38 UNIVERSITY OF COLORADO 



^Yhen zinc is present add an excess of a solution of K 4 Fe(CN) 6 

 before adding the silver nitrate.* 



3. Estimation of Free Cyanide: 



(a). Differential Method. — Where zinc is the only metal pres- 

 ent which is capable of forming easily decomposible double cyan- 

 ides of the character of K 2 Zn(CN) 4 , a determination of the so-called 

 "total cyanide" by the method given below, together with a deter- 

 mination of the zinc, enables us to calculate the free cyanide, 

 assuming one part of zinc equivalent to 4 parts of KCN converted 

 into K 2 Zn(CN) 4 . In this case: 



Free cyanide equals total cyanide minus 4xZn. 



The presence of cyanogen, in the form of ferro-cyanides or thio- 

 cyanates, does not affect this result. 



4. Estimation of "Total Cyanide." — Strictly speaking, the term 

 "total cyanide" should indicate the equivalent of all the cyanogen 

 contained in the solution. Practically, it is generally taken to mean 

 "the equivalent, in terms of KCN, of all the cyanogen existing in 

 the form of simple cyanides, HCN, and certain readily decompos- 

 ible double cyanides, such as that of zinc." Some other double 

 cyanides, such as those of silver and copper, are generally excluded, 

 together with ferro and ferri-cyanides, thio-cyanates and similar 

 bodies. 



Method Based on Use of Alkaline Iodide Indicator. — An indi- 

 cator is prepared by dissolving 40 grams of caustic soda and 10 

 grams of KI in water, and making up to a litre. Fifty cc. of the 

 cyanide solution are taken, and 5 cc. of the above indicator added. 

 The liquid is titrated with standard AgN0 3 (6.519 grams per litre) 

 until a distinct yellow coloration is obtained, disregarding any 

 white turbidity. This latter may sometimes be removed by adding 

 NH 4 OH, which, in moderate amounts, does not affect the accuracy 

 of the test: 



1 cc. of AgN0 3 used equals .01$ KCN (equivalent to total 

 cyanide). 



5. Estimation of HCN.f — Estimation of free HCN in presence 

 of alkaline cyanides and of zinc double cyanides: 



(a) The free cyanide is first estimated in the ordinary way 

 without addition of alkali. It is assumed that the presence of 

 free HCN does not affect this result. 



*Proc. Inst. Min. and Met., volume X, pages 29 to 37, Green. 



t Bettel. Proc. Chem. and Met. Soc. of S. Africa, volume 1, page 165. 



