AN OPTICAL AND SPECTROELECTROCHEMICAL INVESTIGATION OF INDIGO CARMINE

The existence of a monomer-dimer equilibrium for indigo carmine was found in the concentration range < 3 mM in aqueous solution. The extinction coefficients at lambda = 608 nm of the monomer, epsilon(M) = 15000 M-1 cm-1, and of the dimer, epsilon(D) = 18500 M-1 cm-1, were calculated from uv-vis spectroscopic data, and a value of K(D) = 4 . 10(-4) M in 0.1 N H2SO4 was obtained for the equilibrium constant. The diffusion coefficient of the monomer, D(M) = 0.95(+/- 0.3) . 10(-5) cm2 s-1, was determined in a spectroelectrochemical experiment.