Prudent Practices in the Laboratory

Transcription

1 Prudent Practices in the Laboratory Taken from NATIONAL ACADEMY PRESS, Washington, D.C.1995 A selection of useful sections TABLE 3.9 Partial List of Incompatible Chemicals (Reactive Hazards)... 2 TABLE 6.1 Summary of Magnetic Field Effects C.11.8 Handling Spills of Elemental Mercury G.6 Reactive or Explosive Materials Requiring Special Attention... 4 PEROXIDES... 7 Other Oxidizers... 8 Dusts... 8 The "Dirty Dozen" : top suspects for laboratory accidents

2 TABLE 3.9 Partial List of Incompatible Chemicals (Reactive Hazards) Substances in the left hand column should be stored and handled so that they cannot accidentally contact corresponding substances in the right hand column under uncontrolled conditions.) Acetic acid Acetic anhydride Acetone Acetylene Alkali and alkaline earth metals, powdered aluminum Ammonia (anhydrous) Ammonium nitrate Aniline Bromine Calcium oxide Carbon, activated Chlorates Chlorine Chlorine dioxide Chromic acid, chromium trioxide Copper Fluorine Hydrazine Hydrocarbons (benzene, butane, turpentine, ) Hydrocyanic acid Hydrofluoric acid (anhydrous) Hydrogen peroxide Hydrogen sulfide Iodine Mercury Nitric acid (concentrated) Nitroparaffins Oxalic acid Oxygen Perchloric acid Chromic acid, nitric acid, peroxides, permanganates Hydroxyl-containing compounds such as ethylene glycol, perchloric acid Concentrated nitric and sulfuric acid mixtures, hydrogen peroxide Chlorine, bromine, copper, silver, fluorine, mercury Carbon dioxide, carbon tetrachloride, other chlorinated hydrocarbons (also prohibit the use of water, foam, and dry chemical extinguishers on fires involving these metals -dry sand should be employed) Mercury, chlorine, calcium hypochlorite, iodine, bromine, hydrogen fluoride Acids, metal powders, flammable liquids, chlorates, nitrites, sulfur, finely divided organics, combustibles Nitric acid, hydrogen peroxide Ammonia, acetylene, butadiene, butane, other petroleum gases, sodium carbide, turpentine, benzene, finely divided metals Water Calcium hypochlorite, other oxidants Ammonium salts, acids, metal powders, sulfur, finely divided organics, combustibles Ammonia, acetylene, butadiene, butane, other petroleum gases, hydrogen, sodium carbide, turpentine, benzene, finely divided metals Ammonia, methane, phosphine, hydrogen sulfide Acetic acid, naphthalene, camphor, glycerol, turpentine, alcohol, other flammable liquids Acetylene, hydrogen peroxide, flammable liquids, flammable gases, nitratable substances Isolate from everything Hydrogen peroxide, nitric acid, any other oxidant Fluorine, chlorine, bromine, chromic acid, peroxides Nitric acid, alkalis Ammonia (aqueous or anhydrous) Hydrogen fluoride Copper, chromium, iron, most metals or their salts, any flammable liquid, combustible materials, aniline, nitromethane Fuming nitric acid,a oxidizing gases Acetylene, ammonia (anhydrous or aqueous) Acetylene, fulminic acid (Produced in nitric acid-ethanol mixtures), ammonia Acetic acid, acetone, alcohol, aniline, chromic acid, hydrocyanic acid, hydrogen sulfide, Inorganic bases, amines Silver and mercury and their salts Oils, grease, hydrogen, flammable liquids, solids, gases Acetic anhydride, bismuth and its alloys, alcohol, paper, wood, grease, oils (all organics) 2

3 Peroxides, organic Phosphorus (white) Phosphorus pentoxide Potassium chlorate Potassium perchlorate Potassium permanganate Silver and silver salts Sodium nitrite Sodium peroxide Sulfuric acid Acids (organic or mineral), (also avoid friction, store cold) Air, oxygen Alcohols, strong bases, water Acids (see also chlorates) Acids (see also perchloric acid) Glycerol, ethylene glycol, benzaldehyde, sulfuric acid Acetylene, oxalic acid, tartaric acid, fulminic acid (Produced in nitric acidethanol mixtures), ammonium compounds Ammonium nitrate and other ammonium salts Any oxidizable substance, such as ethanol, methanol, glacial acetic acid, acetic anhydride, benzaldehyde, carbon disulfide, glycerol, ethylene glycol, ethyl acetate, methyl acetate, furfural Chlorates, perchlorates, permanganates Hazards in the Chemical Laboratory, 4th edition, L. Bretherick, Ed. (1986). TABLE 6.1 Summary of Magnetic Field Effects Level at Which Effects Occur (gauss) Effects on electron microscopes 1 Disturbance of color computer displays 1-3 Disturbance of monochrome computer displays 3-5 Erasure of credit card and bank card coding 10 Effects on watches and micromechanical devices 10 Lowest known field effect on pacemakers 17 Saturation of transformers and amplifiers 50 Erasure of floppy disks 350 Site Planning Guide for Superconducting NMR Systems, Bruker Instruments (1992). 