The Brunauer, Emmet, and Teller BET isotherm

Brunauer, Emmet, and Teller have worked out a model for multilayer adsorption. They assumed that the first step in the adsorption is 

Next they assumed that additional molecules sit on top of one another to form a variety of multilayers. They interpreted the process as a sequence of chemical reactions, each with an appropriate equilibrium constant  

We can use the equilibrium conditions to calculate the values of the various 0. We have 

Combining the first two we have, 6 = 6i(Kpy. Repeating the operation, we find 

In the second writing we replaced 0 by its equal from Eq. (18.53). If we temporarily set Kp = X, this becomes 

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