6. General Hazards
At a minimum, floods are a nuisance. If however, water enters a hot oil bath, or leaks into electrical equipment or onto the floor, the resulting damage can be considerable, and potentially dangerous. Common flooding sources include condenser hose, steam baths, ice baths, hoses from steam lines, stopped-up sinks, drains too full (inside bench in organic lab), and excessive delivery rate of water.
A common cause of floods is when a rubber hose is improperly attached to a condenser. The hose diameter must match the diameter of the glass tubing (see your instructor), and two hands must be used when attaching the hose to the condenser. In cases of difficulty when attempting to attach the tubing, moisten the glass with water but do not use oil or grease (because the hose will eventually slide off and cause a flood). However, in other cases where tubing must be quickly removed from glass, use a drop of silicone oil for lubrication so that the connection remains flexible and the tubing can be removed at any time. In special cases (for instance, if a condenser must operate overnight or otherwise left unattended) secure the tubing with special clamps; see your instructor. Inspect tubing periodically for pinhole leaks, cracking due to rubber tears, and other damage (such as from very hot equipment). Rubber tubing used in steam lines deteriorates over relatively short periods and should therefore be inspected frequently and replaced, when indicated.
Water flow rates that are too high will cause rubber hoses to swell up ("balloon") and burst. The result will be wasteful splashing, which can cause floors to become dangerously slippery.
In cases in which it is more imperative to monitor the flow rate, use a special "flow watchman," in which a red ball is impelled around a circle by the moving water. Check with your instructor. When you connect steam baths, make sure that the steam enters the top tube and condensed water emerges at the bottom tube. Steam baths will collect undrained water and cause a flood if:
All steam lines must remain connected to a drain, since all of them leak, even when not in use. ALWAYS replace your steam bath, after you have finished using it, with a direct hose from steam valve to drain.
When operating the steam valve, do not run steam at any more than a low flow rate: once a bath gets to 100∞C, it will not get hotter from a faster steam flow. The idea that it might is a very common misconception. Excessive steam can contaminate your experiment as well as others’ experiments.
Floods from ice baths are not very common, but will occur when large vessels are set into a rather full bath. If a large amount of ice is piled up in the bath, and subsequently left unattended for a long period, flooding can also occur.
All sinks are fitted with a strainer to remove large particles and help prevent the drains from becoming blocked. Do not put corks, tubing, glassware, labels, and similar items into any sink. These items can cause the strainer to become blocked. If you find a sink containing these items, carefully remove and discard them using gloved hands.
Finally, use reasonable flow rates of water in condensers and in sinks. If a faucet handle is stiff, obtain assistance: sudden spurts of water might result if you turn it on suddenly. Always observe the exit hose on a condenser when you are adjusting the flow rate; a continuous, strong trickle is satisfactory, since increasing the rate of water does not improve condensation rates. Monitor the flow rates during the first few minutes of operation. Do not turn the water on, inspect the outflow, and then assume that the flow rate will be constant. The washer in the faucet may swell in the initial stages, reduce the flow rate, and require an adjustment.
6.2 Electrical Hazards — 110 v 60 Hz AC; Grounding, D.C., High Voltage
Every piece of electrical equipment used by laboratory workers and operated on 110 volts AC from the laboratory supply must be grounded (and therefore have a three-wire cord terminating in a three-pin plug). Bring to the attention of the instructor any piece of such equipment that is not properly connected. Equipment that has only a two-wire cord may give rise to electrical shocks under certain conditions, especially if:
• The floor is wet. Never step on a damp floor when handling live apparatus.
• If the AC voltage is higher than 110 volts, special plugs are usually necessary. Check to see that the case of this equipment (certain heavy-duty furnaces and large electric motors) is grounded. In general, all DC and high-voltage circuits should have ground wires.
Do not use any piece of electrical equipment where inspection shows a break in the insulation of power cord wires. Report such breaks immediately to your instructor so that the equipment can be sent for repair.
It is also important to place electrical equipment in an area that will minimize the possibility of spills onto the equipment or the presence of flammable vapors around it.
6.3 High Temperatures—Hot Plates, Heating Mantles, and Furnaces
Unsafe operation at high temperatures can cause burns, explosions (from trapped steam or gases, for example), detonations (from pyrolysis of unstable compounds or mixtures or other causes), and fires.
Use only hot plates that are fitted with a light that indicates that they are working. When you switch off a hot plate, do not forget that while it is still hot, it is a hazard to others. Label it appropriately so that the next person who tries to pick it up does not suffer an accident. Do not return hot plates to the side shelves while they are still hot. Similarly, if a dish of hot oil resides on this hot plate, either leave a thermometer in it (so that the next person can judge how hot the oil is) or attach a note. Do not move hot oil baths; consult the instructor if it is necessary to do so. Never connect heating mantles directly to the 110-volt VC supply. Use a voltage control device.
6.4 Superheated Liquids
Liquids will not boil smoothly unless a source of nuclei is present on which the bubbles of vapor may readily form. Boiling liquids that suddenly vaporize can lurch out of vessels with almost explosive force and scatter hot liquid over innocent bystanders. This problem is especially troublesome in vacuum distillations and in systems containing precipitates.
For work at atmospheric pressure, you can overcome most problems by very good stirring. A magnetic stirrer will provide enough nuclei if the liquid is devoid of precipitates. If dense precipitates are present, use a paddle stirrer.