5.C.11.8 Handling Spills of Elemental Mercury Mercury spills can be avoided by using supplies and equipment that do not contain mercury. However, most mercury spills do not pose a high risk. The initial response to a spill of elemental mercury should be to isolate the spill area and begin the cleanup procedure. Those doing the cleanup should wear protective gloves. The cleanup should begin with collecting the droplets. The large droplets can be consolidated by using a scraper or a piece of cardboard, and the pool of mercury removed with a pump or other appropriate equipment. A standard vacuum cleaner should never be used to pick up mercury. If a house vacuum system is used, it can be protected from the mercury by a charcoal filter in a trap. For cleaning up small mercury droplets, a special vacuum pump may be used, or the mercury may be picked up on wet toweling, which consolidates the small droplets to larger pieces, or picked up with a piece of adhesive tape. Commercial mercury spill cleanup sponges and spill control kits are available. The common practice of using sulfur should be discontinued because the practice is ineffective and the resulting waste creates a disposal problem. The mercury should be placed in a thick-wall high-density polyethylene bottle and transferred to a central depository for reclamation. After a mercury spill the exposed work surfaces and floors should be decontaminated by using an appropriate decontamination kit. 3

4 5.G.6 Reactive or Explosive Materials Requiring Special Attention An explosive is any chemical compound or mechanical mixture that, when subjected to heat, impact, friction, detonation, or other suitable initiation, undergoes rapid chemical change, evolving large volumes of highly heated gases that exert pressure on the surrounding medium. The term applies to materials that either detonate or deflagrate. Heat, light, mechanical shock, and certain catalysts initiate explosive reactions. Hydrogen and chlorine react explosively in the presence of light. Acids, bases, and other substances catalyze the explosive polymerization of acrolein, and many metal ions can catalyze the violent decomposition of hydrogen peroxide. Shock-sensitive materials include acetylides, azides, nitrogen triiodide, organic nitrates, nitro compounds, perchlorate salts (especially those of heavy metals such as ruthenium and osmium), many organic peroxides, and compounds containing diazo, halamine, nitroso, and ozonide functional groups. Even a small sample may be dangerous. Furthermore, the hazard is associated not with the total energy released, but rather with the remarkably high rate of a detonation reaction. A high-order explosion of even milligram quantities can drive small fragments of glass or other matter deep into the body. It is important to use minimum amounts of these hazardous materials with adequate shielding and personal protection. A compound is apt to be explosive if its heat of formation is more than about 100 calories per gram (cal/g) less than the sum of the heats of formation of its products. In making this calculation, a reasonable reaction should be used in order to yield the most exothermic products. Scaling up reactions can introduce several hazards. Another situation that can lead to problems is a reaction susceptible to an induction period; particular care must be given to the rate of reagent addition versus its rate of consumption. Finally, the hazards of exothermic reactions or unstable or reactive chemicals are exacerbated under extreme conditions, such as high temperature or high pressure used for hydrogenations, oxygenations, or work with supercritical fluids. The following list is not intended to be all-inclusive. Further guidance on reactive and explosive materials should be sought. Acetylenic compounds can be explosive in mixtures of 2.5 to 80% with air. At pressures of 2 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. Acetylene must be handled in acetone solution and never