The colloid chemistry of dyes: The aqueous solutions of benzopurpurine 4B and its isomer prepared from m-tolidine.—Part II

Abstract

A number of investigators, including Knecht, Bayliss, Biltz and von Vegesack, Pelet-Jolivet, Donnan and Harris, and Zsigmondy and his co-workers have made measurements of the conductivity and osmotic pressures of benzidine dyes, congo red in particular having been much studied. From these results it appears that although the conductivities of the dyes have a high absolute value suggesting a highly dissociated or completely dissociated electrolyte, the osmotic pressure approximately corresponds to the pressure calculated from the molecular weight, that is to say, the pressure is that which would be expected if we assume the dye to exist in solution in single undissociated molecules. A number of attempts have been made to explain this very striking anomaly. Thus Donnan and Harris point out that although the observed values for the osmotic pressure could be explained by assuming complexes dissociated somewhat as follows (Na₂R)_n = nNa⁺ + (NaR)_n^n- this cannot account for the values obtained for the conductivities.