FUMIGATION OF CITRUS TREES. 43 



the proportion between the first two chemicals is practically 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 given amount of sulphuric acid of a fixed degree 

 of purity in order to thoroughly utilize the quantity of cyanid 

 employed, evolving the maximum amount of gas, and carrying the 

 reaction to completion. A quotation from a letter received from 

 Dr. J. K. Haywood, of the Bureau of Chemistry, of this department, 

 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. 1 



Expressed in fluid ounces four-fifths of an 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 

 considerable excess of sulphuric acid. It is perfectly possible, however, that this 

 excess of sulphuric acid is of value in heating up the mixture so tha^ more of the 

 hydrocyanic acid is liberated and not absorbed by the liquid. 



Two series of field experiments were performed which were identical 

 in all respects except that in the first 1 ounce of sulphuric acid was 

 used while in the other 1| ounces were used to each ounce of potas- 

 sium cyanid. Analysis of the residue by the Bureau of Chemistry, 

 of this department, showed that the reaction was as perfect with the 

 smaller proportion of acid as with the larger. The addition of a 

 great excess of acid might even result in an impediment to rapid work 

 in the field. As an explanation of this condition it might be stated 

 that the residue always contains a substance which is soluble in 

 water alone, but the presence of a large excess of sulphuric acid will 

 cause it to crystallize and solidify. This latter condition will fre- 

 quently occur if more than equal parts of acid to cyanid are used, 

 especially so with the smaller dosages. The removal of solidified 

 residue necessitates loss of time. 



Summing up, it may be said that 1 fluid ounce of commercial 

 sulphuric acid (93 per cent) to 1 ounce (avoirdupois) of 98 per cent 

 potassium cyanid is certainly enough to carry the reaction to com- 

 pletion in the liberation of hydrocyanic-acid gas and is perhaps an 

 unnecessarily large amount. In practical field work, where dosages 

 of varying sizes are constantly being used, it is very convenient to 

 reckon the acid in the same number of parts as the cyanid. The 

 use of 1 part (fluid measure) of acid to each part (1 ounce avoirdu- 

 pois) of cyanid is therefore recommended. 



The reaction is as follows: 2 KCN + H 2 SO«=K 2 S04+2 HCN. 



