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D. General Procedures for Work with Flammable and Explosive Substances

Flammable substances are among the most common of the hazardous materials found in the laboratories. Flammable substances are materials that readily catch fire and burn in air. A flammable liquid does not itself burn; it is the vapors from the liquid that burn. The rate at which different liquids produce flammable vapors depends on their vapor pressure, which increases with temperature. The degree of fire hazard depends also on the ability to form combustible or explosive mixtures with air, the ease of ignition of these mixtures, and the relative densities of the liquid with respect to water and of the gas with respect to air.


An open beaker of diethyl ether set on the laboratory bench next to a Bunsen burner will ignite, whereas a similar beaker of diethyl phthalate will not. The difference in behavior is due to the fact that the ether has a much lower flash point. The flash point is the lowest temperature, as determined by standard tests, at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid within the test vessel. As indicated in the following table, many common laboratory solvents and chemicals have flash points that are lower than room temperature and are potentially very dangerous.
Chemical Flash Point (°C)
Diethyl ether -45.0

Pentane -40.0

Carbon disulfide -30.0

Hexane -21.7

Cyclohexane -20.0

Acetone -17.8

Benzene -11.1

Toluene 4.4

Methanol 11.1

Ethanol 12.8


(1) Handling Flammable Substances
The following basic precautions should be followed in handling flammable substances.
1. Flammable substances should be handled only in areas free of ignition sources. Besides open flames, ignition sources include electrical equipment (especially motors), static electricity, and for some materials (e.g. carbon disulfide), even hot surfaces.
2. Never heat a flammable substance with an open flame.
3. When transferring flammable liquids in metal equipment, static-generated sparks should be avoided by electrically grounding the metal container with a grounding strap.
4. Ventilation is one of the most effective ways to prevent the formation of flammable mixtures. A laboratory hood should be used whenever appreciable quantities of flammable substances are transferred from one container to another, allowed to stand or heated in open containers, or handled in any other way.
(2) Handling Explosive Substances
Explosive substances are materials that decompose under conditions of mechanical shock, elevated temperature, or chemical action, with the release of large volumes of gases and heat. Special precautions are required for the safe use of explosive materials. It is the responsibility of researchers to evaluate the explosive hazards involved in their work and to consult with their supervisors to develop detailed standard operating procedures for any work involving explosive substances. Work with explosive materials will generally require the use of special protective apparel (face shields, gloves, lab coats) and protective devices such as explosion shields and barriers.
Organic peroxides are among the most hazardous substances handled in laboratories. As a class, they are low-power explosives, hazardous because of their sensitivity to shock, sparks, and even friction (as in a cap being twisted open). Many peroxides that are routinely handled in laboratories are far more sensitive to shock than most primary explosives such as TNT. All organic peroxides are highly flammable, and most are sensitive to heat, friction, impact, light, as well as strong oxidizing and reducing agents.
Some peroxides in use in the Department are commercial compounds such as m-chloroperoxybenzoic acid, benzoyl peroxide, hydrogen peroxide, and t-butyl hydroperoxide. However, many common solvents and reagents are known to form peroxides on exposure to air, and these chemicals often become contaminated with sufficient peroxides to pose a serious hazard. Classes of compounds that form peroxides by autoxidation include:
(a) Aldehydes including acetaldehyde and benzaldehyde,
(b) Ethers with primary and/or secondary alkyl groups, including acyclic and cyclic ethers, acetals, and ketals. Examples include diethyl ether, diisopropyl ether (especially dangerous!), dioxane, DME, THF, ethyl vinyl ether and alcohols protected as THP ethers. Isopropyl alcohol also frequently forms peroxides upon storage.
(c) Hydrocarbons with allylic, benzylic, or propargylic hydrogens. Examples of this class of peroxide-formers include cyclohexene, cyclooctene, methyl acetylene, isopropylbenzene (cumene), and tetralin (tetrahydronaphthalene).
(d) Conjugated dienes, eneynes, and diynes, among which divinylacetylene is particularly hazardous. Other materials such as 1,4-butadiene (usually supplied in a gas cylinder) form peroxides on contact with air.

Various epoxides and other compounds such as ethylene oxide are also peroxide formers.


(e) Saturated hydrocarbons with exposed tertiary hydrogens; common peroxide formers include decalin (decahydronaphthalene) and 2,5-dimethylhexane.
Compounds belonging to the classes listed above cannot form peroxides without exposure to oxygen (or other oxidizers). Consequently, when storing these materials always flush the container with an inert gas such as nitrogen or argon before sealing. If the compound is not volatile, it may be advisable to degas the sample by vacuum or bubbling techniques. In some cases it may be appropriate to add an oxidation inhibitor such as hydroquinone or BHT (2,6-di-t-butyl-4-methylphenol) to the sample. Containers should be tightly sealed and dated. Do not attempt to open bottles of liquid ethers (e.g., diisopropyl ether) containing crystallized material; contact EHS (777-5269) for assistance in disposal.

