Cationic and related dyes

Abstract

The term ‘Basic dyes’ is conveniently used to distinguish an application class of dyes that, by virtue of the presence of basic groups in their molecules, have affinity for protein fibres and cellulosic fibres mordanted with tannic acid. If acid as well as basic groups are present zwitterions are formed, but the groups are ionised independently, and negatively charged dye ions usually predominate; such dyes therefore function as acid dyes. In a chemical classification the term ‘Cationic dyes’ is usually preferred; this relates to dyes consisting of salts in which the chromophoric system resides in the cation. For the purposes of the present chapter zwitterionic dyes (i.e. acid or mordant cationic dyes) are treated as cationic dyes. This arrangement, which is convenient for presentation, is justified in that a zwitterion is both cation and anion, but does not conform to a recent definition of a cationic dye as one in which the balance of charge on the ion is positive.¹