CHEMISTRY OF FUMIGATION. 93 



siderable amounts of sodium ehlorid. While the amount of cyanid 

 present is equivalent to that found in a high-grade potassium cyanid, 

 owing to the presence of this ehlorid they will yield considerably less 

 gas, as subsequently shown in tins paper, and are therefore not as 

 valuable for fumigating purposes. 



II. PROPORTION OF CYANID, SULPHURIC ACID, AND WATER FOR 



BEST YIELD OF GAS. 



POTASSIUM CYANID. 



When dilute sulphuric acid acts on potassium cyanid, resulting in 

 complete decomposition, the theoretical reaction is as follows: 



2KCX + H a S0 4 = K 2 S0 4 + 2HCX . 



According to this reaction 1 ounce (avoirdupois) of potassium 

 cyanid (KCX 100 per cent) would require 0.75 ounce (avoirdupois) 

 of sulphuric acid, H 2 S0 4 , or 0.81 ounce (avoirdupois) of commercial 

 sulphuric acid containing 93 per cent sulphuric acid, which would be 

 equal to 0.42 fluid ounce. 



The above reaction can not be obtained under the conditions exist- 

 ing in fumigation work, and to get the best yield of gas it is necessary 

 to have a considerable excess of sulphuric acid. Under such condi- 

 tions it would be the acid potassium sulphate that would be formed 

 and the reaction would proceed thus: 



KCX + H 2 S0 4 = KHS0 4 + HCX, 



or for each ounce (avoirdupois) of potassium cyanid there would be 

 required 0.84 fluid ounce of 93 per cent sulphuric acid. This amount 

 has been found in practice to give the best results, or, in round numbers, 

 for convenience in practical field work, 1 part cyanid to 1 part acid. 

 According to laboratory work the proportion of water that should be 

 used with these amounts of cyanid and acid in order to obtain the 

 best yield of gas was 2 parts. 1 In field practice, however, Mr. >\ og- 

 lum has pointed out 2 that it is not always practicable to use this 

 amount on account of the fact that the residue sometimes solidifies 

 in the generating jar, and he has therefore adopted and recommends 

 3 parts of water, or a 1-1-3 formula. 



SODIUM CYANID. 



The action of sulphuric acid on sodium cyanid is identical with its 

 action on the potassium salt, and is as follows: 



2XaCX + H 2 S0 4 = Xa 2 S0 4 + 2HCX. 



i Bui. 79, Bur. Ent., U. S. Dept. Agr., pp. 37-38, 1909. 

 2 Ibid., p. 39. 



