The aggregation of dyes in aqueous solution in the presence of an excess of inert salt has been studied by a polarographic technique. By comparing the limiting current (when all the electroactive ions or molecules reaching the electrode are reduced) given by a dye with that given by the same molar concentration of cadmium ions, the diffusion coefficient of the dye can be compared with that of the (known) diffusion coefficient of cadmium ions. The molecular weight of the aggregate was deduced from its diffusion coefficient by means of an empirical linear relationship between the logarithm of molecular weight and the logarithm of the diffusion coefficient for a large number of substances.Tests with four well-characterized " textile " dyes, viz : Congo Red, Crystal Violet, Methylene Blue and Solway Ultra Blue B have shown that the above technique is reliable and gives reproducible results which agree well with data available for the aggregation of these dyes in the presence of an excess of electrolyte.

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Metachromatic behaviour has been induced in the visible spectra of solutions of ionic dyes by increasing the dye concentration, adding salts, and by lowering the dielectric constant of the solvent from medium to low values. The spectral changes are of the same general type, irrespective of the method used, and are exhibited only by those dyes in which the characteristic charge is an integral part of the chromophoric system. The H-bands which appear are attributed primarily to interaction between the dye ions and counter-ions, although in aqueous solution aggregation is important too, probably because counter-ions are included in the aggregates. A new band (L-band), at even shorter wavelengths than the H-band, was observed in solvents of very low dielectric constant and from which the dye soon precipitated. This L-band is attributed to strong interaction between dye ions and counter-ions to produce an undissociated species in which the ions are not separated by solvent molecules. Non-isosbestic spectral changes which occur in water at high dye concentrations are attributed to the presence of a small proportion of this species. The present hypothesis permits a simple explanation of metachromatic behaviour in accordance with the principles of colour and constitution.

, AND ROB-ERT B. McKAY. Adsorption of cationic (basic) dyes by fixed yeast cells. J. Bacteriol. 89:390-397. 1965.-The adsorption of 10 typical cationic dyes on formalin-fixed yeast cells has been studied by determining isotherms, and the results are consistent with an ion-exchange mechanism. The adsorption on this complex substrate is similar to that on the simpler substrate, alumina. The dyes are probably aggregated when adsorbed, and the size of the aggregates increases with increase in the molecular weight of the dye ion. After considering the possible adsorption sites, and comparing the data with adsorption on simpler substrates, we suggest that the most important adsorption sites may be phosphate or other strongly acidic groups.