Page 162 - Principles of Catalyst Development

150 CHAPTER 7
error, and with c - c - I we have

(7.8)

as the basis of the "one point" method in many automated instruments.
Other approximations used are(21bl

(7.9)
or

and are sufficiently accurate for most purposes. One additional feature must
be emphasized. The "dead" volume correction is a function of the pressure.
For small areas of less than 5 m g - \ this correction becomes significant,
2
and precision suffers. However, by using krypton as the adsorbate, better
accuracy is possible. Krypton has a Po value of 3 torr, compared with 780 torr
for nitrogen. The BET range up to p/Po = 0.3 is covered at pressures 100
times lower, with much less dead volume correction.
In addition to volumetric measurements, gravimetric methods, with
microbalances are sometimes used. A typical assembly is shown in Fig.
7.12. Gravimetric methods are preferred with heavier adsorbates such as
hydrocarbons.
Also, techniques based on flow systems are used. These have the
advantage that high vacuum or pressure measuring devices are not required.
They are rapid and accurate enough for most purposes. A typical system
is shown in Fig. 7.13. Helium carrier gas containing a known partial pressure
of nitrogen passes over a degassed sample at liquid-nitrogen temperature.

MICROBALANCE
L VA.CUUM

_ AND
GAS DOSING
SYSTEM

__ TARE
SAMPLE -

Figure 7.12. Gravimetric adsorption apparatus.