Afterchrome acid dyes

Mordant Dyes. MetaUizable azo dyes are appHed to wool by the method used for acid dyes and then treated with metal salts such as sodium chromate [7775-11-5] sodium dichromate [10588-01-9] and chromium fluoride [1488-42-5] to form the metal complex in situ. This treatment usually produces a bathochromic shift ia shade, decreases the solubUity of the coloring matter, and yields dyeiags with improved fastness properties. The chromium salts can be appHed to the substrate before dyeiag (chrome-mordant or chrome-bottom method), together with the dye ia a single bath procedure (metachrome process), or as a treatment after dyeiag (afterchrome process). 

In afterchroming the dye is allowed to go onto the fiber from an acid bath, and this is followed by treatment with dissolved alkali metal dichromate, which is reduced by the cystine group of the wool to trivalent chromium compounds. The chromium complex formed on the wool fiber contains two azo dye radicals for each metal atom (1 2 complex). 

In the wool dye field Acid dyes were introduced from 1875 onwards. Afterchrome dyes appeared on the market in 1897. 

In mordant dyes, phenols, naphthols, and enolizable carbonyl compounds, such as pyrazolones, are generally the couplers. As a rule, 2 1 metal complexes are formed ia the afterchroming process. A typical example of a mordant dye is Eriochrome Black T (18b) which is made from the important dyestuff iatermediate nitro-l,2,4-acid, 4-amiQO-3-hydroxy-7-nitro-l-naphthalenesulfonic acid [6259-63-8]. Eriochrome Red B [3618-63-1] (49) (Cl Mordant Red 7 Cl 18760) (1, 2,4-acid — l-phenyl-3-methyl-5-pyrazolone) is another example. The equiUbrium of the two tautomeric forms depends on the nature of the solvent. 

Certain dyes when appHed by the afterchrome method are oxidized prior to metal complex formation. Examples iaclude. Chromotrope EB [3567-69-9] (52) (Cl Acid Red 14 Cl 14720) (naphthionic acid — l-naphthol-4-sulfonic acid) ia which a hydroxyl group is iatroduced by oxidation at the... 

In the afterchroming method with chrome-developing dyes, the dyeing liquor is prepared with formic acid (pH 3.5-3.8), sodium sulfate, and an amphoteric leveling agent as a wool protectant. The process is started at 40 °C, and dissolved dye is added. The system is heated, and dyeing is performed at 90°C for 30-... 

USE To dye wool from an acid bath reddish-black, which can be converted to blue-black by afterchroming. As indicator in the determination of the total calcium and magnesi -um content of water. 

---