Mordant Dyes

synthetic dyes that are used with auxiliary substances called mordants. Specific mordant dyes are used for cotton and wool. The mordant applied in the dyeing of wool is Na₂Cr₂O₇. During the dyeing process, hexavalent chromium becomes trivalent and combines with the dye and the wool to form a stable complex compound. Hence the name chrome dyes. Mordant dyes are rarely used for cotton because of insufficient dye stability.

Mordant dyes are classified according to chemical structure as azo dyes and anthraquinone dyes, even though the latter are often used without a mordant. The ability of mordant dyes to form complexes with trivalent chromium is dependent on the presence of specific atomic groups in their composition, for example, two OH groups, or OH and NH₂, or COOH and OH in ortho positions relative to the azo group. One example of a mordant dye is chrome blue 2K:

In wool dyeing, mordant dyes impart color with exceptional fastness and are widely used in those cases where bright colors are not necessary.