Specific Surface Area

Specific Surface Area (SSA)

Determination of the specific surface area (or "BET area") of a sample by means of gas sorption

The specific surface area of a solid is usually determined by means of nitrogen sorption in a defined partial pressure range. For this purpose, AixMinerals offers a five-point measurement in the partial pressure interval from 0.05 to 0.35 p/p0 as standard. The basic analysis method is the Brunauer-Emmett-Teller (BET) method, which is why specific surface area is often referred to as "BET area" in the literature. The specific surface area is given in square metres per gram (m2/g). The most widespread method is the determination of the specific surface area with nitrogen at a Temperature of 77 K (-196 °C), since sorption measurements using Ar are associated with significantly higher operating costs. The IUPAC (International Union of Pure and Applied Chemistry) nevertheless recommends argon, due to its spherical atomic shape compared to nitrogen and its lower specific interaction with surface functional groups (steric effects). Depending on the application, it is also worthwhile for customers to have an entire gas sorption isotherm measured directly, as a wealth of material-specific information is contained in isotherms. The following figure illustrates the relevant partial pressure range ("BET-Area") of an entire sorption isotherm for determining the specific surface area. AixMinerals also always optimises the partial pressure range to be selected for the measured data points using a "Rouquerol plot". This allows us to guarantee our customers the highest accuracy in determining the specific surface area.

Figure 1: Comparison of two sorption isotherms of a claystone measured with nitrogen (N2) and argon (Ar) (left side). The sorbed quantity of the gas used (mmol/g) is shown as a function of the partial pressure (p/p0). The relevant partial pressure range (approx. 0.5 to 3.5 p/p0) for determining the specific surface area is highlighted in yellow. For the selection of the exact partial pressure range for the calculation of the specific surface area, a so-called "Roquerol plot" is used (bottom right-hand side). Based on the maximum of this curve, the optimal partial pressure range is determined and used in a "BET plot" (top right-hand side) for calculating the specific surface area.

Applications

Potential areas of application for BET analyses include:
  • Quantification of reactive surfaces of solids for chemical applications, e.g. catalyst materials
  • Determining the specific surface area of solids for storage applications, e.g. metal-organic frameworks (MOFs) for gas storage
  • Quality control of industrial raw materials and pharmaceutical products
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