PRODUCTION OF DYES 315 



Nitrobenzene and a little hydrochloric acid are placed in the 

 pot, and iron borings are added to the mixture, which is kept 

 warm by steam and stirred by the paddle till the reaction 

 commences. Once started the chemical action, which is attended 

 by the evolution of heat, proceeds till the reduction of the 

 nitrobenzene is complete. At this stage any aniline evaporating 

 from the mass condenses in the upright pipe and returns to the 

 reduction vessel. Lime is then added to neutralise the acid, 

 and the aniline distilled off through the condenser d into the 

 receptacle e. 



Aniline is a colourless liquid which boils at 182-3 C., and 

 becomes brown by exposure to air. It sinks in water but slowly, 

 as its specific gravity is only 1-024 at 16. It is slightly soluble 

 in cold water. 



The toluidines are produced in a similar way from toluene, 

 and, as already explained, the red magenta is formed by oxidising 

 a mixture of these bases. Several oxidising agents have been 

 and are still used. Of these arsenic acid is one of the most 

 convenient, but the use of this reagent involves the careful 

 removal of arsenic from the finished colouring matter. The 

 presence of even a small quantity of arsenate or arsenite in the 

 dye applied to fabrics which are used for any kind of clothing 

 which c mes into direct contact with the skin, leads to irritation 

 of the surface, and even to symptoms of arsenical poisoning. 



To make magenta with the aid of arsenic acid a quantity of 

 this acid, in the form of a strong syrupy solution, is placed in a 

 cast-iron pot provided with a stirrer, and a mixture in due 

 proportions of aniline with the two isomeric toluidines is added. 

 The mixture is heated first to a temperature just above the 

 boiling-point of water, and subsequently to 180-190 C. for 

 several hours. Water and a considerable quantity of oil, con- 

 sisting of the three unaltered bases, evaporate off and are 

 collected by passing through a proper condenser. The residue 

 is a dark pitch-like mass which is fluid while hot, but after being 

 run off from the pot solidifies into a brittle solid. This contains 

 several colouring matters beside the red, and to obtain the latter 

 free from arsenic the mass is ground to powder, extracted with 

 water heated under pressure, and the solution, after being 

 filtered, is mixed with hydrochloric acid and sufficient common 

 salt to replace the arsenic. On cooling the solution the magenta 

 crystallises out leaving the other colouring matters in the liquid 



