Micromeritics GEMINI VII, Operator'S Manual

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6 About Reports
For terms such as the interaction parameter found in the Horvath-Kawazoe calculation
1 )
, the
Dubinin affinity coefficient, or Astakhov exponent
2 )
, the default values as provided by the
software generally are adequate. A search of the technical literature is required if the analysis
involves a gas-solid system other than that covered by the default values.
The t-Plot method plots quantity adsorbed against thickness (t) derived from a thickness equation,
and the Dubinin transform plots quantity adsorbed against log(P/P
0
)n. All of these data reduction
methods were first proposed for specific applications. The user must make a judgment as to the
applicability of the method to a gas-solid system.
If applied appropriately, all transform plots will exhibit a linear range and the regression analysis
must be applied only over the linear range and within the range of application. Fitting a regression
line to surface area transformation plots should yield a correlation coefficient of 0.9999 or better
and for t-plots and Dubinin plots the correlation coefficient should be 0.99 or better.
If the data reduction model does not apply to the gas-solid system under examination, then it may
be that either no linear range exists within the pressure range of validity, or that solutions derived
from the regression line of the linear range are intuitively incorrect, that is, they have no relevance
to the physical situation, such as a negative C-value from a BET transform.
BET C-
VALUE
BET theory assumes uniform surface coverage with no favored adsorption sites and it also
assumes that the gas is more strongly attracted to the surface than to other gas molecules. The
typical range of BET C-values is from about 5 to well over 100. Values much less than 5 imply that
the gas-to-gas affinity is competing with the gas-to-solid affinity which conflicts with the basic
assumptions of BET theory. C-values much greater than 100 indicate very strong attraction for the
surface or preferential adsorption.
Provided the isotherm was determined with negligible error and the regression line to the BET
transformation data was fit properly, then an out-of-range C-value probably indicates that the gas-
solid interaction for the particular sample material does not conform to the BET model. An
inappropriate adsorption model may be indicated also by the coefficient of correlation of the
regression line, 0.999 being about the minimum value expected with five more or less equally
spaced points. In the case of indications of poor conformance to the BET model, the Langmuir
data reduction method should be examined.
1 )
Everett, D.H. and Powl, J.C., J. Chem Soc., Faraday Trans. 1, 72, 619 (1976).
2 )
Dubinin, M. and Radushkevich, L.V., Proc. Acad. Sci. USSR, 55, 331 (1947).
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Gemini VII Operator Manual
239-42828-01 (Rev H ) — May 2021