This chapter provides resources that can help you prevent a laboratory accident due to mishandling explosive substances, or mixing incompatible reactive substances. This chapter details several specific examples of explosive and reactive hazards that are common in laboratories.

The variety of chemicals commonly present in the laboratory poses the potential for accidental hazardous chemical reactions or explosions. A hazardous reaction occurs when two or more chemicals that are incompatible combine, resulting in an undesirable or uncontrolled reaction with adverse consequences. Such reactions may result when incompatible chemicals spill by accident, inadvertently mix as chemical waste, or combine unwittingly during experimental procedures.

Hazardous reactions may cause any one or more of the following:

  • dispersal of toxic dusts, mists, particles
  • explosion
  • fire
  • formation of flammable gases
  • formation of shock or friction sensitive compounds
  • formation of substances of greater toxicity
  • formation of toxic vapors
  • heat generation
  • pressurization in closed vessels
  • solubilization of toxic substances
  • violent polymerization
  • volatilization of toxic or flammable substances

It is easy to become complacent with chemicals used everyday in routine procedures. It is prudent to check for incompatibility whenever making a change in chemical procedures. Chemical incompatibility is the primary reason you do not store chemicals on the shelf alphabetically. If there is an accident, adverse reactions may make the situation worse.

Safety Data Sheets will list reactivity information in Section 10: Stability and Reactivity Data within the form. References of incompatible chemical combinations include:

  • Handbook of Reactive Chemical Hazards; L. Bretherick, 6th edition 1999
  • Manual of Hazardous Chemical Reactions, National Fire Protection Association Manual 491M
Explosives are solid, liquid, or gaseous chemicals that can cause a sudden, almost instantaneous release of pressure, gas, and heat when subjected to shock, pressure, or high temperature. Their acquisition, storage, use, and disposal are highly regulated, and these materials demand the highest safety precautions.

The U.S. Department of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has an extensive set of regulations entitled Commerce in Explosives (27 CFR 555). These rules govern the acquisition, use, storage, and security requirements for a specific list of explosive materials, updated annually. The most recent list is in the January 8, 2010 Federal Register. This list includes obvious explosive materials such as 2,4,6-trinitrotoluene (TNT), lead azide, and mercury fulminate. The list also includes more common laboratory chemicals in dried out or non-reagent form such as dinitrophenol, picric acid, and sodium azide.

Public educational institutions such as UNC are exempt from several of the provisions of 27 CFR 555, except for storage requirements. If you use any substance on the ATF List of Explosive Materials in your research, you might be required to comply with the requirements for magazine storage, depending on the concentration of substance and whether it is packaged in reagent form.

The U.S. Department of Transportation (DOT) classifies explosive (Class 1) materials into one of six divisions. Division 1.1 materials are the most hazardous due to their sensitivity and mass explosion hazard, whereas Division 1.6 materials are insensitive and not a mass explosion hazard. Other divisions fall between these extremes. The DOT also maintains a forbidden explosives list. This list is similar but not identical to the ATF list. Under most circumstances, you cannot receive forbidden explosives from vendors, drive them over the road, ship them to other collaborators, or receive them from other off-site collaborators.

Please contact EHS first if you contemplate receiving, synthesizing (directly or as by-product), using, or shipping any substance on the ATF List of Explosive Materials or the DOT Forbidden Explosives List.

Figure 11.1
Figure 11.1. Common placards and pictograms for explosive materials (left to right): DOT placard for Division 1.1 – 1.3 materials; DOT placard for Division 1.4 materials; European Union glyph for explosive materials; iconic glyph for explosive materials.
Listed below are some specific, representative chemical reactive hazards in laboratories that can lead to fires or explosions. Any solid, liquid, or gaseous chemical substances that have the potential to react rapidly to release relatively large amounts of energy and/or dangerous by-products (e.g., toxic gas) are considered reactive.

Keep in mind, this list is not exhaustive; space does not permit us to list all the potential reactive hazards that could exist in a laboratory. Consult the resources mentioned in Section I of this Chapter for more information.

