Studies in chrome mordanting. VII.The nature of the chromium-dye complex formed during the chrome mordant dyeing of wool

Abstract

When pre-chromed wool is immersed in a boiling solution of a mordant dye, the dye is rapidly absorbed by the wool. This is followed by a slower reaction in which the dye reacts with the chromium(111) in the wool to form either a 1 : 1 or a 1 : 2 chromium(111)-dye complex. With solo-chrome violet RS only a 1 : 2 complex could be detected in agreement with the results of earlier workers, but with solo-chromate fast red both 1 : 1 and 1 : 2 complexes were detected in the wool. The equilibrium [Cr(dye)L(H₂O)₂] + dye ←→ [Cr(dye)₂] + L + 2H₂O lies further to the right when L is a negatively charged ligand such as SO₄^2- or F^- than when L is a neutral ligand such as H₂O. This accounts for the preferential formation of 1 : 2 complexes in wool, where L is a carboxylate ion. It is shown that on average 1 : 1 chromium(111)-dye complexes are bound to slightly more than one carboxyl group. An explanation is advanced for the higher wash-fastness of the 1 : 1 and 1 : 2 chromium(111)-dye complexes relative to the corresponding free dyes.