(3) Control of Fires


USC Policy states that personnel are not required to fight fires. The following guidelines should be followed to prevent and minimize injury and damage from fires.
Be prepared! Know where all of the fire extinguishers are located in your laboratory, what types of fires they can be used for, and how to correctly operate them. Know where the nearest fire alarm is located. Know the location of safety showers and fire blankets.
Fires in small vessels can usually be suffocated by loosely covering the vessel. Never pick up a flask or container of burning material.
A small fire that has just started can sometimes be extinguished with a laboratory fire extinguisher. Extinguishing such fires should only be attempted if you are confident that you can do so successfully and quickly, and from a position in which you are always between the fire and an exit from the laboratory. Do not underestimate fires, and remember that toxic gases and smoke may present additional hazards. Do not attempt to extinguish a fire that has burned for more than 30 seconds.
Small fires involving reactive metals and organometallic compounds (such as magnesium, sodium, potassium, metal hydrides, etc.) should be extinguished with Met-L-X or Met-L-Kyl extinguishers (see Part V.E), or by covering with dry sand.
In the event of a more serious fire, evacuate the laboratory and activate the nearest fire alarm. Be prepared to meet and advise personnel from the Columbia Fire Department and USC Health & Safety Programs Unit with regard to hazardous substances in your laboratory.
(4) Personal injuries involving fires
If a person's clothing catches on fire, they should be doused with water from the safety shower. Minor clothing fires can sometimes be extinguished by immediately dropping to the floor and rolling. Fire blankets should only be used as a last resort measure to extinguish fires since they tend to hold in heat and to increase the severity of burns. Quickly remove contaminated clothing, douse the person with water, and place clean, wet, cold cloths on burned areas. Wrap the injured person in a blanket to avoid shock and get medical attention promptly. See page 2 for the guidelines on emergency notification. If major chemical contamination of the body surface has occurred, continue submersion of the entire body in the safety shower for at least 15 minutes.
(5) Specific Hazards That May Lead to Fires or Explosions
The combination of certain compounds or classes of compounds can result in a violent chemical reaction leading to an explosion or fire. Other compounds pose explosion or fire hazards when exposed to heat, shock, or other conditions. Listed below are some of the specific compounds and combinations of compounds that may pose explosion or fire hazards and may be encountered in laboratories. This list is not intended to be complete, and researchers should always be familiar with the flammability and other properties of the chemicals involved in their research.
1. Acetylenic compounds are explosive in mixtures of 2.5-80% with air. At pressures of 2 or more atmospheres, acetylene subjected to an electrical discharge or high temperature decomposes with explosive violence. Dry acetylides can detonate on receiving the slightest shock. Many heavy metal acetylides are sensitive explosives.
2. Aluminum chloride should be considered a potentially dangerous material. If moisture is present, there may be sufficient decomposition (generating HCl) to build up considerable pressure. If a bottle is to be opened after long standing, it should be completely enclosed in a heavy towel.
3. Ammonia reacts with iodine to give nitrogen triiodide, which is explosive, and with hypochlorites to give chlorine. Mixtures of ammonia and organic halides sometimes react violently when heated under pressure.
4. Dry benzoyl peroxide is easily ignited and sensitive to shock and may decompose spontaneously at temperatures above 50 °C. Benzoyl peroxide is reported to be desensitized by addition of 20% water.
5. Carbon disulfide is both very toxic and very flammable; mixed with air, its vapors can be ignited by a steam bath or pipe, a hot plate, or a glowing light bulb.
6. Chlorine may react violently with hydrogen or with hydrocarbons when exposed to sunlight.
7. Diazomethane and related compounds should be treated with extreme caution. They are very toxic (potent carcinogens), and the pure gases and liquids explode readily. Solutions in ether are safer from this standpoint
8. Dimethyl sulfoxide decomposes violently on contact with a wide variety of active halogen compounds. Explosions from contact with active metal hydrides have been reported, probably because of water impurity, i.e., wet DMSO.
9. Diethyl, diisopropyl, and other ethers (particularly the branched-chain type) sometimes explode during heating or refluxing because of the presence of peroxides. Ferrous salts or sodium bisulfite can be used to decompose these peroxides, and passage over basic active alumina will remove most of the peroxidic material. In general, however, old samples of ethers should be carefully and properly disposed of.
10. Ethylene oxide has been known to explode when heated in a closed vessel. Experiments using ethylene oxide under pressure should be carried out behind suitable barricades.
11. Halogenated compounds such as chloroform, carbon tetrachloride, and other halogenated solvents should not be dried with sodium, potassium, or other active metals; violent explosions are usually the result of such attempts.
12. Hydrogen peroxide stronger than 3% can be dangerous; in contact with the skin, it may cause severe burns. Thirty percent hydrogen peroxide may decompose violently if contaminated with iron, copper, chromium, or other metals or their salts.
13. Liquid-nitrogen cooled traps open to the atmosphere rapidly condense liquid air. Then, when the coolant is removed, an explosive pressure buildup occurs, usually with enough force to shatter glass equipment. Hence, only sealed or evacuated equipment should be cooled.
