METHODS FOR QUANTITATIVE DETERMINATIONS. 267 



of the potassium salts) in 900 c.c. of water. Of this solution, which 

 is too concentrated, transfer 20 c.c. into a bottle of about 250 c.c. 

 provided with a glass stopper. Next add 75 c.c. of water, 5 c.c. of 

 pure hydrochloric acid, and immediately insert the stopper. Shake 

 the bottle a few times to cause the liberation of bromine, then quickly 

 introduce 1 gramme of potassium iodide, taking care that no bromine 

 vapor escape. Gradually an equivalent quantity of iodine is liber- 

 ated from the potassium iodide by the bromine. When this has taken 

 place add to the contents of the flask a little starch solution and from 

 a burette deci-normal hyposulphite solution until the blue color has 

 been discharged. 



The use of bromine solution is directed by the U. S. P. in one case only, viz., 

 for the volumetric determination of phenol (carbolic acid). This substance 

 forms with bromine tribromphenol and hydrobromic acid : 



C 6 H 5 OH + 6Br = C 6 H 2 Br 3 OH + 3HBr. 



The molecular weight of phenol is 93.78, and as it reacts with 6 atoms of 

 bromine, one-sixth of 93.78, or 15.63 grammes of phenol correspond to one liter 

 of normal, and 1.563 grammes of deci-normal bromine solution ; i. e., 1 c.c. of 

 deci-normal bromine solution corresponds to 0.001563 gramme of phenol. The 

 U. S. P. directs the assay to be made as follows: Dissolve 1.563 gramme of the 

 specimen in water to make 1 liter. Transfer 25 c.c. of this solution (0.0391 

 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 hyposulphite, as described above. Deduct the num- 

 ber of c.c. of hyposulphite 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 = 169.55, is used for this solution, which is 

 made by dissolving 16.955 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 

 5.837 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 



