METHODS FOR QUANTITATIVE DETERMINATIONS. 425 



The molecular weight of phenol is 93.36, and as ii reacts with 6 atoms of 

 bromine, one-sixth of 93.36, or 15.56 grammes of phenol correspond to 1 liter 

 of normal, and 1.556 grammes to deci-normal bromine solution i. e., I c.c. of 

 deci-normal bromine solution corresponds to 0.001556 gramme of phenol. The 

 U. S. P. directs the assay to be made as follows : Dissolve 1.556 grammes of the 

 specimen in water to make 1 liter. Transfer 25 c.c. of this solution (0.0389 

 phenol) to a glass-stoppered bottle of about 200 c.c. capacity, and add 30 c.c. of 

 deci-normal bromine solution and 5 c.c. of hydrochloric acid. Shake the con- 

 tents of the bottle repeatedly, during half an hour, then quickly introduce 1 

 gramme of potassium iodide, allow the reaction to take place and titrate the 

 solution with deci-normal thiosulphate, as described above. Deduct the num- 

 ber of c.c. of thiosulphate used from the 30 c.c. of bromine solution. The 

 remainder multiplied by 4 indicates the percentage of phenol in the carbolic 

 acid examined. 



Deci-normal solution of silver. The pure, dry crystallized 

 silver nitrate, AgNO 3 = 168.69, is used for this solution, which is 

 made by dissolving 16.869 grammes of the salt in water to make 

 1000 c.c. The standard of this solution may be found by means of 

 a deci-uormal solution of sodium chloride containing of this salt 

 5.806 grammes in one liter. 



Volumetric silver solution is used directly for the estimation of 

 most chlorides, iodides, bromides, and cyanides, including the free 

 acids of these salts. Insoluble chlorides must first be converted into 

 a soluble form by fusing them with sodium hydroxide, dissolving the 

 fused mass (containing sodium chloride) in water, filtering and neu- 

 tralizing with nitric acid. 



The hydroxides and carbonates of alkali metals and of alkaline 

 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 wi*th 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 



