Mathematical nature of the BET equation

The BET equation is equivalent to the difference between the upper branches of two rectangular hyperbolas [35] and may be written  

For c less than 2 and positive, a type 3 isotherm is generated. Values of c less than unity infer that the cohesive force between the adsorbate molecules is stronger than the adhesive force between the adsorbate and adsorbent. 

The point of inflection of the isotherm may be determined by differentiating twice with respect to x and equating to zero. This gives  

Genot [36] described a more accurate method for determining momlayer capacity by considering the mathematical nature of the BET equation. The treatment is as follows. At point M on the isotherm let V = Vm P =P and X = x. Then  

point M will result from the determination of the tangent of slope -2 on the graph of log(V) against log(l-x). This treatment produces more standardized results than using the conventional BET plot. 

---