Control of Potentially Explosive Chemicals in Laboratories
Many relatively common chemicals can become explosive when stored improperly or for excessive periods of time. This note contains background information on potentially explosive and shock-sensitive chemicals. Table 1 lists functional groups in a number of explosive compounds.
Peroxides and Peroxidizable chemicals
Peroxy compounds contain the characteristic peroxide oxygen-oxygen bond. They present special problems in the laboratory because they can be violently reactive or explosive. Their handling requires careful attention.
They are generally stable but in contact with organic compounds they may generate organic peroxides and hydroperoxides. Their contact with any combustible materials may lead to fire or explosion. They must be stored, handled, and used with caution. Peroxides of alkali metals are not sensitive to shock but are decomposed slowly by moisture and violently by bulk water. The most common inorganic peroxides are sodium peroxide, hydrogen peroxide, sodium perborate, and sodium persulfate. The higher atomic weight alkali metals readily form superoxides or ozonides. Sodium amide and many organometallic compounds can also autoxidize and form hazardous peroxides. These peroxy compounds can pose a threat of fire or explosion when contacted by oxidizable materials. Small spills can be treated cautiously with water and sodium bisulfite solution but larger ones should be taken up with inert solids such as vermiculite, sand, or salt and treated with bisulfite in a safe area.
Organic peroxides are among the most hazardous chemicals handled in the chemical laboratory. The primary types of organic peroxides are hydroperoxides (R-O-O-H) and dialkyl peroxides (R-O-O-R¢ ), where R and R¢ are alkyl groups. Organic peroxides are generally low-power explosives that are sensitive to shock, sparks, shaking, friction, heat or light. They are far more shock-sensitive than most primary explosives such as TNT. Organic peroxides fall largely into four classes: dialkyl or diarylalkyl peroxides, peracids, diacyl peroxides and alkyl or arylalkyl hydroperoxides. Those of low molecular weight can deflagrate or detonate. Examples of the most common ones are: tert-butyl peroxide, tert-butyl hydroperoxide, peracetic acid, benzoyl peroxide, and isopropylbenzene (cumene) hydroperoxide.
Peroxy compounds are unstable and decompose continuously and bulk quantities may generate enough heat to autoaccelerate up to ignition or explosion. Because they can generate free radicals with catalytic power their presence as contaminant in a reaction mixture can change the course of a planned reaction. Organic peroxy compounds are generally more stable when water is present. For example, benzoyl peroxide which is a solid at room temperature, can ignite or explode from heat, impact or friction must be kept moist on storage. The unscrewing of a lid covered with the dry chemical can set off the entire lot.
A wide variety of organic chemicals react with molecular oxygen by a free radical reaction even at low concentrations and ordinary temperatures in a process of autoxidation to form peroxy compounds that are usually hydroperoxides and/or peroxides. Autoxidation of organic chemicals (solvents and other liquids, most frequently) proceeds by a free radical chain mechanism. For a chemical R-H, the chain may be initiated by ultraviolet light, by the presence of a radical source, and by the peroxide itself. Oxygen adds to the R radical, producing the peroxy radical R-O-O. The chain is propagated when the peroxy radical abstracts a hydrogen atom from R-H. The reaction can be initiated by light or by a contaminant. In addition to any other hazards they may have, these chemicals pose a "peroxide threat" especially if the oxygenated product crystallizes out or becomes concentrated by evaporation or distillation of the unoxidized part. Peroxide crystals may form even on the threads of a sealing plug or cap. Ethers are the most notorious peroxide formers. Table 2 lists groups of peroxidizable organic chemicals in order of decreasing hazard. However, not all chemicals which fall into these categories have been shown to form potentially dangerous peroxides. MSDS or other chemical literature for hazard potential of individual chemicals should be consulted.
