Page 57 - Essentials of physical chemistry

Ideal and Real Gas Behavior 19

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FIGURE 1.8 Schematic diagram of a table top supercritical fluid chromatograph. The numbers in the figure
are explained in the Internet site listed where this is an animated GIF picture which simulates the passage of a
sample through the instrument. (Karey O’Leary and Advisor Prof. Andrea Detrich of Virginia Tech University at
http:==www.cee.vt.edu=ewr=environmental=teach=smprimer=sfc=sfc.html)

FLUIDS
The term ‘‘fluid’’ includes not only liquids but also anything that ‘‘flows’’ such as powdered coal or
sand slurries in water and of course airflow. Experimentally, the critical temperature for carbon
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dioxide is 30.988C, the critical pressure is 73.75 bar, and the critical volume is 94 cm =mol [1].
Above the critical temperature, the fluid is called supercritical fluid. We can compare that to the
value estimated from the van der Waals equation using data from Table 1.3. The result is within
0.258 of the experimental value.
2
2
8a 8(3:658 L bar=mol )
¼ 303:88 K ¼ 30:73 C

27Rb 27(0:08314 L bar= K mol)(0:0429 L=mol)
T c ¼ ¼
However, we can see that the van der Waals equation is less accurate for the critical volume and the
critical pressure
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V c ¼ 3b ¼ 3(0:0429 L=mol) ¼ 128:7 cm =mol
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instead of 94 cm =mol and
2
3:658 L bar=mol 2 1000 cm 3 2
2
c
P c ¼ a=3V ¼ 3 2 ¼ 73:61 bar
3(128:7 cm =mol) L
which is close to experiment.
Apparently the van der Waals equation gives good estimates of the critical temperature and
critical pressure that are of practical interest but does poorly for the critical volume so there are other
equations of state chemical engineers use when more accuracy is required!
This critical temperature of CO 2 is only a few degrees above room temperature, which makes it
safe to treat organic compounds with little danger of thermal degradation. Although carbon dioxide
is an excellent solvent for nonpolar compounds it is less good for polar compounds.
It has been found that for nonpolar materials CO 2 is a very good solvent but to keep the mobile
phase (gas) flowing, a column temperature slightly above the critical temperature is used so that the
temperature is ‘‘super’’ critical and the pressure is regulated near the critical pressure to keep the
mobile phase nearly a liquid (Figure 1.9). Under these conditions SCF chromatography has