B.E.T. method

Harkins and Jura (Ref 5) have described an improved modification of the basic B.E.T. method for the calculation of surface area. Eigsti Dwiggins(Ref 16) evaluated this method for the determination of the particle size of chemical delay compns vs other methods, as described earlier in this article (see Table 9)... 

The surface area of a solid material is important in that it provides information on the available void spaces on the surfaces of a powdered solid [48]. In addition, the dissolution rate of a solid is partially determined by its surface area. The most reproducible measurements of the surface area of a solid are obtained by adsorbing a monolayer of inert gas onto the solid surface at reduced temperature and subsequently desorbing this gas at room temperature. The sorption isotherms obtained in this technique are interpreted using the equations developed by Brunauer, Emmett, and Teller, and therefore the technique is referred to as the B.E.T. method [49]. The surface area is obtained in units of square meters of surface per gram of material. 

Measurements of particle porosity are a valuable supplement to studies of specific surface area, and such data are particularly useful in the evaluation of materials used in direct compression processes. For example, both micromeritic properties were measured for several different types of cellulosic-type excipients [53]. Surface areas by the B.E.T. method were used to evaluate all types of pore structures, while the method of mercury intrusion porosimetry used could not detect pores smaller than 10 nm. The data permitted a ready differentiation between the intraparticle pore structure of microcrystalline and agglomerated cellulose powders. 

It Is Important to use highly accurate surface area values of a crystal particle In growth rate equations, when the equations Include a term of the surface area. In practice, however. It Is difficult to measure the surface area unless the crystal possesses a simple geometry (e.g., sphere, cube, octahedron, etc.), or unless the B.E.T. method of measuring surface area Is applied. Therefore, a characteristic diameter Is usually defined, and the area calculated from the diameter Is used. 

It is apparent from the foregoing that the B.E.T. method will give values for the monolayer which are too large when applied to clean metal surfaces using a gas whose heat of adsorption is large relative to that in ordinary van der Waal s adsorption. With a heat of adsorption of 10,000 calories on iron, nitrogen is unsuitable for surface measurements of such metals by the B.E.T. method when absolute best values are of interest. 

At present, the occurrence of stepwise adsorption in the low pressure region is the best criterion for caution in the application of the B.E.T. method. Since it is highly probable that less distinct steps or even several steps (heterogeneous systems) will occur iii other systems from which it may be difficult to discern the monolayer, one is forced to the conclusion that final judgment of the accuracy of the B.E.T. method... 

Figure 5 shows data for catalytic activity, CO adsorption at 23°, surface area by the B.E.T. method using krypton at —196°, and the fast hydrogen adsorption at —196° plotted against the temperature at which the various films were sintered. All quantities were taken as unity for films sintered at 23°C. These experiments clearly indicate that the previously observed slow adsorption of hydrogen on nickel catalysts is not adsorption but is sorption consisting of adsorption and... 

It has been shown in Section III that krypton isotherms at liquid nitrogen temperature give correct values for the surface of evaporated metal films when evaluated by the B.E.T. method. It — has also been shown that these values are in excellent... 

B.E.T. method gives about 20% larger values for oriented films as... 

B.E.T. method using nitrogen, since nitrogen is chemisorbed at — 196°C. The hydrogen adsorption at this temperature measures the surface more accurately and is in close agreement with the chemisorption of carbon monoxide at both liquid nitrogen and room temperature and with the van der Waal s adsorption of krypton. 

One could think about measuring b directly in a separate adsorption experiment. However, it has been found that the adsorption thus measured has no quantitative relation to the adsorption leading to a catalytic reaction. Not only the amounts adsorbed, but even the shape of the isotherm and the sensitivity to poisoning or surface alterations are totally different. The reason is that the adsorption experiment measures the total surface under favorable conditions (B.E.T. method), whereas the catalysis takes place on a quantitatively entirely different active ... 

