Micromeritics Flowsorb II 2300 User manual

Instructions for the Flowsorb II 2300
BET –Brunauer, Emmet & Teller
CALIBRATION
1. Make sure that clean tubes are connected in the test position (TEST) and at the “Cold trap” position.
2. Set the gas to 15 psi (lb/in2) by opening the gas tube’s regulator and flip the switch at the side of the
instrument so that it is pointing straight out.
3. Adjust the FLOW so that the ball and line coincides. The position of the ball shall not very (which
usually is an indication of leakage)
4. Let the He/N2gas mixture flow at least 15 min before the electronics is turned on. This step is
important. The instrument can be damaged if is not performed.
5. Attach the sample in the leftmost holder (DEGAS). Attach the heating mantle in “MANTLE” and
“THERMOCOUPLE”. If the sample is sensitive to heat the temperature should be lowered. Wrap the
mantle around the sample. Heat the sample for 10 min –3 h.
6. DET shall display 000.00, else adjust it with COUARSE ZERO and FINE ZERO.
Note: Sometimes the integration does not work if DET displays -000.00.
7. (If you turn the ZERO-knobs maximum clockwise the display shows the gas’ N2-content in percent. )
8. PATH –most often at SHORT. Only at LONG when very large surfaces (200m2/g) or if there is a
great difference between adsorption and desorption. This since there is not sufficient time if the
surface is large and all gas leaves at once.
9. Suck up 1 ml nitrogen gas from above the surface of the liquid N2container. After 10-15 s. the DET-
value will increase before it slowly decreases again.
10. Wait until DET is under 000.03. Read the surface area.
11. Adjust with CALIBRATE if it does not corresponds to the calculated value (2.84 m2)
12. If it does not correspond, push CLEAR SA DISPLAY and repeat steps 9-11.
13. You can also calibrate using kaolin clay according to MEASUREMENT below.
MEASUREMENT
1. Unplug THERMOCOUPLE and remove the heating mantle
2. Attach the sample tube in the holder “TEST”
3. Immerse the tube in water (in a beaker)
4. Allow DET to reach at least 000.03
5. Reset the surface area value using CLEAR SA DISPLAY
6. Immerse the sample tube in a Thermos with liquid nitrogen by flipping up the metal plate. When the
plate is raised the sample will start adsorbing nitrogen. The DET value will increase and then
decrease.
7. Wait until DET reaches at least 000.03. Read the adsorbed surface are.
8. Again, reset the surface area value with CLEAR SA DISPLAY
9. Remove the nitrogen Thermos (hold the sample tube and lower the metal plate by pressing
RELEASE) and replace the Thermos with a beaker with room temperature water to slowly heat the
sample. Remove any frost from the tube’s upper part.
10. Desorb in room temperature
11. Read the surface area at DET at least 000.03
12. Divide the surface area with the weight of the degassed sample.
13. Turn off the instrument
14. Wait 15-20 minutes before the gas flow is turned off (the same way it was turned on)

FORMULA FOR THE SURFACE:
22
%%
..
100 100
273.2 . 6.023 16.2 .
1 1.565 1
760 22.414 . 15 . 15
NN
atm press atm press
atm press atm press
SVRoomtemp 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):
 
 
cP
P
c
c
PP mm 
11
1/
1
00












(1)
Pand P0are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption,  is
the adsorbed gas quantity (for example, in volume units), and 
mis the monolayer adsorbed gas quantity. c
is the BET constant, which is expressed by (2):






RT
EE
cL1 exp
(2)
E1is the heat of adsorption for the first layer, and
ELis 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[(P0/ P) − 1] on
the y-axis and φ= P/ P0on 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/ P0<
0.35. The value of the slope Aand the y-intercept I
of the line are used to calculate the monolayer
adsorbed gas quantity vmand the BET constant c.
The following equations can be used:
IA
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 Stotal and a specific surface area S are evaluated by the
following equations:
V
Ns
Sm
totalBET


,
(5)
a
S
Stotal
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