The surface area of granulated and powdered solids or porous materials is measured with the FlowSorb III by determining the quantity of a gas that adsorbs as a single layer of molecules, a so-called monomolecular layer, on a sample. This adsorption is done at or near the boiling point of the adsorbate gas. Under this condition, the area covered by each gas molecule is known within relatively narrow limits. The area of the sample is thus calculable directly from the number of adsorbed molecules, which is derived from the gas quantity at the prescribed conditions, and the area occupied by each.
For a nitrogen and helium mixture of 30 volume percent nitrogen, conditions most favorable for the formation of a monolayer of adsorbed nitrogen are established at atmospheric pressure and the temperature of liquid nitrogen. Parameters defining the conditions at which the monolayer forms are incorporated in the multipoint analysis result by the data treatment. Atmospheric pressure and ice water temperature may establish appropriate conditions for a n-butane and helium mixture. Other gases at other conditions are also usable.
In general, a surface area result obtained by the multipoint method using nitrogen as the adsorbate is somewhat more reliable than a single point one, and nitrogen gas is preferable over other gases. Unless special circumstances dictate, nitrogen is recommended.