Micromeritics Gemini VII 2390a, Operator'S Manual

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Appendix C Gemini VII
C-10 239-42828-01 - Dec 2012
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
(Pr), quantity adsorbed (Va) pairs from (Pr
1
, Va
1
) to (Pr
N
, Va
N
) where (Pr
N
= 0, Va
N
= 0) is assumed as
a final point. Each data pair represents an interval boundary (or de-sorption step boundary) for inter-
vals i=1 to i=N-1 where N = total number of (Pr, Va) 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.
Explanation of Terms
A pore filled with condensed liquid nitrogen has three zones:
a.) The
core
- evaporates all at once when the critical pressure for that radius is reached; the
relationship between the core radius and the critical pressure is defined by the Kelvin
equation.
b.) The
adsorbed layer
- composed of adsorbed gas that is stripped off a bit at a time with each
pressure step; the relationship between the thickness of the layer and the relative pressure is
defined by the thickness equation.