Certain solvents will degrade over time requiring special handling and storage considerations. In addition, the products of certain degradation processes pose a potential safety risk if present at sufficiently high levels. For these types of materials, small amounts of stabilizing chemicals are added to slow down or stop material degradation.
It is important to understand the stability of materials in use and if unclear, consult the Sigma-Aldrich Technical Support and Product Safety Center & MSDS Search. Below is a basic summary of the stabilizer systems available for the four most popular unstable solvents.
Organic peroxides are formed in ethers via a mechanism catalyzed by heat, light and oxygen. In addition to potentially interfering with a given method due to their high reactivity, ether peroxides are highly unstable and represent a potential explosion hazard at concentrations above 100ppm. Lower levels of these peroxides may also present a safety hazard during heating and / or evaporative concentration.
A common stabilizer system for ethyl ether is butylated hydroxytoluene (BHT), which is typically added to ethyl ether at low ppm concentrations. This compound scavenges the free radical species responsible for peroxide formation and is a very effective suppressor of peroxide formation. BHT is incompatible with methods requiring high optical purity due to strong UV absorbance resulting from the aromatic functionality of this molecule.
Ethanol is another common stabilizer, which is added at much higher concentrations (1-2%) than BHT. Due to this relatively high stabilizer concentration, the presence of ethanol significantly increases material polarity and may affect certain applications. Sigma-Aldrich offers ethyl ether with BHT, ethanol and BHT / ethanol stabilizer systems.
THF is another popular ether solvent that receives widespread use and like ethyl ether, will form organic peroxides upon storage. BHT is typically used as a stabilizer in THF at levels of 100-300 ppm. Like ethyl ether, BHT stabilized THF is of little utility in methods utilizing UV detection due to the high level background caused by this additive.
Chlorinated solvents degrade with time via mechanisms that, like ethers, are catalyzed by light, heat and oxygen. Degradation products of chlorinated solvents include phosgene and hydrochloric acid that present safety concerns at elevated concentrations.
Chloroform is unstable and is combined with a variety of stabilizers to enhance product shelf life. Sigma-Aldrich offers ethanol and amylene stabilized chloroform. Like ether, ethanol must be added to chloroform at relatively high concentrations (~1%) in order to be effective. This will increase the polarity of the solvent and potentially impact certain applications. Amylene (2-methyl, 2-butane) is added to chloroform in order to scavenge free radicals such as chloride generated by chloroform decomposition. This compound is effective at levels of approximately 100 ppm.
Dichloromethane (Methylene Chloride)
While more stable than chloroform, dichloromethane will also degrade with time and requires a stabilizer. Alkanes such as cyclohexane, cyclohexene or amylene are typically employed as preservatives and are usually present at levels on the order of 100 ppm.
Due to the popularity of dichloromethane as a solvent in environmental labs, it should be noted that over time, alkene stabilizers produce chlorinated byproducts that may interfere with some GC analyses. This is particularly true in the case of cyclohexene since these byproducts are relatively higher boiling and will elute further away from the solvent front and may interfere with target analytes.
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