Choosing Between the Two Models

The new proportionality constant D is the diffusion coefficient. Its introduction implies the other model for diffusion, the model often called Pick s law. 

These assumptions may seem arbitrary, but they are similar to those made in many other branches of science. For example, they are similar to those used in developing Ohm s law, which states that 

the mass transfer coefficient k is analogous to the reciprocal of the resistance. An alternative form of Ohm s law is 

Neither the equation using the mass transfer coefficient k nor that using the diffusion coefficient D is always successful. This is because of the assumptions made in their development. For example, the flux may not be proportional to the concentration difference if the capillary is very thin or if the two gases react. In the same way, Ohm s law is not always valid at very high voltages. But these cases are exceptions both diffusion equations work well in most practical situations, just as Ohm s law does. 

The parallels with Ohm s law also provide a clue about how the choice between diffusion models is made. The mass transfer coefficient in Eq. 1.1-2 and the resistance in Eq. 1.1-4 are simpler, best used for practical situations and rough measurements. The diffusion coefficient in Eq. 1.1-3 and the resistivity in Eq. 1.1-5 are more fundamental, involving physical properties like those found in handbooks. How these differences guide the choice between the two models is the subject of the next section. 

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It was attempted to choose between the two models on the basis of r structures. However, the asymmetric model showed no appreciable change. The angle of rotation was obtained again 75° and the deviation from the symmetric structure was present even at a 99% significance level according to a Hamilton test (Hamilton, 1965) j as seen in Figure 24. A critical remark has to be made here, however, concerning the procedure for vibrational corrections. The correction terms were calculated, as usual, in the harmonic approximation. The conversion is especially... 

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