It has been found by experience that the colour is developed much brighter upon the fibre when the diazo solution contains acetic acid and no free mineral acid. However, the diazotisation is better carried out with hydrochloric acid, and the presence of the latter is necessary to give stability to the solution. If before the diazo solution is used a quantity of acetate of soda be added to it, the free hydrochloric acid liberates acetic acid from the acetate, and the chloride of the diazo body changes into its acetate. It is better to add an excess above the two molecules of acetate of soda which are required.
The combination when aniline and beta-naphthol are used, as the amine and phenol respectively, is shown in the following equations:—
C6H5N:NCl + C10H7OH +
Diazo-benzene chloride, B. naphthol,
NaOH = 2NaCl + C6H5N:NC10H6OH + H2O Caustic soda, Benzene- azo-naphthol, Water.
Or, with naphthylamine and naphthol, thus:—
C10H7N:NCl + C10H7OH + NaOH =
NaCl + C10H7N:NC10H6OH + H2O
Naphthalene azo-naphthol.
By the action of nitrous acid upon amido-azo bodies a group of bodies called diazo-azo compounds are obtained which contain the group N:N twice over, thus:—