Comparison of parameters calculated from the BET and Freundlich isotherms obtained by nitrogen adsorption on activated carbons: A new method for calculating the specific surface area

The empirical linear relationship between the BET surface area SBET and the Freundlich constant KF, calculated from nitrogen adsorption isotherms of activated carbons, SBET = a0KF is mathematically demonstrated. This correlation exists in the relative pressure domain in which the BET equation is valid, whatever the value of c for the BET equation and for values of the Freundlich exponent, α, between 0 and 0.2. This study allows to determine the correlation factor a0 = 1/a   with a=1SN2N(α+1)α+1αα. From this result, a new expression, depending of α and KF, can be deduced for calculating the specific surface area.