Abstract
The acidity of free acid of azo dyes having benzene-azo-benzene and naphthalene-azo-naphthalene as the skeletal structure has been studied.
The pure free dye acid was prepared from the purified sodium salt by passing through the column of ion exchange resin.
The acidities of free dye acids were measured by potentiometric titration with caustic soda. Titration curve of free dye acid is indistinguishable from that of hydrochloric acid. Set the value of dissociation constant of sulphonic acid group be K1, and that of hydroxyl group K2, then the former generally becomes the order of 10-3.
In general, for o-hydroxyazo dyes the pK2=ca. 11, for p-hydroxyazo dyes the pK2=ca. 8.
Amongst o-hydroxyazo dyes, dyes having NW acid (1, 4-Naphtholsulphonic acid) as coupling component are anomalous in titration curve (Fig. 5), e. g. for Naphthionic acid →NW acid pK2=8.1; it accords with the pK2 of NW acid. This observation means that the hydroxyl group scarecely chelate although it lies in the ortho position to the azo linkage.
It is suggested that in dyeing of protein or polyamide fibres, all of p-hydroxyazo dyes and some of o-hydroxyazo dyes in which hydroxyl group is not restricted with inner molecular hydrogen bonding, are able to be adsorbed by a mechanism, probably hydrogen bonding with fibres in addition to the electrostatic attraction between sulphonic acid group of dye and amino group of these fibres. Thus the anomalous dyeing behaviour of these dyes to 6-nylon fibre may be explained.
The value of the standard affinity of some acid azo dyes to 6, 6-nylon decreases with increasing acidity of free dye acid of these dyes. (Fig. 7)
