268 ANALYTICAL CHEMISTRY. 



earths may be converted into chlorides by evaporation to dryness 

 with pure hydrochloric acid, and heating to about 120 C. (248 F.). 

 The chlorides thus obtained may be titrated with silver solution. 



In the case of chlorides, iodides, and bromides, normal potassium 

 chromate is used as an indicator. This salt forms with silver nitrate 

 a red precipitate of silver chromate, but not before the silver chloride 

 (bromide or iodide) has been precipitated entirely. In case free acids 

 are determined by silver, these are neutralized with sodium hydroxide 

 before titration. 



The operation is conducted as follows : The weighed quantity of 

 the chloride is dissolved in 50-100 c.c. of water, neutralized if neces- 

 sary, mixed with a little potassium chromate, and silver solution 

 added from the burette until a red coloration is just produced, which 

 does not disappear on shaking. 



In estimating cyanides, the operation can be conducted as above 

 described, or it can be modified, use being made of the formation of 

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

 takes place thus : 



2KCN + AgNO 3 = AgK(CN) 2 + KN T O 3 . 



If to this soluble double compound more silver nitrate be added, it 



is decomposed with the formation of a precipitate of silver cyanide : 



AgK(CN) a + AgNO 3 == 2AgCN + KNO 3 . 



The estimation of hydrocyanic acid or of simple cyanides, according to this 

 method, is accomplished by first rendering slightly alkaline the solution of the 

 substance to be examined by the addition of sodium hydroxide, and then 

 adding the silver solution until a permanent cloudiness is produced in the 

 liquid, which shows that all cyanogen present has been converted into the 

 soluble double salt. 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 0.016955 

 gramme of AgNO 3 , is the equivalent of: 



Gramme. 



Ammonium bromide, NH 4 Br . 0.009777 



Ammonium chloride, NH 4 C1 0.005338 



Ammonium iodide, NHJ 0.014454 



Calcium bromide, CaBr 2 0.009971 



Ferrous bromide, FeBr 2 0.010770 



Ferrous iodide, FeI 2 0-015447 



Hydriodic acid, HI 0.012753 



Hydrobromic acid, HBr 0.008076 



