Film diffusion interruption tests

Several methods are available to ascertain whether film diffusion is rate determining for a given system (e.g., interruption test, determination of the Helfferich number) but will not be discussed here in more detail. 

All the previously described mechanism criteria rely in some way or another upon comparing the predictions of rate theory with the observations. The interruption test , first described by Kressman, is a favoured means of differentiating between film and particle diffusion rate control since it places no reliance upon theoretical boundary conditions or exchanger properties. 

In the interruption test (Kressman and Kitchener, 1949), the ion exchanger is temporarily separated from the liquid. If the rate is controlled by mass transfer in the liquid, the rate upon reimmersion is the same as at the time of separation, the quasi-stationary state in the film is very quickly re-established. If the rate is controlled by intraparticle diffusion, concentration gradients within the particle have time to relax, and the rate is faster upon reimmersion. This comparison is independent of specific mechanisms and algebraic forms of rate laws. 

Interruption tests indicated that diffusion of the ions across a liquid layer (film) of unstirred solution surrounding the peat particles is the ratecontrolling step (Bunzl, 1974a). Accordingly, rate equations for film diffusion-controlled, ion-exchange processes for the boundary and initial conditions of our experiments are derived and compared to experimental results. 

The above theory will of course not hold, if film diffusion is not the rate-determining step. This can be the case if e.g., the desorption process is very slow or even almost irreversible (e.g., as a result of conformational changes of the polydispersed molecular aggregates). Therefore, before any rate equations are applied, the rate-determining step should be confirmed (e.g., by interruption tests). 

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