PRODUCTION OF DYES 313 



Mauve was produced by adding potassium dichromate to a 

 solution of aniline in dilute sulphuric acid, whereby a black 

 precipitate was formed, which after removal of impurities by 

 boiling it in coal-tar naphtha, was dissolved in alcohol in which 

 it forms a rich purple-coloured solution. 



Dichrornate of potassium is an oxidising agent, and naturally 

 other oxidising agents were tried as soon as the process became 

 known. This led to the discovery of the red dye magenta. 

 Strangely enough pure aniline acted upon by these oxidising 

 agents does not yield a dye, but fortunately the benzene of 

 former days was very far from pure, and contained a varying 

 quantity of toluene, the hydrocarbon which stands next to 

 benzene in the series and possesses very similar properties. These 

 hydrocarbons, which are extracted by distillation from coal-tar, 

 form the starting point for the production of this class of colours. 

 The successive steps in the production of magenta are the 

 following. 



Benzene acted upon by strong nitric acid is converted into 

 nitrobenzene, C 6 H 5 .N0 2 . Toluene, C 7 H 8 , is similarly con- 

 verted into nitrotoluene, C 7 H 7 .N0 2 , or rather into a mixture of 

 two compounds having the same ultimate composition, though 

 differing in the arrangement of the constituent atoms. 



When nitrobenzene is brought into contact with a substance 

 or mixture of substances, such as iron and acetic acid, which is 

 capable of supplying hydrogen, the two atoms of oxygen are 

 removed and two atoms of hydrogen are introduced in place of 

 them, and aniline, C 6 H 5 NH 2 , is the result. The nitrotoluenes, 

 under the same circumstances, are affected in a similar manner, 

 and give rise to bases called toluidines, C 7 H 7 NH 2 . 



Now as the commercial benzene fifty years ago consisted of a 

 mixture of benzene and toluene, the aniline which resulted from 

 using it as the parent material always consisted of a mixture of 

 aniline and ortho- and para- toluidines. Such a mixture acted 

 on by an oxidising agent, yields a deep red mass which contains 

 magenta dye. 



The apparatus employed in these operations is simple ^in 

 principle. The nitration of benzene or toluene is effected by 

 bringing the hydrocarbon into contact with the requisite quantity 

 of a mixture of strong sulphuric and nitric acids, the former being 

 employed to combine with the water generated in the reaction. 



The mixture is made in a cast-iron pot, cooled externally by 



