D-1 Liquid Nitrogen
D‑2 High Pressure Gas Cylinders
D‑3 Explosions, Pressure and Vacuum
Most explosions can be avoided by common sense and adequate precautions. The most common causes of explosions are given below. Please read the following guidelines and take extreme caution when dealing with potentially explosive situations.
D‑1. LIQUID NITROGEN
Liquid nitrogen and other cryogens are dangerous due to their very low temperature; use proper precautions when handling them. Goggles and protective gloves should be worn. A graduate student lost all the skin from his hand by failing to follow simple procedures.
Proper Vacuum Line Techniques
Use liquid nitrogen as a cold trap only with high vacuum lines. Since the temperature of liquid nitrogen is a few degrees below that of liquid oxygen, the immersion of any open vessel into liquid nitrogen will result in the slow condensation of liquid oxygen from the atmosphere into that vessel. Liquid oxygen, a very pale blue liquid, is an extremely powerful oxidant. When condensed into clean vessels liquid oxygen is relatively safe; however, in the presence of any oxidizable substances, such as organic compounds, liquid oxygen can lead to violent explosions. The most common situation leading to the condensation of liquid oxygen onto organic compounds arises when vacuum lines are incorrectly shut down. When shutting down a vacuum line, the liquid nitrogen Dewars on the cold traps should be removed exposing the cold traps. - Only then do you bleed air or nitrogen gas into the system before shutting off the pump motor. If the liquid nitrogen Dewars are NOT removed but the system is opened to the atmosphere and the pump motor turned off then liquid oxygen can condense slowly into the traps on top of the condensate already in the trap. Depending on how easily oxidized the day's condensate is, the resultant mixture may be an extremely powerful, contact-sensitive explosive.
Be sure to double check with someone familiar with vacuum line procedures if you are not sure how to set up or shut down a vacuum line. If you don't feel comfortable using liquid nitrogen to cool the cold traps, you can substitute a dry ice/isopropanol mixture instead.
It is recommended that liquid nitrogen not be used to cool glass tubes prior to sealing unless the tube is attached to a high vacuum line; otherwise air may condense in the tube and cause an explosion when the tube is brought to room temperature. There are prescribed freeze‑pump thaw procedures to be followed when sealing glass tubes under vacuum -‑ be sure that you are familiar with them before starting. If you don't know these procedures, ask. Do not cool carbon steel gas cylinders with liquid nitrogen ‑‑ they might explode. Do not mix liquid nitrogen with any organic solvent to give very low temperature baths, since after some time liquid oxygen may be condensed from the atmosphere.
D‑2. HIGH PRESSURE CYLINDERS
High pressure cylinders, whether or not they contain flammable or explosive gases are potentially dangerous. Rupture or sudden discharge can turn these cylinders into lethal missiles. Breaking the tap off a gas cylinder at 2000 psi results in a "rocket" that can penetrate two concrete walls. Use caution when opening valves on old cylinders of dangerous materials. If the label has deteriorated, be sure of the identity of the material before use. In trying to open corroded or jammed valves, the valve may suddenly release and become jammed open thereby releasing the entire contents of the cylinder. Acetylene has been known to explode under non-extreme conditions of mechanical shock or heating and may form explosive copper acetylide if copper tubing is used.
One should never move a gas cylinder with the working head (regulator) attached, even if the cylinder is simply being moved within a single laboratory. ANY TIME a gas cylinder which is not empty is moved the working head should be removed and the safety cap replaced.
Cylinders must be securely fastened to an approved cart while being transported. They must be anchored by straps or chains at the working site before the safety cap is removed and the regulator is attached. Close the main cylinder valve when not in use. Use the correct regulators and fittings for the particular gas in the cylinder. Use no oil or grease with valves or regulators ‑‑ especially with oxygen. One should never admit gas from a pressurized vessel to a closed system without providing an adequate safety relief. Compressed air from the laboratory line should be used only when necessary, and not routinely for cleaning off machinery, your desk, the floor, or your clothes. The sixty psi blast can easily burst an eardrum or drive particles deep into the eyeball. Never point a compressed air blast at anyone, including yourself. One should never connect the compressed air to a closed system (or one that could accidentally become closed) that is not rated for at least twice the pressure.
