SUBJECTS mordants

Most mordant dyes are monoazo stmctures. The most important feature of this class of dyes is excellent fastness to light and washing. Mordant dyes are available ia aU shades of the spectmm with the exceptioa of bright violets, blues, and greens. To be useful, the metal complexes must be stable, ie, must not demetallize when subjected to dyebath conditions and aU aftertreatment processes, especially repeated washings. Chromium forms stable chelate rings with mordant dyes which are not affected by treatment with either weak acid or alkaU (see Coordination compounds). 

The classical forerunner of metallizable hydroxyanthraquinones was alizarin (1), which was widely used in the production of very fast, bright red shades on cotton by the mordant technique (Section 58.1). Early studies by Pfeiffer140 demonstrated that metal complex formation involved the carbonyl group and the a-hydroxy group (208) rather than the two hydroxy groups and led to intensive research on metal complexes of 1 -hydroxyanthraquinones and related compounds such as 8-hydroxynaphthoquinone. This is the subject of a number of reviews3a Jc141 and will only be considered briefly here. 

The more widely applicable mordants are inorganic basic salts which form compounds with acidic dyes. When cotton is treated with aluminium acetate and subjected to the action of steam, the salt undergoes hydrolysis and basic acetates are formed. Cloth treated in this way can be dyed directly, as the basic salts form insoluble compounds with dyes. In mordanting cloth to be dyed with basic colors, a compound is used which is an acid or yields an acid on hydrolysis. Tannic acid. 

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