Jan. 15, 1920 Cyanogenesis in Sudan Grass 449 



PREPARATION OF THE STANDARD SOLUTION 



Ten cc. of a solution containing 5 mg. of hydrocyanic acid as potas- 

 sium cyanid were placed in an evaporating dish, and i cc. of con- 

 centrated yellow ammonium sulphid and one drop of concentrated 

 sodium hydroxid were added. This was slowly evaporated to dryness 

 on a water bath by passing a current of air over the dish by means of an 

 electric fan running at low speed. The residue was heated to 130° C. 

 for five minutes then dissolved in 10 cc. of warm water acidified with 

 dilute hydrochloric acid, two or three drops being added in excess. A 

 15 per cent solution of cadimum chlorid was added drop by drop until 

 the sulphid ceased to form, and then a 10 per cent solution of ferric 

 chlorid was added until the red color was permanent. This solution 

 was then filtered through a moistened paper and 5 cc. of 10 per cent 

 solution of ferric chlorid added to the filtrate. The volume was then 

 made up to 100 cc. 



PREPARATION OF THE UNKNOWN SOLUTION 



One cc. of concentrated yellow ammonium sulphid was added to the 

 distillate obtained by the process described above and then evaporated 

 slowly on the water bath as in the preparation of the standard solution. 

 The temperature was kept at about 70° C. The residue was treated as in 

 the preparation of the standard solution. The standard and unknown 

 solutions were then compared by means of a Bock-Benedict * colorimeter. 



The method was tested by comparing a solution of potassium cyanid, 

 which had been standardized by precipitation with silver nitrate, with a 

 solution of potassium sulphocyanid, which had been standardized by 

 the Volhard method. Two or three drops of dilute hydrochloric acid 

 were added to each solution before the addition of the ferric chlorid. 



The purpose of heating the residue to 130° C. was to prevent the 

 colloidal sulphur from going back into solution. The maximum intensity 

 of color was obtained with the concentration of ferric chlorid used by 

 the writers. Johnson ^ used 2 cc. of a i per cent solution of ferric chlo- 

 rid to the hundred and Francis-Connell ^ used i cc. of a 10 per cent 

 solution. The writers found, as did Johnson, that the addition of 

 hydrochloric acid to a solution containing but 2 cc. of a i per cent solu- 

 tion of ferric chlorid to the hundred increased the intensity of the color, 

 and that the addition of potassium chlorid to such a solution had a 

 bleaching effect. No change in the intensity of the color could be 

 detected by either of the writers after the addition of the acid and salt 

 to their solution. The results obtained by Johnson are what would 

 be expected when dilute solutions are used. 



' Bock, Joseph C, and Benedict, Stanley R. a NEW form of coudrimeter. In Jour. Biol. Chem. 

 V. 35, no. 2, p. 227-230, 3 pi. 1918. 

 'Johnson, Maxwell O., 1916. op. cit. 

 3 FHANCis, C. K., and Connell, W. B., 1913. op. aT. 



