Analysis - Micromeritics FlowSorb III 2305 Operator's Manual

Performing a Multipoint Surface Area Measurement

Analysis

The objective in multipoint surface area analysis is to obtain the volume of adsor-
bate (nitrogen, usually) at STP taken up by the sample at a series of relative pres-
sures and to treat these data in accordance with the BET equation (refer to
Appendix A, Theory) to yield the surface area. The calibration procedure for mul-
tipoint analysis was designed to set the instrument to register adsorbed gas vol-
umes. Now the purpose is to relate adsorbed gas volumes to conditions for a
specific sample.
A multipoint analysis can be conducted as a series of single-point steps using either
the FlowSorb III 2305 or 2310, progressing from lower to higher nitrogen gas
concentrations. After each concentration is established in the FlowSorb, the in-
strument is adjusted appropriately for the new gas mixture, and the adsorbed gas
volume is measured. The sample surface area is computed in accordance with the
BET equation after three or four measurements are completed.
If not previously obtained, determine the sample weight when all gases have been
employed and the data collected. Divide the measured adsorbed volumes by the
sample weight to obtain specific adsorbed volumes. As an example, the data
might be as shown in Table 3.2.
Now compute the sample specific surface area following the method given in Cal-
culating the Result.
3-14
Table 3.2. Example Adsorption Data
Gas Composition
(% N )
2
4.999
9.997
14.993
22.006
Specific Adsorbed Volume
3
(cm /g, STP)
15.16
17.39
19.16
21.46
FlowSorb III
Mar 2012