Defined by \[D_{i} = \frac{-J_{i}}{\nabla c_{i}}\] where \(J_{i}\) is the amount of species \(i\) flowing through unit area in unit time and \(\nabla c_{i}\) is the @C01227@ of species \(i\). Different @D01716@ coefficients may be defined depending on the choice of the frame of reference used for \(J_{i}\) and \(\nabla c_{i}\). For systems with more than two components, the flow of any component and hence its @D01719@ depends on the concentration distribution of all components. The limiting differential diffusion coefficient is the value of \(D_{i}\) extrapolated to zero concentration of the diffusing species: \[[D_{i}]=\lim _{c_{i}\rightarrow 0}D_{i}\]