Interference measuring surface permeabilities

If we abandon the very unlikely case of anisotropic diffusion with principal tensor axes which are not perpendicular to the crystal faces normal, molecular fluxes may quite generally be assumed to be directed perpendicular to the crystal surfaces. Hence, molecular uptake and release may be considered to proceed via one-dimensional diffusion quite generally, as long as the fluxes stemming from different crystal faces do not superimpose upon each other. This includes in particular the initial phases of uptake and release. We shall see that due to this reason, by measuring surface permeabilities, interference microscopy is in general able to quantify the intensity of surface resistances. 

Fig. 45 Comparison of the transient concentration profiles during methanol uptake by the MOF-type crystal as recorded by interference microscopy (symbols) with the corresponding profiles recalculated from the measured diffusivities with surface permeabilities (full line in Fig. 44) which lead to the best fit to the experimental points...

If more than a single type of ion contributes to the measured potential in Equation 5.4, the potential can no longer be used to quantify the ions of interest. This is the interference in a potentiometric sensor. Thus, in many cases, the surface of the active electrode often incorporates a specific functional membrane which maybe ion-selective, ion-permeable, or have ion-exchange properties. These membranes tend to selectivity permit the ions of interest to diffuse or migrate through. This minimizes the ionic interference. 

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