THEORY AND MEASUREMENT OF THE CONCENTRATION-DEPENDENCE OF THE DIFFERENTIAL DIFFUSION-COEFFICIENT USING A DIAPHRAGM CELL WITH COMPARTMENTS OF UNEQUAL VOLUME

A diaphragm cell consists of two well-stirred solution compartments on opposite sides of a porous membrane, which is usually a sintered glass disk. When the compartments are filled with different concentrations of a solution, diffusion sets in across the membrane. We have integrated exactly the equation of motion for the concentration difference across the membrane for the important general case where the compartment volumes are unequal and the differential diffusion coefficient is an arbitrary function of the concentration. Our integral takes the form of an infinite series involving the concentration difference. We have tested this integral experimentally in the case of aqueous solutions of HCl using diaphragm cells of the Stokes design as well as of a new design which permits the continuous determination of the electrical conductivity of the top compartment. By combining the data according to our new result, we have computed the differential diffusion coefficient and its first two derivatives with respect to concentration for HCl at 25°C and 1 M. © 1990 American Chemical Society.