stored alone in a cylinder. Aluminum chloride (AlCl3) should be considered a potentially dangerous material. If moisture is present, there may be sufficient decomposition to form hydrogen chloride (HC1) and build up considerable pressure. If a bottle is to be opened after long storage, it should first be completely enclosed in a heavy towel. Ammonia (NH3) reacts with iodine to give nitrogen triiodide, which detonates on touch. Ammonia reacts with hypochlorites to give chlorine. Mixtures of NH3 and organic halides sometimes react violently when heated under pressure. Ammonia is combustible. Inhalation of concentrated fumes can be fatal. Azides, both organic and inorganic, and some azo compounds can be heat- and shock-sensitive. Azides such as sodium azide can displace halide from chlorinated hydrocarbons such as dichloromethane to form highly explosive organic polyazides; this substitution reaction is facilitated in solvents such as dimethyl sulfoxide (DMSO). Carbon disulfide (CS2) 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 light bulb. Chlorine (C12) is toxic and may react violently with hydrogen (H2) or with hydrocarbons when exposed to sunlight. Chromium trioxide pyridine complex (CrO3 C5H5N) may explode if the CrO3 concentration is too high. The complex should be prepared by addition of CrO3 to excess C5H5N. Diazomethane (CH2N2) and related diazo compounds should be treated with extreme caution. They are very toxic, and the pure gases and liquids explode readily even from contact with sharp edges of glass (ground glass joints). Solutions in ether are safer from this standpoint. An ether solution of diazomethane is rendered harmless by drop wise addition of acetic acid. Diethyl, diisopropyl, and other ethers, including tetrahydrofuran and 1,4-dioxane and particularly the branched-chain type of ethers, sometimes explode during heating or refluxing because the 4

5 presence of peroxides has developed from air oxidation. Ferrous salts or sodium bisulfite can be used to decompose these peroxides, and passage over basic active alumina can remove most of the peroxidic material. In general, however, old samples of ethers should be disposed of after testing, following procedures for disposal of peroxides (see Chapter 7, section 7.D.2.5). Dimethyl sulfoxide (DMSO), (CH3)2SO, decomposes violently on contact with a wide variety of active halogen compounds, such as acyl chlorides. Explosions from contact with active metal hydrides have been reported. Dimethyl sulfoxide does penetrate and carry dissolved substances through the skin membrane. Dry benzoyl peroxide (C6H5CO2)2 is easily ignited and sensitive to shock. It decomposes spontaneously at temperatures above 50 C. It is reported to be desensitized by addition of 20% water. Dry ice should not 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 they have been sealed with care. When such containers are removed from storage and allowed 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, the stopper should be loosened or the container itself should be wrapped in towels and kept behind a shield. Dry ice can produce serious burns, as is also true for all types of dry-ice-cooled cooling baths. Drying agents, such as Ascarite (sodium hydroxide-coated silica), should not be mixed with phosphorus pentoxide (P2O5) because the mixture may explode if it is warmed with a trace of water. Because the cobalt salts used as moisture indicators in some drying agents may be extracted by some organic solvents, the use of these drying agents should be restricted to drying gases. Dusts that are suspensions of oxidizable particles (e.g., magnesium powder, zinc dust, carbon powder, and flowers of sulfur) in the air can constitute powerful explosive mixtures. These materials should be used with adequate ventilation and should not be exposed to ignition sources. When finely divided, some solids, including zirconium, titanium, Raney nickel, lead (such as prepared by pyrolysis of lead tartrate), and catalysts (such as activated carbon containing active metals and hydrogen), can combust spontaneously if allowed to dry while exposed to air and should be handled wet. Ethylene oxide (C2H4O) has been known to explode when heated in a closed vessel. Experiments using ethylene oxide under pressure should be carried out behind suitable