The internal surface area of a carbon is usually determined by the BET method (Brunauer, Emmett and Teller).This method utilizes the low-pressure range of the adsorption isotherm of a molecule of known dimensions (usually nitrogen).

This region of the isotherm is generally attributed to monolayer adsorption.


Thus, by assuming the species is adsorbed only one molecule deep on the carbon’s surface, the surface area may be calculated using the equation:

S = XmNA/M

  • S = specific surface in m2/g
  • Xm = sorption value (weight of adsorbed N2 divided by weight of carbon sample)
  • N = Avagadro’s Number, 6.025 E+23
  • A = cross-sectional area of nitrogen molecule in angstroms
  • M = molecular weight of nitrogen


Most manufacturers will specify the surface area of their products but as with CTC activity, it does not necessarily provide a measure of their effectiveness, merely demonstrating their degree of activation. It is also impractical to utilize surface area measurement as a means of quality control since this is a very lengthy procedure.