general signifi- 

 cation. When exposed to the oxygen of the air, we know that 

 potassium cyanide is readily converted into potassium cyanate. 

 This reaction will be referred to at a later period. 



5. AMMONIUM CYANIDE. 



1. It was found that the combination of hydrocyanic acid in 

 solution, with ammonia in solution, gives off -J- 1*3 Cal. The 

 solution of freshly prepared ammonium cyanide ( 1 part of salt 

 to 180 parts water) absorbs - 4'36 for NH 4 CN (= 44 grms.). 



2. From these figures it follows that the union of hydrocyanic 

 gas and ammonia gas, with formation of solid cyanide 



HCN (gas) + NH 3 (gas) = NH 4 CN, gives off + 20'5. 

 1 " Annales de Chimie et de Physique," 4 9 serie, torn. xxx. p. 492. 



