C‑2 Other Fire Hazards
C‑3 Extinguishing a Fire
Fire is the principal cause of serious laboratory accidents. Nearly all organic solvents are flammable, some of them extremely so, as are many gases such as hydrogen, acetylene, ammonia, and light hydrocarbons. These gases and solvent vapors can form explosive mixtures with air. The following pages outline the ways to prevent a fire from occurring and the procedures to deal with a fire. Please make sure you know and follow these guidelines. Ask any member of the safety committee questions you might have.
In the case of a fire alarm, safely shut off what apparatus you can, and leave the building immediately by the stairs (do not use the elevator); be sure to close the lab or office door behind you.
C‑1 SOLVENTS AND FLAMMABILITY
The auto-ignition temperature of a liquid is the minimum temperature at which a substance will spontaneously ignite (no spark or flame is required). The flash point of a liquid is the minimum liquid temperature at which the vapor pressure is sufficient to form a flammable mixture with air, so that once initiated the flame will propagate through the vapor.
Flammable limits are the concentration limits of a gas mixture within which a flame, once initiated, will propagate itself. The lower flammable limit for many solvents (CS2, hydrocarbons) in air is as little as one or two percent by volume.
The Ontario Occupational Health and Safety Act defines as flammable all substances with flash points of 100oF (38oC) or below. Under this Act all flammable substances must be stored in proper flammable solvent storage cupboards or in specially constructed rooms. In the Chemistry department the yellow double‑walled steel cabinets should be used for the storage of bottles of solvents. In many laboratories the ovens beneath fumehoods have been converted for additional solvent storage capacity; flammable substances which cannot be accommodated in the steel cabinets should be stored in these areas. Solvent bottles should be stored in proper cabinets at all times except when they are being used in laboratory experiments. Bottles of solvent on the laboratory bench must be tightly capped such that they will not leak if inverted. Flammable substances in quantities greater than 500 mL must be transported in approved safety carriers.
In the absence of a flame, a spark or an incandescent electric heating element, the auto‑ignition temperature is ordinarily of serious concern only with a few substances. Carbon disulfide has an auto‑ignition temperature of about 100oC and the vapors can be ignited by contact with an ordinary low‑pressure steam line; the auto‑ignition temperature for diethyl ether is 180oC, low enough so that use of an electric hot plate has caused ignition. A McMaster chemistry graduate student was seriously injured when ether was ignited by a hot plate. Such liquids should be heated with a water bath or a steam bath in a hood so that vapour from the boiling liquid does not accumulate.
Solvent Flash Points
Of more frequent concern is the flash point. If the flash point of a solvent is below room temperature (25oC), the solvent is termed a Class I solvent. Examples of Class I solvents are the commonly used organic solvents diethyl ether, benzene, methanol, ethanol, acetone, petroleum ether, ethyl acetate. See Appendix 2 for a list of the flash points and boiling points of some common solvents. Precautions to be observed in all operations with Class I solvents include the following:
If large quantities are being handled, set up "NO FLAME" signs. Deliberately scout the working area for lighted burners, pilot lights, electric motors, switches and other sparking electrical contacts, burning tobacco, etc. before beginning your operations and periodically while they are in progress. Operations in which solvents are escaping from the reaction vessel should always be conducted in a fume hood. Solvent vapors are generally more dense than air and may flow for considerable distances along bench tops or floors and may accumulate in depressions.
Conduct recrystallizations on a steam bath or a hot plate (see Precautions above with regard to CS2 and diethyl ether), either in a hood or with a condenser to contain vapors from the boiling liquid. An Erlenmeyer flask (not a beaker or round bottom flask) should be used for recrystallizations.
Before a liquid is heated to boiling, a boiling chip or some other device to should be added to serve as an ebullator. A superheated liquid may suddenly "bump" or boil violently, and often will overflow the container and create a fire hazard. The same may happen if a solid is added to a superheated liquid; therefore never add any solids or boiling chips to a hot liquid.
Reflux and distillation apparatus should be tightly assembled and firmly clamped, with all ground joints well seated. One should be certain that somewhere (at the top of the reflux condenser or at the distillate receiver) the system is open to the air or connected to a N2 line (except in reduced-pressure distillations). A mantle, oil bath, hot plate, or steam should be used for heating. The use of a free flame is never desirable; a heat gun is preferable for this purpose. Care should be exercised when using these hot air guns; they can make the apparatus hot enough to exceed the auto-ignition temperature of some solvents. Be sure to check first. If a microburner must be used to melt solidified distillate in some part of the system or for some other purpose, make doubly sure the joints are tight; in this case the vent to the air should be through a rubber tube with its open end several feet away and below the flame level.
C‑1.5. One should avoid having large quantities of flammable solvents in a work area.
You should eliminate the possibility of sparks of all kinds in the work area. Electric sparks may come from switches. relay contacts, or thermostatic devices; the latter are found in heaters, hot plates, and refrigerators. For this reason these devices, whenever possible, should be sealed so that solvent vapors cannot get in or sparks or flame get out; refrigerators used in the laboratory should be of the "explosion‑proof' type, with switches and thermostat contacts sealed or mounted outside the box. Electric sparks from electric motors can be avoided by employing induction motors for stirrers and pumps instead of series‑wound and other brush‑containing motors. Electric sparks can also arise from the buildup of "static electricity", especially in the dry indoor conditions during the winter. Avoid excessive wiping or swirling of flasks or bottles containing solvents before pouring; when dealing with more than about a liter of Class I solvents in metallic systems, ground the apparatus and the container. Sparks can arise also from metal striking metal or concrete, and, since solvent vapors are denser than air, a fire could be produced from a metal object falling onto a concrete floor or even shoe nails scraping on the concrete. This fact should be remembered if there is any spillage of solvents.
