Micromeritics ASAP 2460, Operator'S Manual

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Appendix C ASAP 2460 Operator’s Manual
C-14 246-42800-01 - Aug 2013
f-Ratio Method
The f-ratio is the quantity adsorbed divided by the quantity adsorbed in a reference isotherm at the
same pressure.
The reference quantity adsorbed is found by spline interpolation of the reference isotherm.
BJH Pore Volume and Area Distribution
For adsorption data, the relative pressure and quantity adsorbed data point pairs collected during an
analysis must be arranged in reverse order from which the points were collected during analysis. All
calculations are performed based on a desorption model, regardless of whether adsorption or desorp-
tion data are being used.
The data used in these calculations must be in order of strictly decreasing numerical value. Points
which do not meet this criterion are omitted. The remaining data set is composed of relative pressure
(
P ), quantity adsorbed ( Q ) pairs from ( P
1
, Q
1
) to ( P , Q
n
) where ( P
n
= 0, Q
n
= 0) is assumed as a final
point. Each data pair represents an interval boundary (or desorption step boundary) for intervals i=1 to
i=n-1 where n = total number of (
P , Q ) pairs.
Generally, the desorption branch of an isotherm is used to relate the amount of adsorbate lost in a
desorption step to the average size of pores emptied in the step. A pore loses its condensed liquid
adsorbate, known as the core of the pore, at a particular relative pressure related to the core radius by
the Kelvin
7
equation. After the core has evaporated, a layer of adsorbate remains on the wall of the
pore. The thickness of this layer is calculated for a particular relative pressure from the thickness equa-
tion. This layer becomes thinner with successive decreases in pressure, so that the measured quantity
of gas desorbed in a step is composed of a quantity equivalent to the liquid cores evaporated in that
step plus the quantity desorbed from the pore walls of pores whose cores have been evaporated in that
and previous steps. Barrett, Joyner, and Halenda
8
developed the method (known as the BJH method)
which incorporates these ideas. The algorithm used is an implementation of the BJH method.
f
i
Q
i
Q
ref
P
i
-----------------
=