C‑1 Solvents
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.
Solvent Storage
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.