The BET Isotherm

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The most widely used isotherm dealing with multilayer adsorption is the BET isotherm:

Where V_ratio = V / V_mon, with V_mon as the volume corresponding to monolayer coverage, c is a constant that is large when the enthalphy of desorption from a monolayer is large compared with the enthalpy of vaporization of the liquid adsorbate, and z = p / p^*, where p^* is the vapour pressure above a layer of adsorbate that is more than one molecule thick and which resembles a pure bulk liquid.

If we rewrite the BET isotherm as

It follows that a plot of y = z / (1-z)V against z should be a straight line of slope, slope(z, y) = (c-1) / cV_mon and intercept, intercept(z,y) = 1 / cV_mon, and

To graph V_ratio against z select the BET isotherm tab, enter the parameters and hit New Plot. Up to 5 plots can be displayed at one time. The Clear button will remove all plots. To see the parameters for each plot hit the Legend on/off button. The Redraw button will refresh the graph. This is useful when the function domain (i.e., z) has been changed. To see the value of each plot at a given point, move your cursor to the desired location then click and hold.

The Data tab contains sample data for the adsorption of N₂ on TiO₂ at 75 K. The volumes have been corrected to 1.00 atm and 273 K and refer to 1.00 g of substrate. The Data Plot tab graphs the data and a "best fit line" and the Linear Regression Analysis tab lists the parameters of this line, and the calculated values of c and V_mon.