Experimental and Theoretical Treatments of Adsorption An

Adsorption experiments are conducted at constant temperature, and an empirical or theoretical representation of the amount adsorbed as a function of the equilibrium gas pressure is called an adsorption isotherm. Adsorption isotherms are studied for a variety of reasons, some of which focus on the adsorbate while others are more concerned with the solid adsorbent. In Chapter 7 we saw that adsorbed molecules can be described as existing in an assortment of two-dimensional states. Although the discussion in that chapter was concerned with adsorption at liquid surfaces, there is no reason to doubt that similar two-dimensional states describe adsorption at solid surfaces also. Adsorption also provides some information about solid surfaces. The total area accessible to adsorption for a unit mass of solid —the specific area Asp — is the most widely encountered result determined from adsorption studies. The energy of adsorbate-adsorbent interaction is also of considerable interest, as we see below. 

The sample tube and gas burette are evacuated and then a nonadsorbing gas — frequently helium —is introduced into the gas burette. The burette is graduated with respect to volume and also serves as one leg of a manometer so that both the volume and pressure of gas in the burette can be measured. Ambient temperature is assumed to apply throughout. 

The three-way stopcock is opened to connect the gas burette with the sample tube. The new pressure and volume are read. From this, the volume of the dead space — the volume beyond the three-way stopcock that is not occupied by sample —can be determined (see Example 9.1). 

The nonadsorbed gas is pumped out and replaced by the adsorbate. Its volume, pressure, and temperature are measured, and from these the number of moles of gas introduced (initially) into the apparatus can be determined. 

The three-way stopcock is opened to connect the burette and sample tube, and volume and pressure are measured again. Since the dead space is known, the (final) number of moles of gas can be calculated. The difference between the initial and final number of moles gives the number of moles adsorbed. 

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