Now,

Substituting this in equation [2]

Or

Or --------------------[3]

The Eq. 3 is known as the BET equation for multilayer adsorption. This equation can explain type II isotherm.

Table 1. Isotherms most commonly used in catalysis

Type	Isotherm equations	Type of adsorption
Langmuir		Chemisorption Physisorption
Freundlich		Chemisorption and physisorption at low coverage
Temkin		Chemisorption
Brunauer-Emmet-Teller		Multilayer physical adsorption