14. Lithium aluminum hydride should not be used to dry methyl ethers or tetrahydrofuran; fires from this are very common. The products of its reaction with carbon dioxide have been reported to be explosive. Carbon dioxide or bicarbonate extinguishers should not be used against lithium aluminum hydride fires, which should be smothered with sand or some other inert substance.
14. Oxygen tanks: Serious explosions have resulted from contact between oil and high-pressure oxygen. Oil should not be used on connections to an oxygen cylinder.
15. Ozone is a highly reactive and toxic gas. It is formed by the action of ultraviolet light on oxygen (air) and, therefore, certain ultraviolet sources may require venting to the exhaust hood. Liquid and solid ozone are explosive substances.
16. Palladium or platinum on carbon, platinum oxide, Raney nickel, and other catalysts should be filtered from catalytic hydrogenation reaction mixtures carefully. The recovered catalyst is usually saturated with hydrogen and highly reactive and, thus, will enflame spontaneously on exposure to air. Particularly in large-scale reactions, the filter cake should not be allowed to become dry. The funnel containing the still-moist catalyst filter cake should be put into a water bath immediately after completion of the filtration. Another hazard in working with such catalysts is the danger of explosion if additional catalyst is added to a flask in which hydrogen is present.
17. Parr bombs used for hydrogenations have been known to explode. They should be handled with care behind shields, and the operator should wear goggles. Hydrogenation bombs should not be filled from a gas cylinder using only a needle valve. If pressure limits (cited in the manufacturer's literature) for a controlled-atmosphere reaction chamber are exceeded, high-pressure rupturing of the container may result. A pressure regulator (preferably two-stage) should be used to avoid over-pressurizing the bomb.
18. Perchlorates: The use of perchlorates should be avoided whenever possible. Perchlorates should not be used as drying agents if there is a possibility of contact with organic compounds, or in proximity to a dehydrating acid strong enough to concentrate perchloric acid to more than 70% strength (e.g., in a drying train that has a bubble counter containing sulfuric acid). Safer drying agents should be used. Seventy percent perchloric acid can be boiled safely at approximately 200 °C, but contact of the boiling undiluted acid or the hot vapor with organic matter, or even easily oxidized inorganic matter (such as compounds of trivalent antimony), will lead to explosions. Oxidizable substances must never be allowed to contact perchloric acid. Beaker tongs, rather than rubber gloves should be used when handling fuming perchloric acid. Perchloric acid evaporations should be carried out in a hood that has a good draft and a built-in water spray for ductwork behind the baffle. Frequent (weekly) washing of the hood and ventilator ducts with water is needed to avoid danger of spontaneous combustion or explosion if this acid is in common use. Metal perchlorates salts that are dried can also detonate upon shock.
19. Permanganates are explosive when treated with sulfuric acid. When both compounds are used in an absorption train, an empty trap should be placed between them.
20. Peroxides (inorganic): When mixed with combustible materials, barium, sodium, and potassium peroxides form explosives that ignite easily.
21. Phosphorus (red and white) forms explosive mixtures with oxidizing agents. White P should be stored under water because it is spontaneously flammable in air. The reaction of P with aqueous hydroxides gives phosphine, which may ignite spontaneously in air or explode.
22. Phosphorus trichloride reacts with water to form phosphorous acid that decomposes on heating to form phosphine, which may ignite spontaneously or explode. Care should be taken in opening containers of phosphorous trichloride, and samples that have been exposed to moisture should not be heated without adequate shielding to protect the operator.
23. Potassium is in general more reactive than sodium; it ignites quickly on exposure to humid air and, therefore, should be handled under the surface of a hydrocarbon solvent such as mineral oil or toluene. Oxidized coatings should be carefully scraped away before cutting the metal. Explosions can occur otherwise.
24. Residues from vacuum distillations have been known to explode when the still was vented to the air before the residue was cool. Such explosions can be avoided by venting the still pot with nitrogen, by cooling it before venting, or by restoring the pressure slowly.
25. Selenium reacts with incandescence with metals (nickel, sodium, potassium, phosphorus, uranium, zinc, and platinum); the particle size of cadmium and selenium must be below a critical size to prevent explosions when making cadmium selenidethis also applies to zinc and selenium; oxidation of recovered selenium with nitric acid is made vigorous by the presence of organic impurities; selenium may react explosively with BrF5, ClF3, N2O2, or Na2O2; it also ignites on contact with fluorine (F2). Selenium initiates a violent and often explosive decomposition of nitrogen trichloride (NCl3). Heating selenium in oxygen in the presence of traces of organic impurities may result in a vigorous explosion.
26. Sodium should be stored in a closed container under kerosene, toluene, or mineral oil. Scraps of sodium (Na) or potassium (K) should be destroyed by reaction with n-butyl alcohol. Contact with water should be avoided because Na reacts violently with water to form hydrogen with evolution of sufficient heat to cause ignition. Carbon dioxide, bicarbonate, and carbon tetrachloride fire extinguishers should not be used on alkali metal fires.
(6) Incompatible Chemicals
When transporting, storing, using, or disposing of any substance, utmost care must be exercised to ensure that the substance cannot accidentally come in contact with another with which it is incompatible. Such contact could result in an explosion or the formation of substances that are highly toxic or flammable or both. The following Table is a guide to avoiding accidents involving incompatible substances.