Peroxidation is generally a problem of the liquid state. Solid peroxide formers present little problem except when finely divided because the reaction, if any occurs only at the surface. Peroxidation does not seem to be a problem in gases or vapours. For liquids, peroxidation usually occurs when containers are not fully sealed and blanketed with inert gas. In some cases, stabilizers or inhibitors, which are free radical scavengers that terminate the chain reaction, are added to the liquid to extend its storage lifetime. Examples of common inhibitors are: hydroquinone, 2,6-di-tert-butyl-p-methylphenol (butylated hydroxy toluene, BHT), diphenylamine which are added to the chemical in trace quantities. Iron will inhibit the formation of peroxides in diethyl ether, which is one reason that this chemical is usually sold in steel containers. However, iron or other metals will not inhibit peroxidation in isopropyl ether and are not known to effective in other chemicals. Actually, iron may catalyze peroxidation in some chemicals. One should, however, be cautious that despite the use of inhibitors, peroxide explosions have occurred. Inhibitors are depleted as peroxides are formed and degraded. Eventually with the total depletion of inhibitors, the peroxide- forming chemical will act as an uninhibited chemical. This may result in rapid accumulation of peroxides in a chemical that has been stable for a long time. Table 3 lists common types of organic chemicals that autoxidize to peroxide and Table 4 provides information about shelf life for groups of peroxidizable chemicals. Table 5 is a fairly comprehensive list of peroxidizable organic laboratory chemicals.
Detection of Peroxides
Add 1 mL of the liquid to be tested to an equal volume of glacial acetic acid in a test tube, add a few drops of freshly prepared 5% aqueous potassium iodide solution and shake. A yellow colour indicates a low concentration of peroxide (40-100 ppm as hydrogen peroxide). A brown colour indicates higher concentration of peroxide. Run a blank to make sure the test is really positive. (The test solution has a very short shelf life and will give high blank values if stored for any length of time). Alternatively, addition of 1 mL of a freshly prepared 10% aqueous solution of potassium iodide to10 mL of an organic liquid in a 25 mL glass cylinder should produce a yellow colour if peroxides are present.
iodide solution and 0.5 mL of dilute hydrochloric acid to which has been added a few drops of starch solution just prior to the test. The appearance of a blue or blue-black colour within a minute indicates the presence of peroxides.
Caution: Alkali metals and their amides may form peroxides on their surfaces. Do not apply standard peroxide tests to such materials because they are both water and oxygen reactive.
For volatile organic chemicals, immerse the test strip in the chemical for 1 sec; move the strip slightly to and fro for 3-30 sec until the solvent evaporates. Then breathe slowly on the test strip for 15-30 sec or until the colour stabilizes. The appearance of any blue colour within 3 minutes indicates the presence of peroxide. The colour is then compared with the scale provided on the bottle. If a deep dark blue to brown colour or a green to brown colour is produced, the peroxide concentration is too high for the colour scale.
Modifications of this procedure are required to test non-volatile organic compounds.
For water-miscible compounds, add 3 drops of water to 1 drop of the chemical to be tested. Wet the dip strip in the mixture, wait 2-3 min or until the colour stabilizes, compare with the colour scale provided and multiply the result by 4. For water-immiscible compounds, mix 3 drops of petroleum ether (boiling range 40-60 C) with 1 drop of the low volatility compound to be tested. Wet the dip strip in the mixture and breathe on the reaction zone of the dip strip for 30-60 sec or until the colour stabilizes, and multiply the measured value by 4.
Dip strips provide the highest sensitivity and most accurate quantitation of peroxide concentration for routine testing. They are easier, faster, and safer to use than other methods and they detect a wider range of peroxides.
Other Shock Sensitive Chemicals
Other shock sensitive chemicals include acetylides, azides, nitrogen triiodides, organic nitrates, nitrocompounds, perchlorate salts (especially those of heavy metals such as ruthenium and osmium) and compounds containing diazo, halamine, nitroso and ozonide functional groups. Table 1 lists a number of functional groups in explosive compounds. Some are set off by friction such as the action of a metal spatula on the solid; some are so sensitive that they are set off by the action of their own crystal formation. For example, diazomethane and organic azides may explode when exposed to a ground glass joint.
Picric acid and other polynitroaromatic compounds
Picric acid is commonly used in laboratories and is relatively safe in the form in which it is sold . It is generally sold with 10% water added to stabilize it. However, picric acid can become explosive when it is allowed to dry out or when it forms some metal salts. The following steps should be taken for safe storage and handling of picric acid :
Other nitro compounds to mention are: nitroglycerine, trinitrotoluene, nitrocellulose, ethyl nitrate, dinitrophenylhydrazine.
Sodium azide, though inherently not unstable, may form highly explosive heavy metal azides if contaminated or used improperly. Improper disposal of sodium azide to the sewer may cause the formation of potentially explosive lead or copper azide in plumbing. Do not store sodium azide in container with metal components.
Figure 1. Sample Label for peroxidizable chemicals
(For internal distribution in the U. of Manitoba only)
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