Micromeritics FlowSorb III 2305 Operator's Manual page 74

Appendix A
which rearranges to
Another way of arriving at the same result is by recognizing that the term
1/(V C) of equation 1 is generally small. Taking it as insignificant changes the
m
slope, and hence the value of V
equation 2, only a small amount. Equation 1 can be rearranged with the contribu-
tion of the intercept term taken to be vanishingly small to give also
Substituting equation 5 into equation 2 yields
from which the sample surface area is readily determined once the volume V of
gas adsorbed (or desorbed, which must theoretically be identical) is measured and
appropriate values for the other terms are incorporated.
For nitrogen gas adsorbed from a mixture of 30 mole % nitrogen and 70 mole %
helium using a liquid nitrogen bath, the values are arrived at as follows:
The volume V of gas with which the FlowSorb III is calibrated is injected at
room temperature and the prevailing atmospheric pressure. This volume must thus
be multiplied by the ratios 273.2/(Rm. Temp., K) x (Atm. Press., mmHg)/760 to
convert it to standard conditions (0
Avogadro's number A is 6.023 x 10
a gas at standard conditions is 22414 cm
The presently accepted value for the area N of a solid surface occupied by an ad-
sorbed nitrogen molecule* is 16.2 x 10-20 m
P is 0.3 x the atmospheric pressure in millimeters of mercury since the gas mix-
ture is 30% nitrogen and adsorption takes place at atmospheric pressure. P
saturation pressure of liquid nitrogen is typically a small amount greater than at-
mospheric due to thermally induced circulation, dissolved oxygen, and other fac-
tors. With fresh, relatively pure liquid nitrogen, the saturation pressure is
typically about 15 mmHg greater than atmospheric pressure. It can be 40 to 50
mmHg greater if the liquid nitrogen is relatively impure. The saturation pressure
should be determined by other means in the latter event.
* Roberts, B.F., J. Coll. Interface Sci. 23, 266 (1967).
A-2
V = V[1-P/P )]
m 0
V = V[1-(P/P )]
m 0
S = VAN [1-P/P ]/M
0
and the sample surface area as calculated by
m
o
C and 760 mmHg).
23
molecules/g mole. The molar volume M of
3
/g mole.
2
(=16.2 Angstroms
FlowSorb III
(5)
(5)
(6)
2
).
, the
0
Aug 00