40 UNIVERSITY OF COLORADO 



A known quantity of the solution is first boiled for about 15 

 minutes with £ gram of tartaric acid in an open flask to get rid 

 of all HCN, cooled and made up to 100 cc. Of this liquid, 10 cc. 

 are then taken for determination of thio-eyanate. A measured 

 amount of N/10 I., more than sufficient for the reaction, is then 

 added, and about one gram of NaHC0 3 . After standing in a stop- 

 pered bottle for £ hour in the dark, the excess of I is titrated with 

 N/10 thio-sulphate. 



Shaking the bottle should be avoided, in order to prevent the 

 evolution of CO a . It is stated that the presence of cyanogen iodide 

 prevents the use of starch as an indicator. It is advisable to work 

 with such quantities that not more than 20 cc. of iodine solution 

 are required. The end of the reaction is shown by the disappear- 

 ance of the yellow color. 



When zinc compounds or ferro-cyanides are present a white or 

 bluish ppt. occurs on boiling with tartaric acid, which must be fil- 

 tered off before adding NaHC0 3 and I. 



8. Estimation of "Total Cyanogen." — The term "total cyano 

 gen" is here taken to imply cyanogen existing in every form, 

 whether as free cyanides, double cyanides, cyanates, thio-cyanates, 

 ferro and ferri-cyanides, etc. 



(a). Estimation by precipitation with AgN0 3 , using chromate 

 indicator. (Based on Vielhaber's method.) 



This method involves the separate determination of the vari. 

 ous cyanogen compounds, and can only be regarded as a check 

 on the combined results. It is not applicable in presence of chlo- 

 rides, unless they also be separately determined and allowed for. 

 Protective alkali must first be neutralized. 



Standard silver solution is then run in until the reddish color 

 of silver chromate becomes permanent on shaking. The indicator 

 consists of a few drops of a strong solution of neutral (yellow) po- 

 tassium chromate; it is advisable to insure the absence of chlorides 

 in the indicator by adding silver nitrate to the chromate solution 

 till a red color is produced, and allowing the ppt. to settle. In this 

 process, cyanides (chlorides), thio-cyanates, iso-cyanates and ferro- 

 cyanides are ppt. as silver salts. 



(b). Total Cyanogen by boiling with oxide of mercury and re- 

 moving mercury by alkaline sulphide. (H. Rose, modified.) 



In cases where ferro-cyanides and similar compounds are pres- 

 ent, the solution is boiled with excess of oxide of mercury until 



