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DESCRIPTION Columns can be scaled up or down as needed, but each must be dedicated to a single solvent. A few solvents--ether, tetrahydrofuran, and methylene chloride--are compatible with activated alumina but not with the copper catalyst. Oxygen must be removed from these solvents by purging them with dry nitrogen or argon.
ECONOMY But even though the column-absorption systems clearly win the safety debate, not everyone has made the switch. Researchers at some colleges and universities, as well as some smaller chemical and pharmaceutical companies, have stuck with conventional solvent stills.

ONE BIG REASON is price. Researchers must pony up approximately $4,000 to $5,000 per solvent to replace their stills with the commercial systems. And although hundreds of liters of solvent can be purified per column, replacing the packing costs about $200 per column. "Money is the deciding factor for departments making these decisions,"
OBSERVATION'S Many organic and organometallic reactions require solvents that are free of water and, sometimes, free of oxygen. Classically, solvent purification is accomplished by refluxing the solvent in the presence of sodi-um or potassium metal and benzophenone in an inert atmosphere. The reactive metal removes moisture from the solvent, and the ketyl intermediate that forms upon reaction of the ketone and the metal helps to sop up any oxygen. The blue color of benzophenone ketyl is used as an indicator that the solvent is ready for use.

Some labs commonly require several liters of each of four or five different water- and oxygen-free solvents per day. To meet demand, many liters of each solvent are allowed to reflux for long periods of time, much of it unattended.

Having large quantities of flammable solvent around the lab is a safety concern in and of itself, but dangers inherent in the reflux/distillation solvent purification process exacerbate the hazard. The setup requires electrical equipment such as a heating mantle and a vacuum pump--either of which can create a spark. In addition, if even a small piece of reactive metal escapes during purification or cleanup, the moisture in the air can be sufficient to ignite the metal.
UPDATE 05.02
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