The term "explosion" is used loosely to denote any reaction in which a pressure buildup is sufficiently rapid and violent to shatter the reaction container. Detonations are explosions in which the decomposition, once initiated by mechanical shock or temperature, propagates at hypersonic velocity through the medium and results in a destructive shock wave. Ordinary protective equipment such as safety shields, and safety glasses provide at best uncertain protection against detonations, even with small quantities of material, though they may be better than nothing. Where detonation is a possibility the reaction should be carried out behind an adequate barricade or in a remote location, and warning signs should be posted throughout the area.
Also under the heading of explosions are non‑detonating reactions which get out of control in such a way that the rapid pressure buildup results in bursting of the reaction vessel and spattering of the contents, or where a flammable gas mixture inside a vessel becomes ignited with similar results. Safety shields and face masks and placement of the apparatus in a hood usually give adequate protection when the quantities involved are not large. However, some reactions which may be carried out safely at ordinary temperatures may result in explosion when the temperature rises and the reaction gets out of control.
D‑3.1. POTENTIAL EXPLOSIVES
Compounds having oxidizing elements ‑‑ oxygen or halogen -- attached to nitrogen or oxygen may be potential detonating explosives. This is particularly true of compounds containing nitrogen since the great stability of the N2 molecule, a common detonation product, contributes substantially to the driving force of the reaction.
Compound types, functional groups or ions which may contribute to the explosiveness of covalent or ionic substances include the following:
amine oxides (=N+‑O-) nitrite salts or ester (NO2 or ‑ONO)
azides (‑N3 or N3-) nitro compounds (‑NO2)
chlorates (Cl03-) nitroso compounds (‑NO)
diazo compounds (‑N=N‑) ozonides (‑O3--)
diazonium salts (-N2+) peracids (-CO3H)
fulminates (ONC-) peroxides (‑OO‑)
haloamines (‑NHX) perchlorates (ClO4-)
hydroperoxides (‑OOH) picrates
hypohalites (OX-) picric acid (dry)
nitrate salts or esters (NO3- or ‑ONO2)
Ethers and conjugated oleflns may form explosive peroxides on prolonged exposure to air: many explosions have resulted from distilling ethers to dryness. Some ethers such as diisopropyl ether readily form explosive hydroperoxides once a reagent bottle has been opened and air admitted to the bottle (see Appendix 5). It is recommended that these ethers not be stored for prolonged periods after opening. A small portion of such a solution should be tested with moist starch‑potassium iodide paper before distillation; the slightest blue coloration indicates the presence of peroxides. Potassium will surface oxidize forming explosive peroxides even when stored under oil. Old samples of potassium have exploded while being cut with a knife. Heavy metal acetylides, fulminates, and azides are highly explosive, a fact to remember if a heavy metal salt is present in a reaction where acetylene is to be used. The presence of ammonia and iodine in the same reaction mixture can lead inadvertently to the formation of nitrogen triiodide, a powerful explosive, which is so sensitive when dry that the slightest shock can set off a violent explosion.
Compounds of the above types vary widely in their sensitivity to shock and temperature. Picric acid, nitrogen triiodide, ether peroxides and heavy metal azides are extremely sensitive, while ammonium nitrate is a powerful explosive that can be set off only with another explosive.
Oxidizing agents such as hydrogen peroxide, sodium peroxide, potassium permanganate, perchloric acid, nitric acid, chromium trioxide, nitrogen tetroxide, tetranitromethane, acetyl peroxide, acetyl nitrate, and Tollens reagent, as well as many compounds of types already mentioned, can yield explosive mixtures with oxidizable substances. Carbon tetrachloride and nitromethane may explode during a sodium fusion test. Liquid oxygen, liquid air, or gaseous fluorine in contact with organic substances may lead to spontaneous explosions.
