Procedures to measure the specific surface area

For all experiments with porous materials or powders we need to know the specific surface area. This is also important for many industrial applications and determination of the specific surface area has become a standard technique in chemical engineering of powders. 

Usually specific surface areas are determined from adsorption experiments. To illustrate this let us assume that adsorption of a specific sample is adequately described by the Langmuir Eq. (9.22). From fitting experimental results we obtain Tmon in units of mol/g. Then we assume a reasonable value for the cross-section area of a gas molecule a a, and obtain the specific surface from J2 = rmon ANA- In most practical applications the BET adsorption isotherm is used instead of the Langmuir Eq. (9.22) because it fits better. From a fit with the BET isotherm we get Tmon or nmon. Some cross-sectional areas for suitable gases in A2 are N2 16.2 02 14.1 Ar 13.8 n-C4Hi0 18.1. 

As a result we plot Vad(i p/Po ) versus P/Po. This should give a straight line with a slope (C — 1) / (CV don) and an intersection with the ordinate at 1/ (FF ). From the slope and intersection C and V/)dm can be determined. 

Porous materials are often analyzed with a mercury porosimeter. With a mercury porosi-meter we can measure the pore distribution of a solid. Thus, we can determine the specific surface area. Mercury is used because of its high surface tension (0.48 N/m) it does not wet 

The pore is modelled as a capillary. Please note isl is larger in this case than is and cos 0 0. Integration results in the entire surface area being wetted by mercury  

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