As the azo colours form at the present day by far the largest group of synthetic organic colouring matters, I have prefaced the sections dealing with these products by certain general methods, as in many cases the diazotization and coupling takes place according to certain well-defined rules. Exact rules, however, cannot be laid down, as each amine and each phenol has its own peculiarities which must first be accurately determined by experiment. As it is not possible to go fully into details in this book, we must content ourselves with a few typical examples. The methods of analysis are given in the analytical portion.

Diazotization Of Amines

Aromatic amines are diazotized, usually at 5-10o, in as concentrated a solution as possible. According to the nature of the amine a greater or less quantity of acid is used, hydrochloric acid being nearly always taken for this purpose, as sulphuric or nitric acids are only of use in exceptional cases. In the works, however, sulphuric acid is frequently used, owing to its cheapness, but it has the disadvantage that, on salting out the finished dye, Glauber salt crystallizes out, which weakens the colour and may even make it unfilterable.