This chapter will discuss the determination of diffusion coefficients in dilute solutions (i.e., the concentration of the solute is dilute). The most important factor in diffusion coefficients is the type of media that the solute is diffusing through: gas, liquid, or solid. Some order-of-magnitude values of diffusion coefficients are presented in Table 3.1, which indicates that the diffusion coefficient of a compound through a gas is four orders of magnitude greater than through a liquid and nine orders of magnitude greater than through a solid. This can make a significant difference in the rate of diffusion in the different media classifications. In fact, the manner in which the important physical process used as a model to describe “transport” is quite different for gases, liquids, and solids.

Diffusion Coefficients in Gases

Diffusion coefficients of a compound through a gas will be described with the simple kinetic theory of diffusion in gases. We could call this the “pinball theory of diffusion.” Although it can be a frustrating game to play, one can enjoy the mechanical aspects of the pinball bouncing around on the table. When we think of diffusion in gases, we can simply think of many pinballs bouncing around simultaneously after hitting the right trigger in the game. The description below may leave you with similar analogies.

Let us assume a linear concentration gradient over a distance Δx, with the change in concentration over this distance equal to ΔC, as shown in Figure 3.1.