CHEMISTRY OF FUMIGATION. 97 



conditions under which all of the hydrocyanic acid may be decom- 

 posed by the hydrochloric acid and none whatever expelled into the 

 medium which it is desired to fill with the gas. 



This action of acids on hydrocyanic acid has been known to chem- 

 ists for years, but the first work the writer has seen in which its bearing 

 upon the subject of fumigation had been brought to the attention of 

 the public was by Xewell, x in which he shows the decomposing action 

 of hydrochloric and nitric acids on hydrocyanic acid and points out 

 the necessity of using pure cyanids for fumigation work. 



ACTION OF SULPHURIC ACID ON HYDROCYANIC ACID. 



When potassium cyanid or sodium cyanid is treated with sulphuric 

 acid, the first action which takes place is the liberation of hydrocyanic- 

 acid gas, as shown by previous equations. Tins is then acted upon 

 to a greater or less extent, depending upon the conditions, and a 

 portion of it decomposed. The principal products of decomposition 

 are ammonia and formic acid, according to the following reaction: 



HCN + 2H 2 = HCOOH + XH 3 . 



The ammonia formed combines with the excess of sulphuric acid 

 present and forms ammonium sulphate. If the sulphuric acid is 

 concentrated, it attacks the formic acid, extracting water therefrom 

 and liberating carbon monoxid, CO, thus: 



[h1 c o|6h]=h 2 o+co. 



Wade and Panting 2 have shown that, on treating potassium cj-anid 

 with sulphuric acid, by suitably varying the concentration of the 

 acid a practically quantitative yield of either hydrocyanic acid or 

 carbon monoxid can be obtained. With dilute sulphuric acid and up to 

 a strength of 1 part acid to 1 part water, which is as strong as it is ever 

 used in fumigation work, nearly pure hydrocyanic acid is formed. 

 With a stronger acid, however, "a certain amount of carbon monoxid 

 is formed, and as the concentration of the acid is increased the volume 

 of gas increases, while the amount of hydrogen cyanid diminishes; 

 and finally, when ordinary concentrated sulphuric acid is allowed to 

 act on the cyanid, nearly pure carbon monoxid is evolved in almost 

 theoretical quantity." Concentrated sulphuric acid at a high tem- 

 perature is reduced by hydrocyanic acid, sulphur dioxid, carbon 

 dioxid, and ammonia being formed. 



HCN + H 2 S0 4 = XH 3 + C0 2 + S0 2 . 

 As sulphuric acid stronger than 1 part acid to 1 part water is never 

 used in fumigation work, we need not concern ourselves with the 



1 Georgia State Board of Entomology, Bui. 15. 

 - Jouru. Chem. Soc, vol. 73, pt. I, p. 255, 1S9S. 



