Harkins and Jura absolute method

In addition to the relative method, Harkins and Jura have also developed an absolute method for surface area measurement which is independent of 

1 g solid with moles of adsorbed vapor and (a —a ) moles of vapor 

The terms AHj, L, AH yUnd i used in Fig. 7.1 are all enthalphy changes defined as follows AHi is the heat of immersion of the solid into the liquid, L is the latent heat of condensation, AH yis the heat of adsorption when the solid is equilibrated with saturated vapor, and i is the heat liberated when solid in equilibrium with saturated vapor is immersed into liquid. Using Hess s law of heat summation 

The quantity AH —aj L) is the integral heat of adsorption. This value as well as the value of i can be measured calorimetrically. The value of i is actually zero if the isotherm approaches the ordinate asymptotically. If the isotherm cuts the ordinate at a finite angle, i will be finite but small. 

---

An important contribution made by the Harkins and Jura absolute method, however, must not be overlooked. Their measurements of some specific surface areas give confirmation to the value of 16.2 for the cross-sectional area of nitrogen. This value, when employed with the BET theory, gave exactly the same specific surface area as the HJ absolute method. 

Figure 6.2. Principle of the Harkins and Jura absolute method for determining the surface area.

The extent of the solid surface for a given liquid—solid system, the immersion energy increases with the surface area (applications measurement of the surface area either by comparison, using a reference material, or by applying a modified absolute Harkins and Jura method). 

The adsorption isotherm yields the amount of gas adsorbed on the surface. Unless the molecular area occupied by the adsorbed gas is known, the adsorption isotherm yields only relative surface areas rather than the absolute values. This is the reason for using only one gas (nitrogen or krypton) to determine the surface areas of different solids. However, Harkins and Jura [45] developed an absolute method of... 

The Harkins and Jura (36) absolute method of calculating specific surface area from adsorption data apparently gives more consistent results than the BET method when different adsorbates are used on dinerent kinds of solids. It is based on an empirical equation ... 

Another method to determine the surface area comes from / theory. The values obtained by this method (as analyzed by Condon [13]) seem to agree with some other methods, such as the absolute method of Harkins and Jura [14] and the conclusions by Kaganer [15, 16], It also consistent with X-ray analysis for some porous samples. For a non-porous, single energy surface the following equation holds according to / theory [17] ... 

Harkins and Jura [7] described a method of obtaining the surface area in an absolute way from a calorimetric measurement. They addressed many of the concerns regarding the method [8] but one must still qualify the method as being very limited. Porosity of any type would significantly alter the answer. 

Harkins and Jura [20] describe a method to obtain the absolute surface area of a solid by the following method. Firstly, the powder is exposed to a high vapor pressure of water. Indeed it is best to expose it in a high-sensitivity calorimeter over a reservoir of water. The powder is then allowed to fall into the reservoir and the amount of heat produced is measured. By doing so, one eliminates the outer surface of the adsorbed film releasing the energy associated with the liquid-gas interface surface tension. Since the liquid-gas surface tension energy is known one may then calculate from the amount of heat released the area of the powder (or at least the outer surface area of the adsorbed film before immersion). 

A more recent investigation has revealed that this problem can be overcome by using water as the liquid since two molecular layers are sufficient to effectively screen the underlying surface of many adsorbents. These results have led to a modification of the original Harkins-Jura "absolute method for surface area determination and they make it possible to apply the technique to mesoporous solids (by avoiding the complication of capillary condensation). Obviously, the approach cannot be used in isolation to study micropore filling, activated entry or molecular sieving, but it becomes a powerful tool when combined with gas adsorption. 

If the solid is first equilibrated with saturated vapor, then immersed in pure liquid adsorbate, the solid-vapor interface is destroyed and the heat liberated should correspond to /,. die surface energy of the pure liquid. The above assumption is made in what is termed the absolute method of Harkins and Jura (HJa) [107] who obtained a heat of immersion of 1.705 kJ kg-1 for titanium dioxide which, when divided by the surface energy of the adsorbent, water (11.8 kJ kg-1) gave a surface area of 14.4 m2 g-1 in agreement with the BET value. For a comprehensive bibliography and description of the calorimeter used, readers are referred to Adamson [30]. The validity of the HJa method may be questioned because exposure to a saturating vapor causes capillary condensation which reduces the available surface. A correction is also required for the thickness of the adsorbed film. 

Harkins W D and Jura G 1944 An absolute method for the determination of the area of a finely divided orystalline solid J. Am. Chem. Soc. 66 1362-6... 

Equations (33) and (34) form the theoretical basis for the absolute Harkins-Jura (HJ) method [76,94] to estimate the solid surface area. However, in the earlier calorimetric experiments applying the Harkins-Jura principle, the term QjJJ, was always neglected. Neglecting it may lead to certain discrepancies between the surface areas determined by the Harkins-Jura and BET methods in the case of water adsorbed on oxides. 

Jagiello J, Olivier J P (2009) A Simple Two-Dimensional NLDFT Model of Gas Adsorption in Finite Carbon Pores. Application to Pore Structure Analysis. Journal of Physical Chemistry C 113 19382-19385 Ravikovitch P I, Neimark A V (2001) Characterization of micro- and mesoporosity in SBA-15 materials from adsorption data by the NLDIT method. Journal of Physical Chemistry B 105 6817-6823 Harkins W D, Jura G (1944) Surfaces of Solids. XII. An Absolute Method for the Determination of the Area of a Finely Divided Crystalline Solid. J. Am. Chem. Soc. 66 1362-1366... 

The method proposed by Harkins et al. (1944) which they called the absolute method , included the previous coverage of the outgassed sample with an adsorbed film (five to seven molecular layers) of the immersion liquid. During the immersion experiment, the liquid sees a surface with an extent equivalent to that of the solid, but with a chemical nature corresponding to that of the bulk liquid. An improvement of this method was later proposed by Partyka et al. (1979) who deduced that, for a number of non-porous solids, the coverage with just 1.5 molecular layers was enough to screen the solid surface without reducing the available surface area. In this modified Harkins-Jura technique, water was used as the immersion liquid for solids with hydrophilic surfaces and pentanol for solids with hydrophobic surfaces. 

---