Examples of Incompatible Chemicals

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Chemical

Is Incompatible With







Acetic acid

Chromic acid, nitric acid, perchloric acid, peroxides, permanganates


Acetylene

Chlorine, bromine, copper, fluorine, silver, mercury


Acetone


Concentrated nitric acid and sulfuric acid mixtures.


Alkali and alkaline earth metals (such as powdered aluminum or magnesium, calcium, lithium, sodium, potassium)


Water, carbon tetrachloride or other chlorinated hydrocarbons, carbon dioxide, halogens


Ammonia (anhydrous)

Mercury (in manometers, for example), chlorine, calcium hypochlorite, iodine, bromine, hydrofluoric acid (anhydrous)


Ammonium nitrate

Acids, powdered metals, flammable liquids, chlorates, nitrites, sulfur, finely divided organic or combustible materials


Aniline

Nitric acid, hydrogen peroxide


Arsenical materials


Any reducing agent


Azides


Acids


Bromine


See chlorine

Calcium oxide


Water

Chlorates


Ammonium salts, acids, powdered metals, sulfur, finely divided organic or combustible materials


Chromic acid and chromium trioxide


Acetic acid, naphthalene, camphor, glycerol, alcohol, flammable liquids in general




















Chemical

Is Incompatible With













Chlorine


Ammonia, acetylene, butadiene, butane, methane, propane (or other petroleum gases), hydrogen, sodium carbide, benzene, finely divided metals, turpentine





Chlorine dioxide

Ammonia, methane, phosphine, hydrogen sulfide





Copper

Acetylene, hydrogen peroxide





Cumene hydroperoxide


Acids (organic or inorganic)





Decaborane


Carbon tetrachloride and some other halogenated hydrocarbons





Flammable liquids


Ammonium nitrate, chromic acid, hydrogen peroxide, nitric acid, sodium peroxide, halogens





Fluorine


Everything





Hydrofluoric acid (anhydrous)


Ammonia (aqueous or anhydrous)





Hydrogen peroxide


Copper, chromium, iron, most metals or their salts, alcohols, acetone, organic materials, aniline, nitromethane, combustible materials





Hydrogen sulfide


Fuming nitric acid, oxidizing gases





Hypochlorites


Acids, activated carbon





Iodine


Acetylene, ammonia (aqueous or anhydrous), hydrogen





Mercury


Acetylene, fulminic acid, ammonia





Nitrates


Sulfuric acid





Nitric acid (concentrated)


Acetic acid, aniline, chromic acid, hydrocyanic acid, hydrogen sulfide, flammable liquids, flammable gases, copper, brass, any heavy metals





Nitrites


Acids





Nitroparaffins


Inorganic bases, amines





Oxalic acid


Silver, mercury





Oxygen


Oils, grease, hydrogen, flammable liquids, solids, or gases





Phosphorous (white)


Air, oxygen, alkalis, reducing agents





Potassium


Carbon tetrachloride, carbon dioxide, water

















Chemical

Is Incompatible With







Potassium perchlorate (see also chlorates)


Sulfuric and other acids


Potassium permanganate


Glycerol, ethylene glycol, benzaldehyde, sulfuric acid


Selenides


Reducing agents


Silver


Acetylene, oxalic acid, tartaric acid, ammonium compounds, fulminic acid


Sodium


Carbon tetrachloride, carbon dioxide, water


Sodium nitrite


Ammonium nitrate and other ammonium salts


Sodium peroxide


Ethyl or methyl alcohol, glacial acetic acid, acetic anhydride, benzaldehyde, carbon disulfide, glycerine, ethylene glycol, ethyl acetate, methyl acetate, furfural


Sulfides


Acids


Sulfuric acid

Potassium chlorate, potassium perchlorate, potassium permanganate, (similar compounds of light metals such as sodium, lithium)


Tellurides

Reducing agents

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