The indicator method for the determination of coefficient of diffusion in gels, with special reference to the diffusion of chlorides

Abstract

Since the original investigations of Thomas Graham on the diffusion of dissolved substances in water a number of methods have been evolved and used for the determination of coefficients of diffusion of solutes in water. The essential Feature of all these methods is the estimation of the quantity of dissolved substance after the lapse of a definite time in different lasers of a cylinder of water through which the solute is diffusing, the calculation of the coefficient then being made on the assumption of Fick's law, which defines the coefficient of diffusion as D in the equationdQ = -DA δc/δxdtwheredQ is the quantity of solute diffusing through a cross-section of the cylinder in the time at a point where the concentration gradient is δc/δxand A is the cross-section of the cylinder. A variety of methods have been used for estimating the concentration of the solution at different levels, these methods including chemical analysis of the lasers, specific gravity determinations, optical methods, such as determination of refractive index or optical rotation, determinations of electrical conductivity, and other electrical methods. These methods for the most part require special apparatus, and the determinations are not always easy to make. These are not, of course, to be regarded as drawbacks, but a glance at the results obtained by their means as summerised, for example, in Landolt and Börnstein's tables, is sufficient to give an impression of unreliability since the determinations of different workers often show very considerable divergence.