448 



Journal of Agricultural Research 



Vol. XVIII, No. 8 



are not aware that this observation has been made before. A compari- 

 son of the results obtained here with those given in the literature for 

 kafir and other varieties of grain sorghums shows that Sudan grass 

 contains about one-third as much hydrocyanic acid as do the grain 

 sorghums. 



Table I. — Percentage of hydrocyanic cu:id in Sudan grass at different stages of growth 



MODIFICATION OF THE FRANCIS-CONNELL METHOD OF DETERMINING 



HYDROCYANIC ACID 



Viehover and Johns* have criticized the Francis-Connell colorimetric 

 method of determining hydrocyanic acid, pointing out (i) that the 

 equilibrium of the reaction upon which the method is based is very 

 sensitive to the presence of electrolytes and hence that the intensity of 

 the color due to the ferric sulphocyanid is varied greatly by salts and 

 (2) that a part of the sulphocyanic acid is volatilized in boiling the 

 acid solution to remove the colloidal sulphur. Johnson^ criticized the 

 Viehover-Johns colorimetric method and used the Francis-Connell 

 method after modifying it. The Viehover-Johns method is objectionable 

 on account of the fact that the intensity of the color due to the Prus- 

 sian blue varies with temperature and is changed by electrolytes. The 

 Francis-Connell method would seem to be the better method if the 

 objections pointed out by Viehover and Johns could be overcome. 

 Johnson's modification of the Francis-Connell method makes the process 

 too long, and the solutions used are more sensitive to electrolytes than 

 are those used in the original method. The writers sought to modify 

 the method and eliminate the objectionable features. 



1 Viehover. Amo, and Johns, Carl O. on the determination op small quantities of hydro- 

 cyanic acid. /)t Jour. Amer. Chem. Soc, V. 37, no. 3, p. 601-607. 1915. 



^ Johnson, Maxwell O. on the deteemination of small quantities of h\t)rocyanic acid. In Jour. 

 Amer. Chem, Soc., v. 38, no. 6, p. 1230-1235. 1916. 



