Determination of the degassing temperature: The higher the system temperature, the faster the molecular diffusion motion, so the better the degassing effect. ? Usually, the degassing station equipped with the instrument can heat up to 400 ° C, but the first principle of selecting the degassing temperature is not to damage the sample structure. In general, the safe degassing temperature of oxides such as alumina and silica can reach 350 ° C; the safe degassing temperature of most carbon materials and calcium carbonate is around 300 ° C; while the hydrate needs to be low. Much degassing temperature. For organic compounds, pretreatment can also be carried out by a degassing station, but most organic compounds have a lower softening temperature and glass transition temperature and must be confirmed in advance. For example, magnesium stearate commonly used in the medical field, the United States Pharmacopoeia (USP) specifies a degassing temperature of 40 ° C. ? If the degassing temperature is set too high, it will cause irreversible changes in the structure of the sample. For example, sintering will reduce the specific surface area of ​​the sample, and decomposition will increase the specific surface area of ​​the sample. However, if the degassing temperature is set too low for insurance, the surface treatment of the sample may be incomplete, resulting in a small analysis result. Therefore, in the case of uncertain degassing temperatures, it is recommended to use chemical manuals such as the?Handbook? of?Chemistry? and?Physical?(CRC,?Boca?Raton,?Florida), and standard methods issued by various standards organizations. As ASTM, as a related reference. The degassing temperature should not be chosen higher than the melting point of the solid or the phase transition point of the glass. It is recommended not to exceed half the melting point temperature. Of course, if conditions permit, the use of a thermal analyzer will result in the most accurate degassing temperature. In general, the degassing temperature should be the temperature of the platform section on the thermogravimetric curve. ? Determining the degassing time: Corresponding to the degassing temperature is the degassing time. The longer the degassing time, the better the sample pretreatment effect. The choice of degassing time is related to the complexity of the sample well. In general, the more complex the pores, the higher the micropore content and the longer the degassing time; the lower the selected degassing temperature, the longer the degassing time required for the sample. The degassing time can be determined by analyzing the change in the BET result of the sample at the same degassing temperature. If the BET results are the same at different degassing times (2 hours, 4 hours and 6 hours), the degassing time must be chosen to be the shortest; if the change is not large, a compromise should be chosen; if the BET results with the degassing time The extension is constantly increasing, indicating that the pores are complex, and there are adsorbed water molecules due to hydrogen bonding at a deep level, exposing the blocked pores and the area. For general samples, IUPAC recommends a degassing time of no less than 6 hours, while those requiring low temperature degassing require much longer degassing time. For some microporous samples, the degassing time may even be more than 12 hours. However, as a special case, the United States Pharmacopoeia USP) stipulates that the degassing time of magnesium stearate is only 2 hours. ? Since the degassing temperature, the degassing time, and the degassing vacuum are all related to the specific surface area value, it is unavoidable that there is an error in the BET result. Therefore, the sample processing conditions need to be fixed for relative comparison. When comparing with literature values, also pay attention to the sample pretreatment and analysis conditions in the literature. ?
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