Significance and Use
This test method is used to measure the surface area of precipitated, hydrated silicas that is available to the nitrogen molecule using the multipoint (B. E. T.) method.
Solids adsorb nitrogen and, under specific conditions, the adsorbed molecules approach a monomolecular layer. The quantity in this hypothetical monomolecular layer is calculated using the BET equation. Combining this with the area occupied by the nitrogen molecule yields the total surface area of the solid.
This test method measures the estimated quantity of nitrogen in the monomolecular layer by adsorption at liquid nitrogen temperature and at several (at least five) partial pressures of nitrogen.
Before a surface area determination can be made it is necessary that the silica be stripped of any material which may already be adsorbed on the surface. The stripping of adsorbed foreign material eliminates two potential errors. The first error is associated with the weight of the foreign material. The second error is associated with the surface area that the foreign material occupies.
1.1 This test method covers a procedure which is used to measure the surface area of precipitated hydrated silicas by the conventional Brunauer, Emmett, and Teller (BET) theory of multilayer gas adsorption behavior using multipoint determinations, similar to that used for carbon black in Test Methods D 4820. This test method specifies the sample preparation and treatment, instrument calibrations, required accuracy and precision of experimental data, and calculations of the surface area results from the obtained data.
1.2 This test method is used to determine the nitrogen surface area of precipitated silicas with specific surface areas in the range of 1 to 50 hm2/kg (10 to 500 m2/g).
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. The minimum safety equipment should include protective gloves, sturdy eye and face protection, and means to deal safely with accidental mercury spills.