Porosity of Porous Substances

Relatively simple methods are available to determine the porosity of porous substances. By definition the porosity Pr is given by  

Vtru the true specific volume, may be obtained from physicochemical tables, but can also be determined pycnometrically by imbibition of the material in helium. 

The total specific volume, Vtoi, is determined pycnometrically by imbibition of the porous material in mercury at 0.1 MPa pressure. Note that most materials are not wetted by mercury. Hence, mercury will not penetrate into the pores at 0.1 MPa pressure. 

In practice the porosity varies from about 14%, as found for some zeolites, to about 70% found for highly activated carbons. 

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