  • Acetylenic Compounds are explosive in mixtures of 2.5-80% with air at pressures of two or more atmospheres. Acetylene (C2H2) subjected to an electrical discharge or high temperature decomposes with explosive violence. Dry acetylides detonate on receiving the slightest shock.
  • Alkyllithium compounds are highly reactive and pyrophoric. Examples include n-butylithium and methyl lithium. tert-Butyllithium solutions are the most pyrophoric of all alkyllithium compounds. They may ignite spontaneously upon exposure to air. Fires or explosion can occur upon contact with water or moist materials. It should be stored under an inert atmosphere free from all ignition sources. For additional information, see Sigma Aldrich Technical Bulletin AL-134 or Sigma Aldrich Technical Bulletin AL-164.
  • Aluminum Chloride (AlCl3) is a potentially dangerous material because if moisture is present, decomposition can produce hydrogen chloride (HCl) and build up considerable container pressure. When opening a bottle that has been stored for a long time, completely enclose it in a heavy towel.
  • Ammonia (NH3) reacts with iodine to produce nitrogen tri-iodide (which is explosive), and with hypochlorites to produce chlorine. Do not mix ammonia-based cleaners with bleach. Mixtures of ammonia and organic halides sometimes react violently when heated under pressure.
  • Aqua Regia is a mixture of nitric acid and hydrochloric acid, and is sometimes used for dissolving noble metals or as glassware cleaner. Try to avoid using aqua regia. If you require it, make and use only what you need in a laboratory hood, and destroy it within the hood after use. Do not store it in closed containers; attempts to store aqua regia will most likely rupture the storage container. Upon generation, the nitric acid begins to reduce, with evolution of toxic nitrogen dioxide gas.
  • Benzoyl Peroxide (C6H5CO2)2 is easily ignited and sensitive to shock. It decomposes spontaneously at temperatures above 50°C (122°F), but can be desensitized by addition of 20% by volume of water.
  • Carbon Disulfide (CS2) is highly toxic and highly flammable; when mixed with air, its vapors can ignite by a steam bath or pipe, a hot plate, or a glowing light bulb. Carbon disulfide catches fire spontaneously upon contact with a hot surface at temperatures approximating or exceeding 80°C (176°F).
  • Chlorine (Cl2) may react violently with hydrogen (H2) or with hydrocarbons when exposed to sunlight.
  • Diazomethane (CH2N2) and related diazo compounds require extreme caution. They are very toxic, and the pure forms (gases and liquids) explode readily. Diazald (a precursor to diazomethane) is a high explosive. Solutions in ether are safer, and are rendered harmless by dropwise addition of acetic acid.
  • Diethyl, Isopropyl, and other Ethers (particularly the branched-chain type) may explode during heating or refluxing due to the presence of peroxides. Ferrous salts or sodium bisulfite can decompose these peroxides, and passage over basic active alumina will remove most of the peroxidic material. Mark containers with the date received, date opened, and date to be discarded, and discard them before they are out of date. For more detail, see Chapter 13: Safe Handling of Peroxidizable Compounds.
  • Diethylzinc [(C2H5)2Zn] is a violently pyrophoric (air-reactive), water-reactive, and light-sensitive liquid, and is generally sold in mixture with toluene, hexane, or other organic solvents. At concentrations above 1.1 molar, store diethylzinc in an inert atmosphere at or below room temperature. Do not use water for extinguishing fires; use dry powder, soda ash, or lime.
  • Dimethyl Sulfoxide [(CH3)2SO] decomposes violently on contact with a wide variety of active halogen compounds. Explosions from contact with active metal hydrides have been reported. Its toxicity is still unknown, but it penetrates and carries dissolved substances through the skin membrane.
  • Dinitrophenols [(NO2)2C6H3OH] such as 2,4-DNP and 2,6-DNP are sensitive to light, heat, friction, and shock, and should never be allowed to dry out. 2,4-DNP forms explosive salts with alkalis and ammonia. Oxidative decomposition can produce nitrogen oxides. At water concentrations less than 15%, DNPs are explosive and subject to the storage requirements of the ATF regulations.
