Surface Area Determination and Pore Size Analysis by BET and Its Applications by LSM Analytical Services

Background

LSM Analytical Services offer a large and diverse range of modern analytical testing facilities that cater for a vast range of industry sectors. LSM’s success has been achieved by building on a strong reputation for low cost, fast accurate turnaround. The laboratory activities are backed up by accreditation to the ISO 17025 (UKAS) and 9001:2000 laboratory and quality management standards. LSM is able to offer complete analytical solutions from its internationally recognised team of technical experts, with in depth knowledge, allowing its customers to benefit from impartial guidance.

LSM’s range of analytical services includes:

• X-Ray Fluorescence Spectrometry (XRF)
• X-Ray Diffraction (XRD)
• ICP Optical Emission Spectrometry (ICP-OES)
• Atomic Absorption Spectrometry (AAS)
• Carbon, Sulphur, Nitrogen, Oxygen Analysers
• Boron analysis by the Neutron Transmission method
• Colorimetric and Volumetric Analysis
• Particle Size Analysis

Surface Area Determination by BET

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.

Practical Applications for Surface Area Measurement

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 metres² per gram, and is the result of measuring surface roughness as well as quantity and size distribution of open pores.

Surface Area Determination by Gas Adsorption

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°C. At this stage the sample is ready to attract gas molecules onto it when they are admitted to the sample tube.

Calculating Surface Area by BET

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.

Final Remark

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.”

Source: LSM Analytical Services

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