Full Description

This document describes a method for the evaluation of porosity and pore size distribution by physical adsorption (or physisorption). The method is limited to the determination of the quantity of a gas adsorbed per unit mass of sample as a function of pressure at a controlled, constant temperature^[1]-[9]. Commonly used adsorptive gases for physical adsorption characterization include nitrogen, argon, krypton at the temperatures of liquid nitrogen and argon (77K and 87K respectively) as well as CO₂ (at 273K). Traditionally, nitrogen and argon adsorption at 77K and 87K, respectively, allows one to assess pores in the approximate range of widths 0,45nm to 50nm, although improvements in temperature control and pressure measurement allow larger pore widths to be evaluated. CO₂ adsorption at 273K293K can be applied for the microporous carbon materials exhibiting ultramicropores. Krypton adsorption at 77K and 87K is used to determine the surface area or porosity of materials with small surface area or for the analysis of thin porous films.

The method described is suitable for a wide range of porous materials. This document focuses on the determination of pore size distribution from as low as 0,4nm up to approximately 100nm. The determination of surface area is described in ISO9277. The procedures which have been devised for the determination of the amount of gas adsorbed may be divided into two groups:

those which depend on the measurement of the amount of gas removed from the gas phase, i.e. manometric (volumetric) methods;

those which involve the measurement of the uptake of the gas by the adsorbent (i.e. direct determination of increase in mass by gravimetric methods).

In practice, static or dynamic techniques can be used to determine the amount of gas adsorbed. However, the static manometric method is generally considered the most suitable technique for undertaking physisorption measurements with nitrogen, argon and krypton at cryogenic temperatures (i.e. 77K and 87K, the boiling temperature of nitrogen and argon, respectively) with the goal of obtaining pore volume and pore size information. This document focuses only on the application of the manometric method.