KBr (potassium bromide) is commonly used in IR spectroscopy for several reasons.
Firstly, KBr is optically transparent for the light in the range of IR measurement. This means that it allows the IR radiation to pass through it without interfering with its absorbance. KBr has a transmittance of 100% in the range of wave numbers (4000-400 cm-1), which is the range typically used in IR spectroscopy. This transparency ensures that KBr does not exhibit absorption in this range, which could interfere with the accurate measurement of the sample's IR spectrum.
Secondly, KBr is used as a carrier for the sample in IR spectroscopy. The sample needs to be transparent to IR radiation in order to obtain an accurate IR spectrum with sharp peaks, good intensity, and high resolution. Therefore, salts like KBr, NaCl, and AgCl are commonly used to mix with the sample and create a transparent medium for the IR radiation to pass through.
In terms of sample preparation, KBr pellets are commonly used for solid samples in IR spectroscopy. The sample is dispersed in KBr by pressing it into a disc-shaped pellet. The typical conditions for KBr pellet preparation include a 100:1 KBr to sample ratio (by weight), a 13 mm pellet die, and a pressing load of 10 tons (or as little as 2 tons for FTIR applications). The concentration of the sample in KBr should be in the range of 0.2 to 1 percent to ensure clear pellets and avoid noisy spectra.
Overall, KBr is used in IR spectroscopy because it is optically transparent for IR radiation, does not exhibit absorption in the IR range, and can be easily prepared into pellets for sample analysis. Its use as a carrier for the sample ensures accurate and reliable results in IR spectroscopy.
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