Boiling chips are either porous, solid lumps that contain trapped air that is released on heating, thus providing a stream of nuclei for even boiling, or they have sharp edges such as carborundum chips. It is important to realize that boiling chips have several limitations:
In a vacuum distillation, the easiest way for providing nuclei is to stir the solution in a magnetic stirrer. Caution: Never add a boiling chip or initiate stirring when the liquid is hot. An eruption will result.
6.5 Stoppers—Swelling in Solvents
Rubber stoppers will swell when exposed to the vapors of many organic solvents. In some cases, the action is extreme, and the stopper will be forced out of the flask. The action can occur at room temperature or when hot. If it is important that samples are tightly sealed, use glass or some other kind of stopper. Consult your instructor.
6.6 Heating Closed Systems—Unintentional and Intentional
The unintentional assembly of a sealed system (no opportunity for the escape of expanding gases upon heating) and consequent rise in internal pressure is much more common than one usually realizes. There are special adapters for use in distillations (the most common example of this danger) which allow the escape of air through side-arm S.
Another unintentional construction of a sealed system is by the use of drying tubes (such as those on condensers) that contain drying agent that has become clogged (because of overexposure to atmospheric moisture, for example). Store anhydrous calcium chloride, commonly used in drying tubes, in a sealed condition. For example, you can use a rubber stopper in the wide end, and with rubber tubing and a screw clamp over the narrow end to prevent it from becoming fused into a block.
Before using any of these different types of equipment, the student must consult a faculty member, preferably the faculty member in charge of the equipment.
You will use a specific construction for the successful resistance to the mechanical deformations expected. In the case of high-pressure autoclaves, the equipment is impressively sturdy, with thick steel walls and special heads that must be screwed down in a definite sequence. Inspect the special seals frequently.
Since so many commonly used reagents and solvents are toxic in some way, it is not possible to avoid entirely the use of toxic materials. For this reason, students must become familiar with the safe techniques appropriate for the use of each substance.
The following is a rough classification according to the type of toxicity and hazard posed. Some substances are immediately lethal, while others are very slowly cumulative in their effects. These lists are not comprehensive, but include substances that you are most likely to encounter in your work.
Toxicity data alone are not sufficient to indicate the degree of hazard involved in the use of these reagents. For example, note that carbon monoxide is not as toxic as many of the compounds listed; however, it is actually more dangerous than many of the others since it is odorless, colorless, and tasteless, and there is no obvious indication of a build-up of its concentration to a lethal level. Phosgene is doubly dangerous, in that it is lethal at low levels, and it is not as irritating as substances such as chlorine or bromine. An individual exposed to phosgene may show no sign of its effects until 8–12 hours after exposure, at which time pulmonary edema (respiratory failure) may have become fatal. Hydrogen cyanide has a characteristic odor, but it is not as unpleasant as hydrogen sulfide, which is detectable by odor in very low concentrations. On the other hand, hydrogen sulfide produces fatigue in the olfactory sensory apparatus. A person who has been exposed to very high concentrations cannot detect the difference between sublethal and lethal levels.
Note that some of the common solvents, such as chloroform, carbon tetrachloride, and benzene, are not very toxic on immediate and one-time exposures; however, their toxicity is far more insidious. In the case of CCl4, two large exposures on consecutive days produce a cumulative effect on the liver (hepatotoxicity) and other organs that may not be fatal until hours after the second exposure. Benzene, which has been commonly used in large amounts as a solvent and reagent, is a known human carcinogen that causes aplastic anemia (bone marrow failure). Similarly, hydrazine and HMPA (hexamethylphosphoramide) have only recently been clearly identified as potent carcinogens. Therefore, treat all chemicals with caution as if they were potentially toxic.
Methyl bromide and dimethyl sulfate represent a class of compounds that become toxic as a result of their "methylating" effect in biological systems. You may not encounter other members of this class as frequently, such as CH3Cl, methyl chloride, and F-SO2-OCH3 methyl fluorosulfonate (magic methyl), but always recognize the danger of any structure that shares the ability to methylate susceptible functional groups.
Diazomethane, CH2N2, is not only a powerful methylating agent, but as noted above it is also extremely sensitive to spontaneous detonation. If you have an occasion to use CH2N2, consult an instructor before proceeding.
6.8 Vesicants, Sternutators, Lachrymators, Allergens, and Burning Agents
While many compounds with these properties are fatal if an overexposure occurs, in general, they are so unpleasant they provide ample warning of their presence. Just as with the more toxic reagents, use these in fume hoods and with suitable trapping arrangements. With many of these reagents it is not sufficient to aspirate their vapors into the sink, since they will escape and create a very unpleasant atmosphere for everyone in the building. Use trap systems that are appropriate for the compound with which you are working. These compounds produce particularly unpleasant reactions if carried from your hands to sensitive areas of the skin, such as mouth, nose, and eyes. Frequent hand washing is essential in order to avoid such contamination.
Based on the compounds listed below, you can begin to predict candidates for an adverse unpleasant reaction:
6.9 Stinkies (a.k.a unpleasantly odoriferous)
While many of the toxic and vesicant compounds also have unpleasant odors, many compounds that may not be particularly harmful may have odors that cause nausea, or at least create a very unpleasant work area. The spreading of such substances from your own work area into areas used by others is inconsiderate. You must avoid this by every means possible. Certainly, use a fume hood. Other preventative measures include:
Compounds of this type—almost all low molecular weight aliphatic acids up to about C10—include: