PHYSICAL PARAMETERS
The value of physical parameters which are used only in data reduction routines should be
reviewed to assure that they agree with experimental conditions. These parameters can be
changed and the experimental data recalculated if an error is discovered or if exploring an
alternate value is desired. Analysis condition values used in the calculation of quantity adsorbed
can be changed also. These are typically the manually entered free space(s), nonideality
correction factor, and bath temperature.
The area occupied by a single adsorbed molecule is a required parameter in the calculation of
surface area by the BET and Langmuir methods. The software provides a default value, but other
values are found in the literature. McClellan and Harnsberger
1 )
provide a comprehensive review
of such values.
The volume of pores of a specific size range is calculated from the gas quantity adsorbed in them
by converting the quantity to its liquid equivalent volume. This is achieved through use of a density
conversion factor calculated from the ratio of molar densities of the condensed adsorbate at bath
temperature to the gaseous phase at STP. The necessary information is found in handbooks. The
software contains default values for common adsorptives; values for other adsorptives must be
calculated.
The terms for liquid surface tension , contact angle between solid and liquid phase , molar
volume of the adsorbate , gas constant , and sample temperature are treated as one
constant, the adsorbate property factor expressed by:
using which, the Kelvin equation
2 )
reduces to
Either surface tension, contact angle, or molar volume can be revised individually to give a new
value for the factor , or can simply be altered arbitrarily for exploratory purposes.
The thermal transpiration correction requires two parameters which may be adjusted from those
of the default values. The first is the inside stem (neck) diameter of the sample holder, and the
second is the hard-sphere diameter of the adsorptive molecule. The sample holder inside
diameter is available from the documentation provided with it or is measurable. Information on
hard-sphere diameters of molecules may be obtained from handbooks.
1 )
McClellan, A.L., and Harnsberger, H.F., Journal of Colloid and Interface Science, 23, 577
(1967).
2 )
Thomson, W., Phil. Mag. S., 42, 448 (1871).
Evaluate Report Results
TriStar II Plus Operator Manual
303-42800-01 (Rev M ) — Sep 2023
7 - 15
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