For this purpose it is usually only necessary to prepare a solution containing the diazo compound.
The dyed colour here plays the part of a phenol or amine, and reacts with the diazo compound to produce a new colour.
diazo compounds are therefore important intermediate substances in the manufacture of azo dyes.
The diazo compounds formed on the fibre are not very stable bodies.
It is usually desirable to keep the solutions cool with ice, owing to the very unstable nature of the diazo compounds produced.
This diazo liquor will keep for some days, but it decomposes in time, so that it should not be kept too long.
Ordinarily the diazo appears a little earlier than the Widal reaction—about the fourth or fifth day—but it may be delayed.
When the para-position is occupied, the diazo group takes the ortho-position.