Interaction of copper ion with some food dyes.

The nature of the complexes formed by Cu²⁺ with some food dyes has been studied spectrophotometrically under varied conditions. The results indicated formation of 1:1 metal: dye species at pH 4.5 and of 2:1 metal: dye species at higher pH. The latter result is unexpected and surprising since in most cases azo dyes form 1:1 or 1:2 metal: dye species and no example of a metal complex having more than one metal atom per dye molecule has so far been reported. This curious observation has been rationalised in terms of a salt forming Cu²⁺ ion and a coordinated Cu²⁺ ion per dye molecule; tentative structures for these species have also been proposed. It is hoped this novel finding will stimulate further investigations into the metal-dye complexes.