  • Dry Ice, solid carbon dioxide (CO2), is not to be kept in a container that is not designed to withstand pressure. Containers of other substances stored over dry ice for extended periods generally absorb carbon dioxide (CO2) unless sealed with care. When removing such containers from storage and allowing them to come rapidly to room temperature, the CO2 may develop sufficient pressure to burst the container with explosive violence. On removal of such containers from storage, loosen the stopper, or wrap the container in towels and keep it behind a shield. Dry ice can produce serious burns; this is also true for all types of cooling baths. Do not store dry ice in walk-in cold rooms, as this may result in oxygen-deficient atmosphere.
  • Drying Agents-Ascarite must not mix with phosphorus pentoxide (P2O5) because the mixture may explode if warmed with a trace of water. Because organic solvents may extract the cobalt salts used as moisture indicators in some drying agents, the use of these drying agents shall be restricted to gases.
  • Ethylene Oxide (C2H4O) can explode when heated in a closed vessel. Carry out experiments using ethylene oxide under pressure behind suitable barricades.
  • Fulminic Acid (HCNO), its metal salts, and other compounds that contain the fulminate ion (C≡N+-O) are highly unstable due to the weak single N-O bond. Fulminates are friction-sensitive primary explosives. The DOT Forbidden Explosives List includes fulminic acid, and both the DOT List and the ATF List of Explosive Materials include mercury fulminate and silver fulminate (not to be confused with fulminating silver, Ag3N, an explosive decomposition product of Tollens Reagents that is also on the DOT List – refer to section on Tollens Reagents below). Contact EHS if your research must involve fulminates.
  • Grignard Reagents (R-Mg-X) are alkyl- or aryl- magnesium halides that are highly reactive with oxygen and carbonyls. They can spontaneously ignite in moist air; handle Grignard reagents under inert gases such as argon or nitrogen, or in solvents such as tetrahydrofuran or ethyl ether.
  • Halogenated Compounds such as chloroform (CHCl3), methylene chloride (CH2Cl2), carbon tetrachloride (CCl4), and other halogenated solvents shall not be dried with sodium, potassium, or other active metal; violent explosions can result.
  • Hydrogen Peroxide (H2O2) stronger than three percent (3%) can be dangerous; in contact with the skin, it may cause severe burns. Thirty percent H2O2 may decompose violently if contaminated with iron, copper, chromium, other metals or their salts. Stirring bars may inadvertently bring metal into a reaction, so use with caution.
  • Liquid-Nitrogen Cooled Traps, when open to the atmosphere, rapidly condense liquid air. With coolant removal, a pressure buildup may occur sufficient to shatter glass equipment. Only cool sealed or evacuated equipment with liquid nitrogen.
  • Liquid Nitrogen Storage Dewars are common for cryopreservation of samples. Cryopreservation vials stored in the liquid phase of liquid nitrogen can rupture upon warming if liquid nitrogen has infiltrated them, as the liquid nitrogen expands more than 600 times during evaporation. Store vials in the gaseous phase above the liquid nitrogen to avoid infiltration.
  • Lithium Aluminum Hydride (LiAlH4) shall not be used to dry methyl ethers or tetrahydrofuran; fires from this are common. The products of its reaction with CO2 can be explosive. Do not use carbon dioxide or bicarbonate extinguishers against LiAlH4 fires; use sand or a Class D extinguisher.
  • Figure 11.2
    Figure 11.2. This explosion within a storage cabinet resulted from nitric acid mixed with an organic solvent in a closed container. The pressure build-up ruptured the container and blew the cabinet doors open.
    Nitric Acid (HNO3) is a powerful oxidizing agent that ignites on contact or reacts explosively with a variety of organic substances including acetic anhydride, acetone, acetonitrile, many alcohols, benzene, DMSO, and methylene chloride (Figure 11.2). Do not store nitric acid with combustible organic acids such as acetic acid or formic acid. Nitric acid can also react violently with many inorganic substances including bases, reducing agents, alkali metals, copper, phosphorus, and ammonia.