barricades. Halogenated compounds, such as chloroform (CHCl3), carbon tetrachloride (CC14), and other halogenated solvents, should not be dried with sodium, potassium, or other active metal; violent explosions usually result. Many halogenated compounds are toxic. Oxidized halogen compounds chlorates, chlorites, bromates, and iodates and the corresponding peroxy compounds may be explosive at high temperatures. Hydrogen peroxide (H2O2) stronger than 3% can be dangerous; in contact with the skin, it can cause severe burns. Thirty percent H2O2 may decompose violently if contaminated with iron, copper, chromium, or other metals or their salts. Stirring bars may inadvertently bring metal into a reaction and should be used with caution. Liquid nitrogen-cooled traps open to the atmosphere condense liquid air rapidly. Then, when the coolant is removed, an explosive pressure buildup occurs, usually with enough force to shatter glass equipment if the system has been closed. Hence, only sealed or evacuated equipment should be so cooled. Lithium aluminum hydride (LiAlH4) should not be used to dry methyl ethers or tetrahydrofuran; fires from reaction with damp ethers are often observed. The reaction of LiAlH4 with carbon dioxide has reportedly generated explosive products. Carbon dioxide or bicarbonate extinguishers should not be used for LiAlH4 fires; instead such fires should be smothered with sand or some other inert substance. Nitrates, nitro and nitroso compounds may be explosive, especially if more than one nitro group is present. Alcohols and polyols can form highly explosive nitrate esters (e.g., nitroglycerine) from reaction with nitric acid. Organometallics are hazardous because some organometallic compounds burn vigorously on contact with air or moisture. For example, solutions of t-butyl lithium can cause ignition of some 5

6 organic solvents on exposure to air. The pertinent information should be obtained for a specific compound. Oxygen tanks should be handled with care because serious explosions have resulted from contact between oil and high-pressure oxygen. Oil or grease should not be used on connections to an O2 cylinder or gas line carrying O2. Ozone (O3) 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. Ozonides can be explosive. Palladium (Pd) or platinum (Pt) on carbon, platinum oxide, Raney nickel, and other catalysts present the danger of explosion if additional catalyst is added to a flask in which an air-flammable vapor mixture and/ or hydrogen is present. The use of flammable filter paper should be avoided. Parr bombs used for hydrogenations should be handled with care behind a shield, and the operator should wear goggles and a face shield. Perchlorates should be avoided insofar as possible. Perchlorate salts of organic, organometallic, and inorganic cations are potentially explosive and have been set off either by heating or by shock. Perchlorates should not be used as drying agents if there is a possibility of contact with organic compounds or of proximity to a dehydrating acid strong enough to concentrate the perchloric acid (HClO4) (e.g., in a drying train that has a bubble counter containing sulfuric acid). Safer drying agents should be used. Seventy percent 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, will lead to serious explosions. Oxidizable substances must never be allowed to contact HClO4. This includes wooden benchtops or hood enclosures, which may become highly flammable after absorbing HClO4 liquid or vapors. Beaker tongs, rather than rubber gloves, should be used when handling fuming HClO4. Perchloric acid evaporations should be carried out in a hood that has a good draft. The hood and ventilator ducts should be washed with water frequently (weekly; but see also section 8.C.7.5) to avoid danger of spontaneous combustion or explosion if this acid is in common use. Special perchloric acid hoods are available from many manufacturers. Disassembly of such hoods must be preceded by washing of the ventilation system to remove deposited perchlorates. Permanganates are explosive when treated with sulfuric acid. If both compounds are used in an absorption train, an empty trap should be placed between them and monitored for entrapment. Peroxides (inorganic) should be handled carefully. When mixed with combustible materials, barium, sodium, and