Calculating Free-Space Values For Micropore Analyses - Micromeritics ASAP 2020 Confirm Operator's Manual

Appendix E
E. CALCULATING FREE-SPACE VALUES
Many microporous materials, such as zeolites and activated carbons, trap and hold helium in
their complex pore structures for many hours after being exposed to helium. Helium trapped
in micropores can interfere with the analysis at low pressures, causing an "S"-shaped curve at
the lower end of the isotherm. For this reason, it is recommended that you enter the warm and
cold free-space volumes when performing micropore analyses, therefore avoiding exposure
of the sample to helium. Two techniques can be used for determining warm and cold free-
space values.
The first method is to perform a short analysis on the sample after partial degassing (one pres-
sure point with no incremental dosing), but prior to final sample preparation. Measure the free
space during this analysis. The measured free-space values will be printed on the report and
may then be entered into the sample file after more thorough sample preparation.
The second method requires prior tests using empty tubes that will be employed later for the
sample analyses. The measured free-space data can be used thereafter on every analysis per-
formed using these sample tubes. This small initial investment of time will save considerable
time later. Perform an empty tube analysis on each sample tube you intend to use for micro-
pore analysis. Measure the free space of each sample tube, taking only one pressure point.
Four important things to remember are: 1) be consistent in using seal frits; for example, use
the same seal frits for the analysis as you did for the empty tube test; 2) since the cold free
space is dependent on bath temperature, perform a test for each bath temperature to be used;
3) the isothermal jacket must be in the same position for the sample analysis as it was for the
empty tube test; and 4) correct the free-space volumes obtained for the volume displaced by
the sample when you use them.
To make the correction, subtract the amount of gas displaced by the sample. The calculations
are simple and are given here with a brief explanation of their derivation. Remember to
employ the appropriate free-space values for each bath temperature used, stating all temper-
atures in Kelvin.
To correct warm free space:
V
ws
where
V
ws
V
wm
M
s
r
s
T
amb
T
std
202-42811-01 - Dec 2013
FOR MICROPORE ANALYSES
M T
 
s std
------ -  ----------- -
= V –
 
wm 
T
s amb
= calculated warm free space with sample present (in standard cm
= warm free space measured for the empty tube (in standard cm
= mass of sample to be analyzed (in grams)
= approximate sample true density (in grams/cm
= ambient temperature (in Kelvin)
= standard temperature (273.15 Kelvin)
ASAP 2020 Confirm
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