  • Nitrocellulose [(C6H7O11N3)n] in dry, unstabilized form is explosive when heated or subjected to sudden shock. Synonyms include Pyroxylin, Parlodion®, and Guncotton. Store moist, away from heat sources and sunlight, and segregated from other materials. Nitrocellulose in membrane filters with polyester backing and mixed cellulose ester (MCE) filters is more stable, but can still spontaneously combust when exposed to oxidizing agents or sources of heat. Do not store filters where exposure to direct sunlight could occur.
  • Nitroglycerin [C5H3(NO3)3] for research purposes is usually in tincture form, mixed with alcohol. Do not allow the carrier to evaporate, as this will result in high explosive nitroglycerin.
  • Oxygen Tanks can explode due to contact between oil and high-pressure oxygen. Do not use oil on connections to an oxygen cylinder or regulator. Do not use soap-based leak detector compounds on the connection threads of an oxygen cylinder.
  • Ozone (O3) is a highly reactive and toxic gas. It forms by the action of ultraviolet light on oxygen (air) and, therefore, certain ultraviolet sources may require venting to the exhaust hood.
  • Palladium or Platinum on Carbon, Platinum Oxide, Raney Nickel, and other Catalysts must be carefully filtered from catalytic hydrogenation reaction mixtures. The recovered catalyst is usually saturated with hydrogen and highly reactive; thus, it will ignite spontaneously on exposure to air. Particularly for large-scale reactions, do not allow the filter cake to dry. Place the funnel containing the still-moist catalyst filter cake into a water bath immediately after completion of the filtration. Another hazard in working with such catalysts is the potential of explosion when adding additional catalyst to a flask in which hydrogen is present.
  • Parr Bombs used for digestions or hydrogenations have failed and exploded. Handle all high-stress equipment such as bomb calorimeters with care behind bench top shields, and wear proper eye protection.
  • Perchlorate use should be avoided whenever possible. Do not use perchlorates as drying agents if there is a possibility of contact with organic compounds, or in proximity to a dehydrating acid strong enough to concentrate the perchloric acid (HClO4) to more than 70% strength (e.g., in a drying train that has a bubble counter containing sulfuric acid). Use safer drying agents.
    Seventy percent (70%) HClO4 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 serious explosions. Do not allow oxidizable substances to contact HClO4. Use beaker tongs, rather than rubber gloves, when handling fuming HClO4. Carry out perchloric acid evaporations and digestions in a dedicated hood that has a good draft, and that is washable. Frequent (weekly) washing out of the hood and ventilator ducts with water is necessary to avoid the danger of metal perchlorate buildup, which could lead to spontaneous combustion or explosion. Contact Environment, Health and Safety to determine if you have a washable hood.
  • Permanganates are explosive when treated with sulfuric acid. When both compounds are in an absorption train, place an empty trap between them.
  • Peroxides (inorganic), when mixed with combustible materials, barium, sodium, and potassium, form explosives that ignite easily.
  • Phosphorus (P), both red and white, forms explosive mixtures with oxidizing agents. White (also called yellow) P should be stored under water, in glass, because it is pyrophoric. The reaction of P with aqueous hydroxides gives phosphine (PH3), a highly toxic gas that can also ignite spontaneously in air or explode.
  • Phosphorus Trichloride (PCl3) reacts with water to form phosphorous acid, which decomposes on heating to form phosphine, which may ignite spontaneously in air or explode. Take care when opening containers of PCl3, and do not heat samples that were exposed to moisture without adequate shielding to protect you.
  • Figure 11.3
    Figure 11.3. A bottle of dried-out picric acid, discovered in a UNC laboratory in 2003, which has become a highly shock-sensitive Division 1.1 explosive.