potassium peroxides form explosives that ignite easily. Phosphorus (P) (red and white) forms explosive mixtures with oxidizing agents. White phosphorus should be stored under water because it ignites spontaneously in air. The reaction of phosphorus with aqueous hydroxides gives phosphine, which may either ignite spontaneously or explode in air. Phosphorus trichloride (PC13) reacts with water to form phosphorous acid with HCl evolution; the phosphorous acid decomposes on heating to form phosphine, which may either ignite spontaneously or explode. Care should be taken in opening containers of PCl3, and samples that have been exposed to moisture should not be heated without adequate shielding to protect the operator. Potassium (K) is much 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 (see Sodium). Potassium can form explosive peroxides on contact with air. If this happens, the act of cutting a surface crust off the metal can cause a severe explosion. Residues from vacuum distillations have been known to explode when the still was vented suddenly 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. Sudden venting may produce a shockwave that can detonate sensitive materials. Sodium (Na) should be stored in a closed container under kerosene, toluene, or mineral oil. Scraps of sodium or potassium should be destroyed by reaction with n-butyl alcohol. Contact with water should be avoided because sodium reacts violently with water to form hydrogen (H2) with evolution of sufficient heat to cause ignition. Carbon dioxide, bicarbonate, and carbon tetrachloride fire extinguishers should not be used on alkali metal fires. Metals like sodium become more 6

7 reactive as the surface area of the particles increases. Prudence dictates using the largest particle size consistent with the task at hand. For example, use of sodium ''balls" or cubes is preferable to use of sodium "sand" for drying solvents. Sodium amide (NaNH2) can undergo oxidation on exposure to air to give sodium nitrite in a mixture that is unstable and may explode. Sulfuric acid (H2SO4) should be avoided, if possible, as a drying agent in desiccators. If it must be used, glass beads should be placed in it to help prevent splashing when the desiccator is moved. To dilute H2SO4, the acid should be added slowly to cold water. Addition of water to the denser H2SO4 can cause localized surface boiling and spattering on the operator. Trichloroethylene (Cl2CCHCl) reacts under a variety of conditions with potassium or sodium hydroxide to form dichloroacetylene, which ignites spontaneously in air and detonates readily even at dry ice temperatures. The compound itself is highly toxic, and suitable precautions should be taken when it is used. PEROXIDES Organic peroxides are among the most hazardous substances handled in the chemical laboratory. They are generally low-power explosives that are sensitive to shock, sparks, or other accidental ignition. They are far more shock-sensitive than most primary explosives such as TNT. Also potentially hazardous are compounds that undergo autooxidation to form organic hydroperoxides and/or peroxides when exposed to the oxygen in air (see Table 3.12). Especially dangerous are ether bottles that have evaporated to dryness. A peroxide present as a contaminant in a reagent or solvent can be very hazardous and change the course of a planned reaction. Autoxidation of organic materials (solvents and other liquids are most frequently of primary concern) proceeds by a free-radical chain mechanism. For the substrate R H, the chain is initiated by ultraviolet light, by the presence of a radical source, and by the peroxide itself. Excluding oxygen by storing potential peroxide-formers under an inert atmosphere (N2 or argon) or under vacuum greatly increases their safe storage lifetime. In some cases, stabilizers or inhibitors (free-radical scavengers that terminate the chain reaction) have been added to the liquid to extend its storage lifetime. Because distillation of the stabilized liquid will remove the stabilizer, the distillate must be stored with care and monitored for peroxide formation. Note that 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! Types of Compounds Known to Autooxidize to Form Peroxides * Aldehydes * Ethers, especially cyclic ethers and those containing