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By Ed Perley
Electronic absorption spectra in the visible and near ultraviolet light are useful in the identification of soluble compounds. They also provide valuble information concerning their electronic structures. In my study of Molybdenum V solutions, I made heavy use of this method of analysis for obtaining the electronic absorption spectra of Molybdenum V compounds in a wide variety of solvents. It quickly became evident that I could not assume that the compounds in solution were the same as they were in the solid state.
I needed a way to obtain spectra without exposing my compounds to any kind of solvent. So I used diffuse reflectance spectroscopy of finely ground up samples. Spectra were obtained that roughly matched some of the solution spectra, but the absorption bands were broad and indistinct, and the base line was nonlinear and hard to duplicate. Also, the instrument was not sensitive to ultraviolet bands.
Out of the desire to obtain higher quality absorption spectra, I devised a method of obtaining them from finely divided samples pressed into potassium bromide pellets. The procedure is described below.
Dry potassium bromide (.2 gm) was ground to a powder with an agate mortar and pestle. Enough sample was added (.5 mg to 1.0 mg) to give the salt a noticeable tint. The mixture was ground to a very fine powder and pressed for about 45 minutes (twice the time used for infrared absorption spectra) at 15,000 psi in a special die designed for that purpose. A water aspirator was used to remove water forced out of the salt.
After being removed form the die, the resulting pellet was immediately covered with nujol (a thick transparent oil used to suspend samples for infrared absorption spectral analysis). The moistened pellet was allowed to stand for five minutes before the excess was wiped off. This treatment improved transparency and protected the pellet from fogging. Pellet thickness was about 1.5 mm. Many of the pellets were transparent between 230 nm and 1000 nm.
Much thicker pellets were used to record the much weaker visible and near infrared absorption bands of these compounds. Approximately .2 mg of sample and up to 2 gm of potassium bromide were used to make these pellets.
A double beam recording spectrophotophotometer was used to record the spectrum of the pellet. The thin pellets were mounted in holders made from card paper and inserted into the cell holders. The thick pellets were mounted directly in the cell holders. A pure potassium bromide pellet of comparable thickness and transparency was mounted in the reference beam. The photomultiplier detector was set to maximum sensitivity.
To see how this method was used to attempt to identify molybdenum(V) species in hydrobromic acid, look go to the page about Molybdenum (V) Solutions in Hydrobromic Acid. Additional information is given in the page on Preparation of Oxobromomolybdate Compounds..
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