Determination of specific surface by the BET method

The specific surface determined by BET relates to the total surface area (reactive surface) as all porous structures adsorb the small gas molecules. The surface area determined by BET is thus normally larger than the surface area determined by air permeability. The samples are dried with nitrogen purging or in a vacuum applying elevated temperatures. Unless otherwise instructed we use P/P0  of 0,1, 0,2 and 0,3 as standard measurement points. The volume of gas adsorbed to the surface of the particles is measured at the boiling point of nitrogen (-196°C). The amount of adsorbed gas is correlated to the total surface area of the particles including pores in the surface. Physical adsorption results from relatively weak forces (van der Waals forces) between the adsorbate gas molecules and the adsorbent surface area of the test powder. The determination is usually carried out at the temperature of liquid nitrogen. The amount of gas adsorbed can be measured by a volumetric or continuous flow procedure.A minimum of 3 data points is required. Additional measurements may be carried out, especially when non-linearity is obtained at a P/Po value close to 0.3. Because non-linearity is often obtained at a P/Po value below 0.05, values in this region are not recommended. Before the specific surface area of the sample can be determined, it is necessary to remove gases and vapours that may have become physically adsorbed onto the surface after manufacture and during treatment, handling and storage. If outgassing is not achieved, the specific surface area may be reduced or may be variable because an intermediate area of the surface is covered with molecules of the previously adsorbed gases or vapours. BET plots, a constant weight of test powder, and no detectable physical or chemical changes in the test powder. The outgassing conditions defined by the temperature, pressure and time should be chosen so that the original surface of the solid is reproduced as closely as possible. Outgassing of many substances is often achieved by applying a vacuum, by purging the sample in a flowing stream of a non-reactive, dry gas, or by applying a desorption-adsorption cycling method. In either case, elevated temperatures are sometimes applied to increase the rate at which the contaminants leave the surface. Calculate the weight of the sample. Attach the sample tube to the volumetric apparatus. The determination of dead volume may be avoided using difference measurements, that is, by means of reference and sample tubes connected by a differential transducer. Brunauer, Emmett and Teller is reviewed, together with the experimental techniques available for measuring physical adsorption of gases by solids. Brunauer-Emmett-Teller, que l’on examine ici, de meme que les techniques experimentales disponibles: methodes statiques (volumetriques ou gravimetriques) et dynamiques plus ou moins complexes. En s’aidant d’exemples, on discute l’application de la theorie de B.E.T. a la determination de la surface specifique des materiaux de construction. Our lab is well equipped with instruments capable of either adsorption technique, and we possess the technical expertise to provide meticulously executed surface area analysis using the most appropriate gas and procedure. We are a trusted advisor to the world's leading pharmaceutical, industrial, public sector and scientific organizations. We work with, and provide advice to, major organizations across the private, public, and scientific research sectors. SA-9600 series of surface area analyzers brings exceptional speed and convenience to BET surface area analysis. SA-9600 series automatically create the necessary mixtures of nitrogen and helium for BET surface area analysis. This lowers the total cost of ownership by eliminating the need to purchase expensive gas mixtures. Microphase Co., Ltd. for simply measuring the specific surface area of nano-materials by the BET method. Because the measurement method is based on a relative comparison with standard samples, the measurement results can be achieved simply and quickly. K is most commonly used adsorptive for determining the surface area. At this temperature nitrogen gas is below the critical temperature and so condensable. It is assumed that it condenses onto the surface in a monolayer and so once you know the size of the gas atoms/molecules and the amount adsorbed the surface area can be estimated. The BET method is applicable only to adsorption isotherms of type II (disperse, nonporous or macroporous solids) and type IV (mesoporous solids, pore diameter between 2 nm and 50 nm). Inaccessible pores are not detected. This is generally done either using a vacuum system or by flushing the sample with an inert gas often at elevated temperature. The temperature used depends on the stability of the sample. A temperature of 110°C is quoted [6] for nitrogen isotherms where the sample is stable to this temperature. BET methodologies will carry out a dead-volume measurement using an inert gas such as He. This result is used to correct the quantity of adsorbate adsorbed. Adsorption of the gas on to the sample occurs, and the pressure in the confined volume continues to fall until the adsorbate and the adsorptive are in equilibrium. The amount of adsorbate at the equilibrium pressure is the difference between the amount of gas admitted and the amount of adsorptive remaining in the gas phase. The other common problem is insufficient degassing (of gas or water) of sample. This may result in degassing during the measurement and problems with your results.

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