Measurement of Pore-related Properties

Porosity of adsorbents is determined by several alternative methods. When true solid density, p, (g/cm ), is known, total porosity, Ci (—), or specific pore volume, Vp (cm /g), is calculated from p, and particle density, pp (g/cm ), as 

Particle density, pp, can be determined using a mercury pycnometer by assuming that mercury does not enter any pore of the porous sample. 

When the sample is first evacuated and the evacuated sample is exposed to mercury at atmospheric pressure, P , then mercury can penetrate pores with radius larger than 7.5 pm. Most porous adsorbents have intraparticle pores smaller than this and one atmosphere is enough to fill the void between particles with mercury. Thus, by comparing the weight of the empty glass pycnometer filled with mercury, Wa, and that of the pycnometer with the sample of weight, W tilled with mercury after evacuation, l p, the particle density, pp, is calculated as 

True solid densities, p, are often found in the literature. But some porous bodies are not easy to find in the tables or to make an estimation because they have confined pores which are not accessible from outside. In such cases, the true density must be determined by direct measurement. 

One practical method is to use the same method as described for measuring particle density using mercury, except that mercury is replaced by a liquid which penetrates pores. Water and organic solvents are often used. The same equation can be used by replacing ph with the density of the liquid employed. 

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