Potassium bromide (KBr) is a chemical compound made up of the elements potassium (K) and bromine (Br). It is commonly used in the preparation of pellets for studying the molecular structure of powder samples using infrared spectroscopy.
To prepare a KBr pellet, a small amount of the sample is added to KBr powder in a ratio of 1 part sample to 100 parts KBr. The mixture is then well mixed using a mortar and pestle. It is important not to crush the KBr powder too finely, as this can cause it to absorb moisture from the environment.
Once the sample and KBr powder are mixed, they are transferred into a pellet die and compressed using a hydraulic press. The uniform force of the press produces a solid pellet that is mostly transparent to infrared light but contains a dilute quantity of the sample within the detection range of the Fourier transform infrared (FTIR) instrument.
The pellet method exploits the property of alkali halides, such as KBr, to become plastic when subjected to pressure and form a transparent sheet in the infrared region. The pellets are typically prepared with a diameter of 13 mm.
During the preparation of the pellets, it is important to eliminate air and moisture from the KBr powder. This is done by applying a force of approximately 8 tons under a vacuum of several mm Hg for several minutes. Inadequate vacuum can result in easily broken pellets that scatter light.
Before forming the KBr powder into pellets, it is recommended to pulverize it to a maximum mesh size of 200 and then dry it at approximately 110 °C for two to three hours. Rapid heating should be avoided, as it can cause oxidation of the KBr powder to KBrO3, resulting in a brown discoloration.
When performing measurements using KBr pellets, it is common to measure the background with an empty pellet holder inserted into the sample chamber. Additionally, background measurements on a pellet holder with a KBr pellet that contains no sample can help correct for infrared light scattering losses in the pellet and for moisture adsorbed on the KBr.
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