32 FUMIGATION INVESTIGATIONS IN CALIFORNIA. 



acid in the sulphuric acid. The impurities, including nitric acid, 

 may be eliminated by refining. This, however, requires extra 

 expense, and, as these impurities are of little or no importance in 

 some of the lower uses to which sulphuric acid is put, the acid is not 

 usually refined. Such acid is unsuitable for use in fumigation. 



Taking all things into consideration it is safer, in purchasing 

 ordinary commercial sulphuric acid on the market, to order that 

 made from brimstone rather than that made from pyrites ore. It is 

 possible, however, to secure quite as good a product from pyrites as 

 from brimstone, if the former be sufficiently refined. If the fumi- 

 gator demands that it be free from nitric acid, arsenic, etc., and refuses 

 to accept it unless the product is of the grade required, there is no 

 reason why he should not be able to secure satisfactory material. 



PROPORTION OF MATERIALS LTSED BY FUMIGATORS. 



With each dry ounce of potassium cyanid most fumigators use 

 1 fluid ounce of sulphuric acid, although some use 1} ounces. The 

 proportion of water used varies all the way from 2 to 8 times the 

 amount (by bulk) of acid, the majority using between 3 and 4 parts 

 of water. 



THE AMOUNT OF SULPHURIC ACID NECESSARY. 



Chemical combinations take place with definiteness ; that is, when 

 one chemical acts on another in the production of a third substance, 

 the proportion between the first two chemicals is always the same. 

 Such is the case when sulphuric acid acts upon potassium cyanid in 

 producing hydrocyanic-acid gas. A given amount of cyanid requires 

 a certain amount of sulphuric acid of a fixed degree of purity to 

 carry the reaction to completion. A quotation from a letter received 

 from J. K. Haywood, of the Bureau of Chemistry of this Depart- 

 ment, illustrates this point: 



In the action of sulphuric acid on potassium cyanid approximately four-fifths of an 

 ounce (avoirdupois) of 93 per cent acid is used up for every ounce of 98 per cent 

 cyanid. a 



Expressed in fluid ounces four-fifths of p.n ounce avoirdupois equals about 0.42 of 

 a fluid ounce. We may say that theoretically 1 ounce avoirdupois of 98 per cent 

 potassium cyanid needs 0.42 of a fluid ounce of ordinary commercial sulphuric acid 

 (93 per cent) to convert it entirely to hydrocyanic acid. Since it is always best to 

 have some excess of the acid to carry the reaction to completion, it is probable that 

 three-fourths of a fluid ounce of commercial sulphuric acid is ample in practice to 

 convert 1 ounce avoirdupois of 98 per cent potassium cyanid to hydrocyanic acid. 

 If 1 fluid ounce of the commercial sulphuric acid is used it will certainly leave a con- 



oThe reaction is as follows: 



2KCN-fH,S04=K,,S(J4+2HCN. 



