A simple test for peroxides in water-immiscible solvents:
Shake 2 ml of solvent with 1 ml of a freshly prepared KI solution (10% in water) and a drop of starch indicator solution, a blue color is rapidly developing if peroxides are present (a faint yellow/brown color after addition of KI indicates low levels of peroxides).
Diethyl ether, dioxan, isopropyl ether and tetrahydrofuran form peroxides on storage and must be kept in dark bottles or in dark areas. These solvents should be routinely tested to prove that peroxide levels are kept low before use. It must be emphasized that peroxides, even at low levels, may present an explosion hazard after concentration of solvents by evaporation or distillation.
Chloroform is a popular solvent, it is not very stable, forming phosgene and HCl in air, it should be stored in brown bottles. It is sold after addition of preservatives, ethanol, amylene or cyclohexene. Dichloromethane (or methylene dichloride) is a similar extractant but less oxidizable. Chloroform and dichloromethane are currently stabilized by addition of ethanol (up to 1%) or methyl-2-butene-2 (about 20 mg/l).
Evaporation of solvents: When a large amount of solvent must be evaporated, it should be done in a rotary film evaporator, the flask containing the extract being maintained at no more than 50°C with a water bath. The reduced pressure must be generated by a pump running without oil (motorized pump with metallic or Teflon membranes, diaphragm pumps) or conveniently with a water aspirator achieving a vacuum of 10-20 mm Hg. Last traces of water may be removed by adding 1 or 2 ml ethanol and evaporating again. LT/
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