CR Scientific: Articles: Chemical Accidents

Chemical Safety, Part II:
Chemistry Accidents

     Practically all chemists (this writer included) have their share of accident 'horror stories' and 'near misses' to share with anyone interested. Everyone makes occasional mistakes in the laboratory... it's just a question of seriousness and scale. Chemistry-- like walking down stairs, woodworking, or driving a car-- has its set of inherent risks. A person could spill a beaker of HF on his leg and die of fluoride poisoning, or he could fall down the stairs and break his neck. Few things on this earth are entirely safe.
     Sometimes things happen that no one could reasonably have foreseen, though far more often they happen because the experimenter took a shortcut or forgot to do some seemingly minor thing. However, proper safety gear and lab hygiene can mimimize the damage if something goes awry. It's not that hard to understand: if you work with HF, you wear a full suit of protective gear; if you work with a power saw, you are extra-careful when that blade is spinning; when you travel up and down stairs, you don't run in your stocking feet.

This section is by no means a substitute for a standard safety text such as Prudent Practices in the Laboratory, but it's a start to get the reader thinking of safety. Let's consider some common accident types in the laboratory, then:

1. Hot or Boiling liquids: Even water can cause severe injury when it's hot (don't underestimate this! Burns stay painful for a long time.)   Many chemical solutions, such as acids and bases, are much worse.

This writer once made the mistake of adding a boiling chip to an already-hot liquid. (Even back then, he knew better in theory, but fatigue brings carelessness.) The liquid instantly boiled over. Guess what the liquid happened to be? It was strong hydrochloric acid. Fortunately, no bystanders were present, and the author was wearing a full face shield, an apron, a lab coat, and neoprene gloves. (There was also a sink and an eye wash nearby in case these didn't stop the acid).

The vessel, however, sat in a mineral oil bath. When the acid boiled over, it landed in the hot mineral oil.

Without a fume hood, this would have resulted in acid / mineral oil aerosol and smoke filling the entire place. As it was, there was a corrosive mess to clean up.

So we arrive at the point: Don't add anything to an already-hot liquid. Keep an eye on the temperature. Don't use an open, liquid heating bath if there's a likelihood of a boil-over or a spill, and avoid using reactive materials as bath media. The best kind of 'bath' (in that it's potentially the least damaging) is a sand bath; just remember it lacks the upper limit to temperature inherent in a water bath or oil bath.

Where possible, anchor all ringstands and support bases to the lab bench with bolts or C-clamps.

2. Fires & Fire Hazards (see also, "Refrigerators", below).

Heating flammable liquids with an open flame is not smart. Heating them with an open flame and in an open container is really not smart.

Get in the habit of putting the cap back on the container of flammable liquid as soon as you're done dispensing the necessary amount of liquid. Then, put the container back where it belongs. Don't leave it sitting on the bench.

There have been plenty of accidents related to someone's leaving the cap off a bottle of acetone, ether, etc. Combustible vapors flow invisibly from the container, sometimes reaching ignition sources many feet away. The flame travels all the way back into the bottle of liquid. Sometimes the source of ignition is a spark caused from static electricity.

Don't forget about spontaneous combustion. Do some reading and familiarize yourself with what not to mix. Be sure those rags you're throwing away aren't full of something that'll burst into flames at some later time. Linseed oil with Japan drier is a good example.

3. Acids and Bases: Concentrated acids and concentrated alkalis will each react with water to produce a great deal of heat. Some of the water will vaporize, carrying with it some acid or base. It may spatter as actual droplets. This can blind you permanently. All it takes is a tiny drop.

Concentrated sulfuric acid in particular is prone to sudden boiling on contact with water, as it has a very high heat of solution.

Strong acids and bases mixed together will produce even more heat and danger of spattering. Do not store strong acids and bases near each other. Do not keep both of them on the lab bench at once.

Hot acids and hot alkali solutions are VERY dangerous. This writer once accidentally knocked over a glass setup containing hot, concentrated sulfuric acid. The glass broke, because glassware is quite fragile when it's at high temperatures¹. Where the acid landed, it made a six-inch, charred circle... instantly. The acid would have immediately burned through clothing and turned flesh into a charred mass if it had gotten on there, but fortunately it missed... and fortunately there were no papers or other combustible materials nearby. The author had on a rubberized apron, a face shield, and neoprene gloves, but these do not cover the entire body.

