McGill Alert / Alerte de McGill

Updated: Thu, 07/18/2024 - 18:12

Gradual reopening continues on downtown campus. See Campus Public Safety website for details.

La réouverture graduelle du campus du centre-ville se poursuit. Complément d'information : Direction de la protection et de la prévention.

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Why Scary Lab Accidents Happen

No matter how simple and controllable a reaction seems on paper, when it’s carried out in real life many other factors come into consideration.

If a chemist has never been in a lab accident, he has been lucky. Of course, luck is more likely to come to those whose mentors have learned from bad experiences and to those who have taken preventive measures seriously, despite their anal nature. Chemical reactions create products with behaviors that differ from those of the ingredients. That’s what makes them intriguing, and it’s also what makes them potentially dangerous. No matter how simple and controllable a reaction seems on paper, when it’s carried out in real life, the exact conditions determine its rate. And when gases or acids acquire too much kinetic energy, no one wants eyes, lungs and flesh in their way.

As an adolescent I played with my chemicals more than my instructors did. Rarely did they carry out demonstrations while lecturing. Seldom did they deviate from the tight parameters of cookbook labs. So, I unconsciously associated accidents with amateurs or with large scale industrial processes. But after a freshman year of chemistry, I got my first summer job in the lab after a metallurgical company did not rehire a chemistry student previously involved in a serious analytical lab accident.

One of the most ineffective ways of teaching something is to present it as a rule without explaining why: “Add concentrated acid to water; never water to acid.” But using a small quantity to show how things can go wrong will get the message across.  Just do it on the side of a two-way fume hood while making sure that the glass is lowered on the side of observing students.  As the less dense water remains on top, the exothermic reaction on the surface brings the top of the mixture to a boil, and of course acidic solution splashes all over the place. The acid is quite strong because the reaction occurs so quickly that little dilution occurs.

A university student working as a replacement for vacationing technicians had been pressured into quickly preparing a standard solution. To save time, so he thought, he measured concentrated sulfuric acid and poured it into a glass jug. He added water to it, screwed the cap on and shook it vigorously from side to side, across his chest. The heat, steam and closed container combined to increase the pressure enough to rupture the glass, and most of the acid ended up on his body. He was in such pain that coworkers had a hard time dragging him to the emergency shower. He ended up with scarring, third degree burns. What happened was consistent with a keen observation by one of the authors (Jacques Vilain) of a book about reducing chemical mishaps:

Each accident is a unique, often bizarre, event, albeit with repetitive patterns but great ‘variability’ in terms of cause/consequences ratio.

About 15 years ago, I wanted to show my students how soft a metal sodium is, and of course I planned to subsequently demonstrate its classic, fiery reaction with water. Luckily I was wise enough to use the two-way fume hood setup. I cut the block, exposing its lustrous interior–the surface slowly oxidizes with time even when it’s kept under oil. But I had made the mistake of not taking tongs with me. I had actually used my pocket knife to pull it out of its bottle, and after cutting it, I poked my knife into one of the halves and placed the knife and sodium into a beaker of water.

Any time an exothermic reaction is carried out, the vessel should be checked for cracks. I only noticed the dark line in the glass after I dropped in the knife and sodium. At the time I was also unaware of the precautionary trick of placing sand at the bottom of a beaker. It helps prevent overheated pyrex from cracking. My last error was my worst one: sticking a knife into too large a piece of sodium is a bad idea because it causes the chunk to sink into water. Without the added weight, sodium, with a density of 0.97 g/cm3, would not have sunk and not all of its surface would have come into contact with water; the reaction would have been slower and more controlled. The hydrogen would have ignited, but the reaction would not have generated as much pressure as when sodium is submerged.

To complete the disaster, our technician had recently left methanol in an open beaker in the fume hood. There was an immense explosion heard across our small school. The observing students gaped in astonishment but were unharmed since the protective pane on their side could not be opened. The fume hood glass on my side did not break, but all the glassware within it did.  I had never totally lowered my window, and sodium hydroxide (NaOH) produced by the reaction along with bits of glass projected towards my belly. A couple of pieces actually got stuck in my shirt. When I unbuttoned it, I saw no blood but found NaOH reacting with my belly.

Enrico Uva is a retired chemistry teacher who still enjoys learning chemistry, other sciences and mathematics. 

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