C‑1.7. One should never place beakers or unstoppered flasks containing solvents in a refrigerator.
Do not smoke, or permit others to do so, at any time in the laboratory. Municipal laws and University regulations permit smoking only in designated areas. The maximum fine is $2000.
C‑2 OTHER FIRE HAZARDS
C‑2.1. PERSONAL HAZARDS
Clothing and hair can catch fire from a forgotten Bunsen burner, or be ignited by a flash fire. Avoid fluffy or floppy or ragged clothing, especially of rayon or cotton, and unrestrained hair or necktie. A person with hair or clothing on fire may suffer very serious or even fatal burns unless prompt action is taken. Douse the person with water at the safety shower and/or roll him/her in a fire blanket immediately. You should know where the deluge showers and fire blankets are, so that the victim will not be a cinder by the time they are found. Showers are normally located by the laboratory exit doors. Fire blankets are located by the elevators (except on floor 1, where the blanket is in the hall opposite ABB107). If you are too far from either, use any available source of water or roll the victim on the floor to snuff out flames. A person whose clothing is afire should not run as this fans the flames.
C‑2.2. SPONTANEOUS COMBUSTION DANGERS
A number of chemical substances and mixtures are spontaneously combustible. These include white phosphorus, pyrophoric metals (including hydrogenation catalysts such as Raney nickel or platinum whose surface is saturated with hydrogen, palladium and methanol, platinum oxide and alcohol vapors or hydrogen, finely divided alkali metals), metal alkyls such as Grignard or organolithium reagents, low molecular weight phosphines and boranes, arsines, and iron carbonyl. White phosphorus can be transported and stored for a time under water, but after long periods acidity builds up in the water due to slow air oxidation. Beware of storing phosphorus under water of high alkalinity; if the pH of the water is above 9, the poisonous and spontaneously flammable gas phosphine, PH3, may be evolved.
C‑2.3. ALKALI METAL FIRES
Alkali metals are spontaneously combustible in the presence of water (owing to evolution of both hydrogen and heat) and certain other substances such as chlorinated hydrocarbons. One should be sure that a bucket of sand is available in the laboratory for extinguishing fires involving alkali metals or metal hydrides. Alkali metals should be stored under purified kerosene or mineral oil (Nujol). Metallic sodium may be used to dry certain solvents (e.g. ether, dioxane) that contain no active hydrogen or halogen; for this purpose the metal is usually introduced in the form of wire extruded from a sodium press directly into the solvent bottle. Scraps of sodium wire in an empty solvent bottle should be immediately destroyed under a flow of nitrogen by cautious addition of ethanol or methanol to the bottle, which is contained in a clean, dry pan or pail; other alkali metal scraps can be disposed of similarly or placed under mineral oil in a bottle provided for that purpose. CAUTION: Never put alkali metals into water, CCl4 or other chlorinated hydrocarbons! See Appendix 3 for procedures to follow for the destruction of small quantities of alkali metals.
C‑3. EXTINGUISHING A FIRE
If a fire breaks out, retreat to safety. You should not approach to extinguish the fire until you are sure that it is safe to do so. The fire extinguishers are usually at (or just inside) the laboratory doors. The instrument labs are equipped with Halon fire extinguishers which fill the room with Halon to extinguish the fire. In case of fire in these labs, an alarm will sound. You will have 30 seconds to leave the room before discharge of the Halon. If there is a false alarm, a reset button on the control panel near the exit door can be activated to stop the discharge. When approaching to extinguish the fire you should (a) be very careful to leave yourself an avenue of retreat, (b) take into account the possibility of explosion or rapid spread of the fire, and (c) be alert for any sign of toxic gases, particularly phosgene which can be present when chlorinated hydrocarbons are involved. Unless the fire is very minor and burns itself out very quickly, call the McMaster emergency number (88) and, if necessary, activate the fire alarm. For procedures to follow in case of personal injury, see Section B.
C‑3.1. FIRE EXTINGUISHER TYPES
The principal classes of fires and the appropriate extinguishers for them are listed in Appendix 1. The "dry‑chemical" fire extinguisher (dry NaHCO3 type) is the preferred all‑round extinguisher, and the most effective extinguisher for Class B type fires (i.e., burning flammable liquids and gases, rubber, plastics, etc). It is also the best all‑round extinguisher for home and automobile purposes. However, some instrument laboratories are equipped with the CO2 type extinguishers, which are reasonably effective. In a confined area the CO2 gas released may add somewhat to the hazard of suffocation that always exists to some degree with a fire. A small class D fire can be smothered by other inert (non‑reducible), dry materials: dry sand is recommended.
Whenever an extinguisher has been used, the usage must be reported without delay (call Steve Kirk ext. 23347) so that the extinguisher can be refilled. All extinguishers are inspected monthly to ensure their readiness for use at any time.