The Boyd and Harkins reference might be considered the first careful calorimetric work taking into account the special problems of immersional calorimetry it was also the first such work in which surface areas could be assessed (the B.E.T. method) as a necessary auxiliary for putting values on a imit area basis temperatiue changes were measured with a 36-junction thermocouple and a White double potentiometer with a sensitive galvanometer. 

The adsorption of hafnium species on glass was found to increase with the solution pH and hafnium concentration. The effects on the adsorption of the solution preparation and age were studied and the equilibration time for the adsorption process was determined. The surface area of the glass sample was determined by the B.E.T. method using water vapor. The results are discussed in terms of the hydrolyzed hafnium(IV) species. At equilibrium, nearly monolayer coverage was obtained at pH > 4.5. Under these conditions hafnium is in the solution in its entirety in the form of neutral, soluble Hf(OHspecies. In the close packed adsorption layer the cross-sectional area of this species is 24 A which is nearly the same as for water on silica surfaces. 

Surface areas were determined by single point B.E.T. method of nitrogen adsorption. 

The surface area measurements were performed by adsorption, using nitrogen as the adsorbate. The samples were previously degassed to below 50 mmHg at room temperature and the analyses were performed at 77 K, using liquid nitrogen. The equilibrium interval was 5 s. The surface area was calculated using the Brunauer, Emmett, and Teller (B.E.T.) method. Pore volume and area distributions based on BJH calculation (8) were evaluated by the B.E.T. apparatus software (NOVA 1200-Quantachrome). 

This explains the success of the well-known B.E.T. method for this analysis. After the excellent discussion by Hill (244) of the B.E.T. and the Hiittig theories, in which he points out the weaknesses of the first and the fallacy of the latter, and after the analysis by Halsey (245), who indicates when a B.E.T. isotherm of satisfactory character is obtained on a heterogeneous surface, little need be said here. 

From Table X it is evident that fcc0 decreases with increasing granule size, and is inversely proportional to the granule size in the range of 0.55 mm. to 1.82 mm. The assumption that breakdown time is dependent on an increase in surface area is not confirmed. In Table XI, measurements of the surface area by the B.E.T. method are given for... 

The oxide was identified by the occurrence of major peaks at 4.18, 2.69, and 2.44 A (Cu radiation with curved crystal monochrometer) in the X-ray diffraction pattern (6). The surface area was 48.5 + 0.2 m g l as determined by N2 adsorption by the B.E.T. method (21). SEM pictures of the oxide revealed needle-shaped crystals approximately 1 micron in length and 0.2 microns wide. The value for the total surface sites (FeO-p) was taken from Yates (22) work on aFeOOH. Yates (22) determined FeO to be equal to 27.8 ymol m by tritum exchange. Further details of the solid s preparation, identification, and basic surface characteristics can be found in Balistrieri (23). 

In the preceding section, we have attempted to point out, on the basis of statistical-mechanical interpretations of the adsorption isotherm and of the B.E.T. method for the measurement of the surface areas of cata-... 

Three different sieved size fractions (20-45, 74-105 and 105-450 pm) of a SDDP batch have been characterized for their in vitro dissolution profiles and the textural properties of the three fractions and the original unsieved batch have been investigated by gas adsorption (B.E.T. method). Mercury Intrusion Porosimetry (MIP), light scattering and microscopy (optical and electron) techniques. 

Specific surface area was obtained using the B E T. method (Sbet)[1] In this method no assumption is made on the particles shape and the whole surface developed by the powder (including internal porosity of particles) is measured except for sealed pores. 

Surface areas were recorded for the whole series of each solid solution by the standard N2 B.E.T. method. The results are listed in Table I. Especially notable are the relatively high surface area of those compounds rich in aluminum and Al2(Mo0l )3 itself. 

Textural analysis was performed by N2 physisorption at -196°C with a Carlo Erba 1990. The surface area of calcined samples was calculated with B.E.T. method and the pore distribution with Dollymore-Heal model. 

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