Analytical Services:
Specific Surface Area
LSM Analytical Services large range of analytical techniques includes instrumentation for surface area measurement by BET.
BET is a well known rule for the physical adsorption of gas molecules on a solid surface, and is the basis for an analysis technique for the measurement of the specific surface area of a material.
Surface area measurement is required for a number of very different applications, principally where the materials surface is the key active part for example in catalysts, the inner surface of hardened cement paste, activated carbon and polishing compounds. Surface area is expressed in metres 2 per gram, and is the result of measuring surface roughness as well as quantity and size distribution of open pores.
Gas adsorption is the most widely used and accurate technique for total surface area measurements. Gas molecules of known sizes are condensed onto the unknown sample surface. By completely covering the surface and opening the pores of each particle with a condensed gas the surface area analyser can characterise the surface, including irregularities and pore interiors down to an atomic level. The technique requires a clean surface, as the sample has to be taken to an elevated temperature under vacuum to “outgas” as a necessary step.
Nitrogen is often the gas used as its molecular size is well established, it is inert and is available in high purity at a reasonable cost. The “outgassed” sample under high vacuum in its sample tube is immersed in a coolant bath of liquid nitrogen at –195 o C. At this stage the sample is ready to attract gas molecules onto it when they are admitted to the sample tube.
The amount of gas adsorbed and the resultant sample pressure are recorded. This data is subjected to a number of calculations to obtain surface area. A widely accepted calculation is BET (after Brunauer, Emmett and Teller – 1938). Although the theory is complex BET measurements are relatively simple to obtain, widely applicable and results are highly reproducible.
Chief chemist Paul Hurditch says: “There is an increasing demand for BET measurement from a wide range of industry sectors for surface active applications. LSM Analytical Services has a long history of providing accurate results at economic rates for BET.”
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Oxygen, Hydrogen Analysers
