A KBr disc, also known as a KBr pellet, is a thin, transparent disc made primarily from potassium bromide (KBr) that is used in infrared spectroscopy. This method involves mixing a sample with KBr powder in a specific ratio, typically between 0.2 to 1 percent concentration of the sample, to ensure clarity and optimal absorption of the infrared beam. The mixture is then pressed under high pressure to form a transparent disc, which is used to analyze the sample's infrared spectrum.
The preparation of a KBr disc involves several steps:
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Sample/KBr Ratio: The sample is mixed with KBr powder in a ratio that maintains the sample concentration between 0.2 to 1 percent. This low concentration is necessary because the pellet is thicker than a liquid film, and a higher concentration could lead to noisy spectra due to complete absorption or scattering of the IR beam.
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Mixing and Grinding: The sample and KBr are briefly ground together to mix thoroughly. This process is ideally done in a glove box to minimize moisture absorption, which can affect the clarity of the final pellet. Over-grinding should be avoided as it increases the surface area and potential moisture absorption.
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Pressing: The mixed powder is placed in a pellet press die sleeve on a base plate, and a plunger is used to apply pressure, forming the disc. The amount of KBr used typically ranges from 200 to 250 mg for a standard ½ inch diameter disc, with adjustments made for smaller discs based on their area.
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Applications: KBr discs are primarily used in Fourier transform infrared spectroscopy (FTIR). In FTIR, the disc is exposed to infrared light, and the resulting light is analyzed to identify the chemical bonds and their vibrations in the sample. This technique is crucial for identifying and analyzing various substances in fields ranging from chemistry to materials science.
In summary, a KBr disc is a fundamental tool in infrared spectroscopy, enabling the analysis of various samples by converting them into a transparent, infrared-compatible form. The careful preparation and handling of these discs are essential for obtaining clear and accurate spectra.
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