The Nature of the Dye‐Chromium‐Fibre Complex in the Case of Wool Dyed with Certain Chrome Mordant Dyes
Authors: RACE E. , BOWE F. M. , SPEAKMAN J. B. 
Published: July 2010

By applying a technique of dyeing wool with a limited, known amount of pure dye by the onchrome, metachrome and afterchrome processes, removing excess chromium from the wool by continuous extraction with N‐oxalic acid solution at room temperature and relating the residual chromium on the fibre to the known amount of dye on the fibre, the constitutions of the dye‐chromium complexes formed when wool is dyed with 4‐chloro‐2‐aminophenol‐6‐sulphonic acid → β‐naphthol, 3:4:6‐trichloro‐2‐aminophenol → 1‐benzoylamino‐8‐naphthol‐4‐sulphonic acid (alkaline coupled) and Metachrome Brilliant Blue BL (I. G.) until complete exhaustion of the dye is obtained, have been established. In the case of the two last‐named dyes, whichever of the three chroming processes is applied, the complex which is formed on the fibre has 2 mol. dye associated with 1 atom of chromium. In the case of 4‐chloro‐2‐aminophenol‐6‐sulphonic acid → β‐naphthol, a similar complex is formed by the onchrome and afterchrome processes, whereas by the metachrome process either a true dye‐chromium lake is not formed or that which is formed is not stable to N‐oxalic acid solution at room temperature.

The complex in which 2 mol. dye are associated with 1 atom of chromium is probably a tribasic acid; e. g. in the case of 3:4:6‐trichloro‐2‐aminophenol → 1‐benzoylamino‐8‐naphthol‐4‐sulphonic acid (alkaline coupled) its structure is‐→

When, in accordance with these results, stoichiometric amounts of 3:4:6‐trichloro‐2‐aminophenol → 1‐benzoylamino‐8‐naphthol‐4‐sulphonic acid (alkaline coupled) and chromium for the formation of the 2:1 complex are applied to wool by each of the three chroming methods, it is found that the whole of both dye and chromium is present in the dye‐chromium complex on the fibre, and the resultant shades are considerably brighter than those produced by the normal chroming methods in which up to a 10 times excess of chromium is used. Conversely, when wool which has been dyed with this dye by any of the three chroming methods using a normal quantity of potassium dichromate or chromate is extracted with N‐oxalic acid solution at room temperature until chromium is no longer stripped from the fibre, the resultant shades are considerably brighter than those produced during dyeing, and approximate very closely to those produced by applying stoichiometric amounts of dye and chromium to the fibre.

The acid 2:1 complexes of 3:4:6‐trichloro‐2‐aminophenol → 1‐benzoylamino‐8‐naphthol‐4‐sulphonic acid (alkaline coupled) and Metachrome Brilliant Blue BL are readily formed in substance by refluxing stoichiometric amounts of dye and chromium sulphate. They are only sparingly soluble in water and, in the absence of assistants in the dyebath, dye wool in a blue shade similar to that produced in a dyeing using stoichiometric amounts of dye and chromium for the formation on the fibre of the 2:1 complex.

Since the only charges associated with the 2:1 complex are the anionic valencies of chromium and sulphonic groups, the only possible combinations of complex and fibre appear to be salt linkages between chromium and sulphonic groups in the complex and amino‐groups in the fibre. Thus, the complex, once it is formed in the fibre, is, in effect, an acid dye of high mol. wt., and the increased fastness properties of the chrome dyeings as compared with corresponding unchromed dyeings must be attributed (a) to the fact that the lake, having a molecular size more than double that of the parent dye, is bound to the fibre by van der Waals forces which are commensurate with its size, and (b) to the possibility that the lake molecules, having been formed in situ, are too large to escape from spaces which were large enough to allow initial entry of individual dye molecules and chromium ions.