Concentrated, hot acids or alkalis must be treated with the utmost care.

4. Glass Tubing, Glass Rods, Capillaries, and Thermometers.... These are made of glass. Glass breaks. When the glass is long and thin, it can go through a finger, a hand, etc.

When trying to put a glass tube, thermometer, or other glass object through a stopper, lubricate it with glycerin or soapy water. DO NOT try to force it. It can break and jab into your hand. A shard of glass can go in your eye. Wear thick glove and goggles to prevent these.

These items, as well as test tubes, also roll off lab benches all too often. Do not place them on their sides and expect them to stay put. Cylinders roll.

5. Thermometers in particular: A common mistake is to use a thermometer with too low a maximum rating for the anticipated temperature. A thermometer that has a maximum temperature of, say, 100 degrees C will burst if it's heated much above the boiling temp of water. With a mercury thermometer there is an extra danger: highly toxic Hg vapors will contaminate nearby surroundings (and people).

Many novices make the mistake of overheating thermometers.

6. Compressed Air: It is not a good practice to dry glassware with compressed air. People have become blinded, maimed, and even killed from using compressed air improperly. It's not just in the laboratory that this happens. It has happened in mechanics' garages and other places too. High p.s.i. can kill.

Let glassware air-dry after the final rinse with distilled water; avoid using compressed air.

The disposable "dusters", however, are acceptable if everyone nearby is wearing safety goggles. These dusters don't make enough force to propel large pieces of glass through flesh (as in the infamous "burette accident" that's said to have happened at a college some years ago), but they can still cause eye injury. The most likely cause there is from small particles (chemicals, sand, grit, etc.) blown into the eyes.

7. Refrigerators:

Let's face it, even professional labs sometimes use domestic refrigerators for storing reagents and temporary solutions. The practice probably won't change as long as explosion-proof refrigerators are so expensive. That said, DON'T store flammable liquids or highly reactive materials in a domestic refrigerator... don't store flammable liquids even in the same room as a domestic refrigerator.

Every chemist seems to know of an incident involving ether and some bad wiring, especially in connection with a refrigerator. This is not an urban legend. In fact, it's happened many times in many different labs. It goes like this: Bottles of flammable solvent were kept in a domestic refrigerator, or in some cases just near one. Fumes built up, because that's what fumes do. The fumes reached sparks made by a thermostat or a motor, because thermostats and motors in domestic refrigerators tend to make sparks. Guess what happened? In all the incidents this author knows of, the lab was destroyed by the resulting fire or explosion.

A variant on the scenario is equally dangerous. Some reagents are not stable or convenient to handle at room temperature... so where better to store them than in a refrigerator or freezer? The problem is, what happens when the power fails or the motor quits some night? The temperature rises. Said compounds suddenly warm up. Vapor pressures rise. Lids pop. Cracks appear. Reactions occur. Other materials in the fridge get in on the act. By the time someone comes in the next morning, the building is gone.

Keep track of what's going in that refrigerator. Don't put flammable solvents in the refrigerator unless the manufacturer specifically made it to be explosion proof.

8. Unforeseen Reactions / Runaways: There are plenty of materials, even common ones, that can react in [un]favorable circumstances to produce heat, fire, toxic gases, and even explosions.

With regard to even the most simple and common materials, the author once tried to fuse a mineral sample of finely-crushed pyrite and hematite / goethite. In hindsight it might be easy to point out the problem, but at the time it really didn't seem out of the ordinary.

The mixture exploded into a cloud of sulfur dioxide and dust particles² when heated for perhaps twenty or thirty seconds. Fortunately there was only a small amount of material involved; imagine if this had been an industrial-scale reaction involving several hundred kilograms.

One common, unwanted (but entirely foreseeable) reaction involves sulfides and acids. Metal sulfides in acid generally yield H₂S. This gas is toxic and can cause death if there's enough of it. The lethal does does not smell any worse than the merely annoying dose. It's one thing if you're preparing H₂S on purpose and are equipped to handle it, but 'surprise' generations of toxic gases are never good.

Even highly-experienced professional chemists sometimes have accidents with unforeseen or runaway reactions.

It's also easy to forget the 'little' things... like surface area. A solid chunk of a given chemical might produce a slow reaction, while the same chemical in finely-powdered form might cause the whole thing to boil over uncontrollably. It can also generate enough heat to cause reactions that might not even have happened had the temperature stayed lower. Some of these runaways are deadly.

