4-26 ANALYTICAL CHEMISTRY. 







soluble double cyanides of silver and an alkali metal. The reaction 

 takes place thus : 



2KCN + AgN0 3 = AgK(CN), + KNO 3 . 



In the process adopted by the U. S. P., a suitable quantity of hydrocyanic 

 acid, or of a cyanide, is diluted with water, and 5 c.c. of ammonia water and 

 a few drops of potassium iodide solution are added. Silver solution is then 

 added until a slight permanent cloudiness is produced, at which point half of 

 the cyanide is converted into silver cyanide, which is held in solution by the 

 other half of the cyanide as a double salt. The least excess of silver solution 

 after this stage is indicated by the insoluble silver iodide formed. As but one- 

 half of the silver solution has been added which is needed for the complete 

 conversion of the cyanogen present into silver cyanide, the number of c.c. of 

 the standard silver solution employed will indicate exactly one-half of the 

 equivalent amount of cyanide present in the solution. 



One c.c. of deci-normal silver nitrate solution, containing 0.016869 

 gramme of AgNO 3 , is the equivalent of: 



Gramme. 



Ammonium bromide, NH 4 Br . 0.009729 



Ammonium chloride, NH 4 C1 . 0.005311 



Ammonium iodide, NH 4 I . . 0.014383 



Calcium bromide, CaBr 2 ..,.,. . 0.009926 



Ferrous bromide, FeBr 2 0.010711 



Ferrous iodide, FeI 2 0.015365 



Hydriodic acid, HI 0.012690 



Hydrobromic acid, HBr . 0.008036 



Hydrochloric acid, HC1 . . . . . . . . 0.003618 



Hydrocyanic acid, HCN, to first formation of precipitate . . 0.005368 



Hydrocyanic acid, HCN, with indicator 0.002684 



Lithium bromide, LiBr 0.008634 



Potassium bromide, KBr ........ 0.011822 



Potassium chloride, KC1 0.007404 



Potassium cyanide, KCN, to first formation of precipitate . . 0.012940 



Potassium cyanide, ECN, with indicator 0.006470 



Potassium iodide, KI 0.016476 



Potassium sulphocyanate, KCNS 0.009653 



Sodium bromide, NaBr 0.010224 



Sodium chloride, NaCl 0.005806 



Sodium iodide, Nal . 0.014878 



Strontium bromide, SrBr 2 .6H 2 O . 0.017647 



Strontium iodide, SrI 2 .6H 2 O 0.022301 



Zinc bromide, ZnBr 2 0.011181 



Zinc chloride, ZnCl 2 0.006763 



Zinc iodide, ZnI 2 0.015835 



Deci-normal solution of sodium chloride is made by dissolving 

 5.806 grammes of pure sodium chloride in enough water to make 

 1000 c.c. The titration is made in neutral solution, normal potas^ 

 sium chromate being used as an indicator. (See explanation in 

 previous paragraph on silver solution.) 



