ASTM D1993

ASTM D1993 – Standard Test Method for Precipitated Silica-Surface Area by Multipoint BET Nitrogen Adsorption

Description:

Significance and Use

3.1 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. Single point nitrogen surface area is measured in accordance with the Test Methods D5604.

3.2 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.

3.3 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.

3.4 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.

Scope

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)2 theory of multilayer gas adsorption behavior using multipoint determinations, similar to that used for carbon black in Test Method D6556. 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 10 to 500 m²/g.

1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.