    Picric Acid [(NO2)3C6H2OH], also known as 2,4,6-trinitrophenol, can form explosive compounds with many combustible materials. Do not store in metal containers, as this can cause the formation of highly explosive metal picrate salts. Picric acid in saturated aqueous solution is relatively stable, but becomes less stable with age; in solutions of 10% to 40% water, it is considered a flammable solid. If picric acid dries to less than 10% water (Figure 11.3), it is a high explosive (DOT Class 1, Division 1.1), and must not be touched or disturbed except by trained high-hazard removal specialists.
  • Piranha Solutions (mixtures of sulfuric acid and hydrogen peroxide) used for removal of organic materials must never be stored, as they are likely to pressurize and explode their container. Make only what you need, and discard immediately after use. The use of a vented cap should be employed when storing as waste. Solutions are very energetic and heat to over 100 °C (212°F) during mixing; handle with care.
  • Potassium (K) is in general more reactive than sodium, and ignites quickly on exposure to humid air; therefore, handle it under the surface of a hydrocarbon solvent such as mineral oil or kerosene (see Sodium). Potassium may also form peroxides even while stored under oil.
  • Propargyl Bromide (C3H3Br), also known as 3-bromopropyne, is an unstable water-insoluble compound that is usually stored in a solvent such as toluene. Do not allow propargyl bromide to dry out, do not store it in an area near heat sources, and do not expose it to mild mechanical shocks.
  • Residues from Vacuum Distillations (for example, ethyl palmitate) have exploded when the still was vented to the air before the residue was cool. Avoid such explosions by venting the still pot with nitrogen, cooling it before venting, or restoring the pressure slowly.
  • Sodium (Na) shall be stored in a closed container under kerosene, toluene or mineral oil. Destroy scraps of Na or K by reaction with n-butyl alcohol. Avoid contact with water, as sodium reacts violently with water to form hydrogen with evolution of sufficient heat to cause ignition. Use sand or Class D extinguishers on alkali metal fires. Do not use CO2 fire extinguishers.
  • Sodium Amide (NaNH2) can rapidly absorb water and carbon dioxide from humid air. Oxidation can produce sodium nitrite in a mixture that is unstable and may explode. Store sodium amide in a cool, dry place in a tightly-sealed container under inert gas blanket.
  • Sodium Azide (NaN3) can react with copper and lead (including copper and lead in plumbing) to produce explosive copper or lead azide. Use caution when drain disposing substances that contain ANY amount of sodium azide. Even the trace amounts (<1%) used as an antimicrobial in many chemical mixtures and reagent test kits can react with copper or lead in areas such as P-traps; there is the potential for prolonged contact between the azide and lead/copper that might be in these traps. Do not drain dispose of any substances with trace amounts of sodium azide. Sodium azide is also highly toxic, and can explosively decompose due to heat, shock, concussion, or friction. Do not mix with sulfuric or nitric acid.
  • Sulfuric Acid (H2SO4) should be avoided, if possible, as a drying agent in desiccators. If used, place glass beads in it to prevent splashing when the desiccator is moved. Avoid using H2SO4 in melting point baths, use silicone oil instead. To dilute H2SO4, add the acid slowly to cold water.
  • Tollens Reagents, which contain an aqueous diamine silver complex [Ag(NH3)2+] and are used to test for aldehydes, must be freshly prepared and NEVER stored for longer than 1-2 hours. Stored Tollens Reagent can form explosive fulminating silver (Ag3N). Acidify with dilute acid before disposal.
  • Trichloroethylene (Cl2CCHCl) reacts under a variety of conditions with potassium or sodium hydroxide to form dichloroacetylene (ClC≡CCl), which ignites spontaneously in air and detonates readily even at dry-ice temperatures. The compound itself is toxic, so take suitable precautions when using as a degreasing solvent. Methyl chloroform (1,1,1-trichloroethane) is a less toxic substitute.