Harkins-Jura relative method

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

Figure 7. Comparison of PSDs obtained by tbe HK method with the A(w) relationships given by equations (4) (circles) and (5) (lines). While in equation (4) the experimental t-curve was used, equation (5) was obtained by using the Harkins-Jura fit for the t-curve, which is much less accurate for relative pressures smaller than 0.1.

To separate contributions due to micropore filling on one hand and the formation of mono- and multilayers on the other hand which are superimposed at relative pressures below 0.2- 0.3, the f-plot or the aj-plot approach can be applied. Both methods use empirical reference isotherms to be compared with the isotherms taken for the sample under investigation. In the f-plot method, the statistical layer thickness t of a nonmicroporous material is related to the relative pressure plp. One of the most frequently used relationship for the layer thickness is the empirical Harkins-Jura equation [67] derived for metal oxides ° ... 

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... 

A remarkable attempt was made by Harkins and Jura nearly fifty years ago to overcome some of these difficulties. Their idea was to cover a non-porous adsorbent with a multilayer thick enough to have a liquid-like surface. The pre-coated adsorbent should therefore exhibit a surface enthalpy identical to that of the liquid and immersion in the same liquid should liberate an amount of energy equivalent to the removal of this surface. In principle, therefore, it would seem a fairly simple matter to calculate the surface area from the heat of immersion of the coated solid. The measurements Harkins and Jura appeared to indicate that 5-7 molecular layers were required to overcome the influence of the solid surface and reduce the surface energy to that of the liquid. Since this layer thickness would correspond to a very high relative pressure, it would be difficult to avoid some capillary condensation -even with non-porous powdered materials-and therefore the method appeared to have very limited applicability. 

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