Use the library to research as much as possible before commencing an unfamiliar reaction. Stay alert during the procedure. Ask yourself while proceeding: "Is this the right step in the right order? Did I forget anything?"

Closely tied to #8 is...
9. Nasty Stuff.

We'll use the term "nasty stuff" as a catch-all term for things like hydrazine, mercuric salts, HF, chromyl chloride, nickel carbonyl, NCl₃, organic peroxides, etc.

Don't make nasty stuff when there's no good reason.

Don't make nasty stuff when there's a less-nasty substitute that will do the same job with less danger.

Learn how to avoid making nasty stuff during whatever reaction you're conducting.

Don't toy with alkali metals. Though a pinhead-sized bit of potassium in a beaker of water can make a dramatic demonstration, too large a chunk can make a horrific disaster.
Don't try to melt alkali salts and make the metals by electrolysis unless you know how to handle the products without getting hurt.
Don't use unregulated current for electrolysis.
Don't use wall current for electrolysis.
Don't try to distill ether³.
Don't keep ether around that lacks peroxide inhibitors.

Some syntheses use routes that, if you do them incorrectly, can make extremely toxic or unstable by-products. Avoid these syntheses unless absolutely necessary.

Don't keep or use water-reactive substances (TiCl₄, AlBr₃, P₂O₅, Na, K, etc) without a reliable, moisture-free way to store them. Some of these will become dangerously hot or emit toxic fumes (chlorine, for example) simply from exposure to moist air.

The same precaution goes double for compounds that react with air (white phosphorus, for example).

Don't keep or use hydrofluoric acid (HF) unless specifically needed. Of all the common lab reagents, HF is probably closest to being Death in a Bottle. Non-fatal encounters with it are also extremely painful.
Do not mix fluorides with sulfuric acid unless prepared to handle the HF that will be generated.

Don't store chromium trioxide (chromic acid, chromic anhydride) in the same place as alcohols or other flammable, oxidizable liquids.

If something really hazardous is necessary for a particular experiment, handle it in the smallest amount that's practical. Bigger containers make a bigger mess when they spill...

For more accounts of chemical accidents, see http://www.crhf.org.uk/index.html. Some of them unfortunately had tragic endings. The point, obviously, is not to harp on the misfortunes of others, but to learn from such stories so they won't be repeated.

Notes:

¹ This is especially so when the heat is distributed unevenly, as it nearly always will be to some extent. There develops strain within the glass between the hotter and the cooler regions, and it doesn't take much force to "relieve" the strain... by breaking. There is no glass this author knows of which has zero coefficent of thermal expansion, and therefore no glass is immune to this effect. Back to article

² ...and probably SO₃ / sulfuric acid as well. Fe₂O₃ is an oxidizer; it just takes more energy to release that oxygen than it would for KNO₃ or some more familiar oxidizer. Pyrite when heated can sometimes become pyrophoric, even by itself. A "pyrophoric" material is something that ignites on contact with oxygen in the air. Pyrite has produced some very serious industrial accidents (see http://www.cheresources.com/ironfires.shtml).

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³ In any case where ether must be distilled, doing it in an argon atmosphere is probably best. The distilled ether should promptly be put into a bottle having minimal air space and a tight-fitting cap.

Di-isopropyl ether is the most prone to forming explosive peroxides, but diethyl and many other types can also do it.

Many lab workers have died or received grievous injuries as a result of peroxidized ethers. Even opening a bottle can set off ether peroxides. They are very powerful; it doesn't require much to be dangerous. If there are any old bottles of ether sitting around in the lab that don't contain peroxide inhibitors, don't move them. Just look at them. If there are crystals in there, call the disposal team. Ether peroxides in small concentrations are soluble in the ether, but at higher concentrations they actually crystallize. It is these crystals that are the biggest source of danger, because they can explode from the slightest perturbation (such as moving or opening the bottle).

As noted in many sources, a piece of clean copper wire in the bottle can inhibit peroxide formation. Transition metals tend to catalyze peroxide breakdown. That said, it could be risky to introduce these metals into ether where peroxide is already suspected. The decomposition could conceivably happen all at once. Of course, if there are already crystals, it's too late anyway.

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Chemical Safety, Part II: Chemistry Accidents

Chemical Safety, Part II:
Chemistry Accidents