Diffusion coefficients of ferricyanide ions in polymeric solutions - comparison of different experimental methods

The molecular diffusion of ferricyanide ions in polymeric Newtonian and non -Newtonian solution has been studied at 25 degrees C with different experim ental methods: rotating-disc flow, Couette-flow and unsteady diffusion. It is shown that the Levich equation established for the determination of diff usion coefficients with the rotating-disc electrode method cannot be applie d for Reynolds number values less than 30 for Newtonian and power-law fluid s, when the momentum boundary layer thickness is of the same order of magni tude than the disc radius. The unsteady diffusion method seems to be the mo st suitable technique to determine the diffusion coefficient in highly or v iscous non-Newtonian electrolytes. For the studied polymer solution, it is shown that the decrease of diffusion coefficient is much slower that the in crease in dynamic viscosity. Then, the classical Stokes-Einstein equation, D mu/T = cst, is not valid for electrolyte solution with high viscosity. (C ) 2000 Elsevier Science Ltd. All rights reserved.