The Diffusion of Dye Solutions

The apparent diffusion coefficient of congo red in water, obtained by Auerbach-Ostwald’s method, changes with concentration. Solving the Fick’s equation with the assumption that the. diffusion coefficient is a function of concentration, the apparent diffusion coefficient can be recalculated to differential diffusion coefficient. From the value extrapolated to infinite dilution, the particle radius of congo red was calculated using Einstein-Sutherland’s equation. On the assumption that the particle is spherical and not hydrated, the particle weight and association degree were obtained. The diffusion coefficient, obtained by this method for congo red solution containing various concentration of NaCl, agrees well with the data reported by others. With the increase of salt concentration, the particle size increases gradually. No decrease was observed in the particle size with a small addition of electrolyte.