primary and secondary alkyl groups (never distill an ether before it has been shown to be free of peroxide) * Compounds containing benzylic hydrogens * Compounds containing allylic hydrogens (C = C CH), including most alkenes; vinyl and vinylidene compounds * Compounds containing a tertiary C H group (e.g., decalin and 2,5-dimethylhexane) Classes of Chemicals That Can Form Peroxides Upon Aging Class I: Unsaturated materials, especially those of low molecular weight, may polymerize violently and hazardously due to peroxide initiation. Acrylic acid Tetrafluoroethylene Acrylonitrile Vinyl acetate Butadiene Vinyl acetylene Chlorobutadiene (chloroprene) Vinyl chloride Chlorotrifluoroethylene Vinyl pyridine Methyl methacrylate Vinylidene chloride Styrene Class II: The following chemicals are a peroxide hazard upon concentration (distillation/ evaporation). A test for peroxide should be performed if concentration is intended or suspected. Acetal Dioxane (p-dioxane) Cumene 7

8 Ethylene glycol dimethyl ether (glyme) Cyclohexene Furan Cyclooctene Methyl acetylene Cyclopentene Methyl cyclopentane Diacetylene Methyl-i-butyl ketone Dicyclopentadiene Tetrahydrofuran Diethylene glycol dimethyl ether (diglyme) Tetrahydronaphthalene Diethyl ether Vinyl ethers Class III: Peroxides derived from the following compounds may explode without concentration. Divinyl ether Potassium metal Divinyl acetylene Potassium amide Isopropyl ether Sodium amide (sodamide) Vinylidene chloride Other Oxidizers Oxidizing agents may react violently when they come into contact with reducing materials, and sometimes with ordinary combustibles. Such oxidizing agents include the halogens, oxyhalogens and organic peroxyhalogens, chromates, and persulfates as well as peroxides. Inorganic peroxides are generally stable. However, they may generate organic peroxides and hydroperoxides in contact with organic compounds, react violently with water (alkali metal peroxides), and form superoxides and ozonides (alkali metal peroxides). Perchloric acid is a powerful oxidizing agent with organic compounds and other reducing agents. Perchlorate salts can be explosive and should be treated as potentially hazardous compounds. For many years, sulfuric acid dichromate mixtures were used to clean glassware (a sulfuric acid peroxydisulfate solution is now recommended because disposal of chromate is a problem). Confusion about cleaning baths has led to explosions on mixing potassium permanganate with sulfuric acid and nitric acid with alcohols. Dusts Suspensions of oxidizable particles (e.g., flour, coal dust, magnesium powder, zinc dust, carbon powder, and flowers of sulfur) in the air can constitute a powerful explosive mixture. These materials should be used with adequate ventilation and should not be exposed to ignition sources. Some solid materials, when finely divided, are spontaneously combustible if allowed to dry while exposed to air. These materials include zirconium, titanium, Raney nickel, finely divided lead (such as prepared by pyrolysis of lead tartrate), and catalysts such as activated carbon containing active metals and hydrogen. The "Dirty Dozen" : top suspects for laboratory accidents 1. Organic azides - Explosion hazards, especially with ground glass joints 2. Perchlorate salts of organic, organometallic, and inorganic complexes - Explosion hazards 3. Diethyl ether - Fires (see also entry 10 below) 4. Lithium aluminum hydride - Fires on quenching 5. Sodium, potassium - Fires on quenching 6. Potassium metal - Fires on quenching 7. Sodium-benzophenone ketyl still pots - Fires on quenching 8. Palladium on carbon - Fires on removal from the inert atmosphere, especially if wet with organic solvent or when contacting combustible materials such as filter paper 9. Heat - Exothermic reactions causing violent spills on scale-up due to inadequate provision for heat removal 10. Ethers with α-hydrogen atom - Dangerous peroxide concentration during distillation; explosion hazards, especially with ground glass joints 11. Carbon monoxide - Toxicity and role in forming nickel tetracarbonyl from steel gas lines and autoclaves 12. Organic peroxides Sensitivity to shock, sparks, and other forms of accidental detonation; sensitivity to heat, friction, impact, and light, as well as to strong oxidizing and reducing agents 8