Micromeritics Flowsorb II 2300, Instructions For Use

Page: 2 / 2

FORMULA FOR THE SURFACE:
2 2
% %
. .
100 100
273.2 . 6.023 16.2 .
1 1.565 1
760 22.414 . 15 . 15
N N
atm press atm press
atm press atm press
S V
Room temp atm press room press atm press
 
   
   
 
   
 
   
   

  
   
     
   
   
  
 
  
    
   
   
 
   
 
 
 
 
 
 
 
BET Theory
The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular
adsorption, to multilayer adsorption with the following hypotheses:
(a) gas molecules physically adsorb on a solid in layers infinitely;
(b) there is no interaction between each adsorption layer; and
(c) the Langmuir theory can be applied to each layer.
The resulting BET equation is expressed by (1):
 
 
c P
P
c
c
P P
m m
  
1 1
1 /
1
0 0










(1)
P and P
0
are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption,

is
the adsorbed gas quantity (for example, in volume units), and

m
is the monolayer adsorbed gas quantity. c
is the BET constant, which is expressed by (2):








RT
E E
c L 1 exp
(2)
E
1
is the heat of adsorption for the first layer, and
E
L
is that for the second and higher layers and is
equal to the heat of liquefaction.
Equation (1) is an adsorption isotherm and can be
plotted as a straight line with 1 /
v [( P
0
/ P
) − 1] on
the y-axis and φ = P / P
0
on the x-axis according
to experimental results. This plot is called a BET
plot . The linear relationship of this equation is
maintained only in the range of 0.05 < P / P
0
<
0.35. The value of the slope A and the y-intercept I
of the line are used to calculate the monolayer
adsorbed gas quantity
v
m
and the BET constant c .
The following equations can be used:
I A
m


1

(3)
I
A
c
 
1
(4)
BET plot
The BET method is widely used in surface science for the calculation of surface areas of solids by physical
adsorption of gas molecules. A total surface area S
total
and a specific surface area S are evaluated by the
following equations:
V
Ns
S m
total BET


,
(5)
a
S
S total
BET

(6)
N : Avogadro's number,
s : adsorption cross section,
V : molar volume of adsorbent gas
a : molar weight of adsorbed species
Adsorption of Gases in Multimolecular Layers , Stephen Brunauer, P. H. Emmett, and Edward Teller, J. Am.
Chem. Soc.; 1938; 60 (2) pp 309 - 319