BENZENE SERIES. AROMATIC COMPOUNDS. 567 



Methylene-blue should not be confounded with the commercial article, which 

 is often the zinc chloride double salt of methylthionine, is employed as a dye or 

 stain, and is unfit for medicinal purposes. The presence of zinc can be told by 

 incinerating 2 grammes of the substance and testing the ash in the usual way 

 for zinc. Methylene-blue should also not be confounded with methyl blue, 

 which is the sodium salt of triphenyl-pararosaniline-trisulphonic acid. A solu- 

 tion of the latter with alkalies changes to reddish-brown, 



Methylene azure, C 18 H 18 N 3 S0 3 C1, is derived from methylene-blue by the ad- 

 dition of oxygen. It is present in "ripened" methylene-blue and almost 

 always in even the best specimens of the medicinal article. It may be detected 

 by adding ammonia to a solution of methylene-blue and then shaking with 

 ether ; the methylene-azure passes into the ether, which is colored red. 



Diazo compounds of benzene. 



When fatty amines are treated with nitrous acid the ammo group 

 is replaced by hydroxyl, thus : 



C 2 H 5 .NH 2 + HONO = C 2 H 5 OH -f 2N + H 2 O. 



When, however, an aromatic amine is treated with nitrous acid, 

 in acid solution, a new class of compounds is formed, known as 

 diazo-compounds, thus : 



C 6 H 5 .NH 2 HC1 + HONO = C 6 H 5 .N a .Cl -f- 2H 2 0. 



Aniline Diazo-benzene 



hydrochloride. chloride. 



The characteristic diazo grouping is expressed thus : R N 2 , and 

 this group combines with acid residues to form diazo-salts, such as 

 diazo-benzene nitrate, C 6 H 5 .N 2 .NO 3 , sulphate, C 6 H 6 .N 2 .SO 4 H, etc. 



The diazo-compounds are colorless, crystalline, unstable, and even explosive 

 substances ; soluble in water, insoluble in ether. They are of great scientific 

 and technical importance, as they form the starting-point for a large class 

 of dyes. Diazo-compounds are decomposed by water, either in the cold or upon 

 heating, the N 2 group being usually replaced by the (OH) group, thus : 



C 6 H 5 .N 2 .NO 3 + H 2 O = C 6 H 5 OH -f N 2 + HNO 3 . 



It is thus possible to introduce hydroxyl into the benzene nucleus through the 

 medium of nitre-compounds, and obtain substances belonging to the class of 

 phenols. 



Diazo-compounds have a marked tendency to react with other substances, 

 especially amino-compounds and phenols, to form a class known as azo-com- 

 pounds, which are characterized by having the group N = N in combina- 

 tion with two residues. Azobenzene, C 6 H 5 N = N C 6 H 5 , is the mother- 

 substance of all azo-compounds, most of which are highly colored, and many 

 are used as dyes. The formation of colored azo-compounds is involved in the 

 test for nitrites in drinking-water by meta-phenylene-diamine (see page 432), 

 which gives triamino-azobenzene, NH 2 .C 6 H 4 N = N C 6 H 3 (NH 2 ) 2 , a dye 

 which has been on the market since 1866 and known as Bismark brown ; also 