Liquid Nitrogen Traps Remember, traps cooled with liquid nitrogen are capable of condensing liquid oxygen inside them, which will create a hazard if organic substances are already present or are later condensed; a trap on a vacuum system should not be chilled with liquid nitrogen until the pressure has been reduced below atmospheric pressure. Vacuum line traps should be cleaned routinely to remove organic residues then allowed to dry thoroughly before use on the vacuum line. (See D-1 above).
An unusual danger is presented to those doing perchloric acid digestions of samples prior to inorganic analyses for metals. Perchloric acid fumes from this procedure can react with components in the fumehood (even stainless steel) producing perchlorate salts on the inside walls of the fumehood and ducts. Since many dry perchlorate salts are sensitive contact explosives, there are examples of violent explosions of these accumulated salts in fumehoods. The problem is alleviated by routine washing of the fumehood walls with water since all perchlorates are soluble in water. Indeed, some fumehoods now contain a washdown system built into the back of the flue. Large scale use of perchloric acid should only be carried out in such a fumehood.
D‑3.2. REACTIONS WHICH MAY GET OUT OF CONTROL
Among reactions which may require careful attention to prevent a possible explosion are nitrations, oxidations (especially with per‑acids, per‑salts, or peroxides), condensations (Friedel-Crafts, Claisen, or Reppe), reductions (Wolff‑Kishner, metal hydride), and polymerizations (such substances as butadiene, acrolein, and acrylonitrile can polymerize spontaneously and explosively in the presence of a catalyst which may be an unintended impurity; the same is true of liquid HCN and liquid acetylene). Some intrinsically slow reactions can be speeded up explosively by the presence of a solubilizer (e.g., NaOH + CHCl3 in the presence of methanol).
D‑3.3. REASONABLE PRECAUTIONS TO OBSERVE
It is not possible to avoid altogether working with potentially explosive substances. Explosion hazard may however be reduced substantially by following sensible precautions:
D‑3.3.1. Ascertain the degree of hazard, where possible, by reference to the literature.
D‑3.3.2.Try any unknown reaction with small quantities and/or low concentration, with all reasonable precautions, then scale up. (Be aware, however, of the essential unpredictability and irreproducibility of detonations.)
D-3.3.3 Try Small Scale Reactions Initially
Compounds which may be prone to detonation should be prepared and handled only in dilute solution; if they should then decompose, even violently, the energy of decomposition is largely absorbed by the solvent. Be sure that transfers are quantitative and washings meticulous so that explosive residues are not left in vessels or on the desk top by evaporation.
D‑3.3.4 Be Prepared to Moderate Vigorous Reactions
Have adequate means on hand for moderating the reaction (control heat, cooling water, rate of addition of reagents or quenchers); if working behind a barrier, controls should be outside. Remember that the reaction rate will double or triple with each 10oC rise in temperature. Continuous cooling is safer than periodic cooling. Always arrange the apparatus with enough space below it so that the heating device can be quickly lowered and a cooling bath substituted without moving the apparatus itself.
D‑3.3.5 Add Reagents
Try to avoid adding a reagent faster than it is consumed, especially in oxidation, free‑radical, and heterogeneous reactions. Never add organic or other oxidizable materials to a strong oxidant; rather, add the oxidant slowly and with caution to the other substances.
D‑3.3.6 Be Alert for Reactions Going Out of Control
Be especially alert for indications that something is about to get out of control. For example, a sudden rise in temperature or pressure, the appearance of fumes or discoloration, evolution of gas, unexpected boiling, or reflux high in the condenser. Any of these may be sufficient cause to quench the reaction if this can be done in time and safely (the procedure having been well thought out ahead of time). Otherwise it should be the signal to quickly retreat to safe cover, warning others as you go. The same may be said for a situation where a cracked flask, flame around a joint, or a loose connection or stopcock indicates immediate serious trouble. From a safe place, with aid present, careful appraise and attempt to control the situation at a distance by disconnecting heat and/or discontinuing the addition of reagents. With suitable protective equipment and with help present, take whatever additional steps can be done safely to reduce the explosion and fire hazards.