The reversible addition of molecular oxygen to the tetrasodium salt of cobalt phthalocyanine-4,4?,4?,4??-tetrasulphonic acid in aqueous solution has been confirmed. Visible absorption spectra of the monomeric and dimeric species and of the oxygen adduct have been determined. A monomer-dimer system prevails at neutral pH, low ionic strength, and low dye concentrations. The oxygen adduct and the dimeric form of the dye predominate in alkaline solution.

The aggregation of the tetrasodium salt of copper phthalocyanine-4,4',4",4??? tetrasulphonic acid has been investigated by equilibrium spectrophotometric measurements at 25�. At low concentrations this can be treated in terms of a monomer-dimer equilibrium. Owing to the strong aggregating tendency of this molecule in water, it was necessary to evaluate the dissociation constant together with the monomer and dimer spectra from measurements made in 20% ethanol solutions. By means of these data, the corresponding dissociation constant in water has been determined.

The relevant thermodynamic parameters for the dimerization of copper(II) phthalocyanine-4,4?,4?,4??-tetrasulphonic acid ion have been measured in a variety of aqueous solvents. It has been found that both the enthalpy and entropy of dissociation are increased in the presence of compounds which enhance the structure of water. Conversely, both of these parameters are decreased by structure-breaking compounds. The results are discussed in terms of the bonding forces responsible for dimerization.

The effect of urea and thiourea on the aggregation of copper phthalocyanine-4,4',4",4"'-tetrasulphonic acid in aqueous solution has been studied using the capillary diffusion technique. The addition of excess inert electrolyte was necessary in order that the measurements were not complicated by facilitated diffusion. The extent of aggregation has been estimated empirically from the dependence of the diffusion coefficient on molecular weight. The significance of both the changes in diffusion coefficient and the dissociation of the dye, in relation to dyeing behaviour, is discussed.

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