Note: this was a post on my old blog site, and never made the migration over to the current "In the Pipeline". I was reminded of it this morning, and thought I'd bring it more out into the light.
There are reports (updated here - DBL) that Mars may have hexavalent chromium compounds in its surface dust, which is already being brought up as a concern for future human exploration. I agree with comments I've seen that this is putting the cart in front of the horse a bit, but it also means that I probably wouldn't be a good candidate for the expedition. I've already had my lifetime's exposure to Cr(VI).
Back in grad school, I had an undergraduate assistant one summer, a guy who was pretty green. I'll refer to him by an altered form of his nickname, henceforth as Toxic Jim. I shouldn't be too hard on him, I guess: I was a summer undergrad in my time, too, and I wasn't a lot of help to anyone, either. But TJ did manage to furnish me with some of my more vivid lab stories in his brief time in my fume hood.
One morning I showed him how to make PCC. That's pyridinium chlorochromate for the non-organic chemists out there, an oxidizing agent that doesn't seem to be used as much as it was 15 or 20 years ago. Even in '85, you could buy it, but the freshly-made stuff was often better. It certainly looked nicer. Like all the Cr(VI) salts, it has a vivid color, in this case a flaming orange. I shouldn't say "flaming;" that's getting ahead of the story. . .
It's not hard to make. You take chromium trioxide, a vicious oxidant in itself which comes as clumpy fine purple crystals, and dissolve it in 6N hydrochloric acid. That's an easy solution to whip up, since it's just concentrated HCl out of the jug cut 1:1 with water. I had Toxic Jim do all this - weighing out the chromium compound, making the HCl. During that part I couldn't resist quoting the ancient adage, which works well in the East Arkansas accent of my youth: "Do like you oughter, add acid to water." Most chemists either remember that one, or they remember the syrupy conc. acids splattering all over their arm when they did it (once!) the other way around.
We set up a three-neck flask with an overhead stirrer to run this in. That's just a motor mounted above the flask, turning a shaft with a paddle on the end of it. Works well for really thick mixtures, which this was supposed to turn into. As things turned out, it was even thicker than planned, for a brief exciting interlude.
In went the HCl, out of a big Erlenmeyer flask, and in went the chromium trioxide. Here's where the wheels began to come off. Instead of a vivid red-orange solution, the stuff got dark and began to thicken. I could tell it was getting hot, too, since you could see the clear wavery solvent vapors coming out of the open necks of the flask. And that was wrong, too - you don't get that so much with water vapor. It's the mark of organic solvent fumes, with their different density and refractive index.
And so it was. TJ had indeed grabbed the wrong Erlenmeyer. Not the one he'd just mixed up the HCl in, but one from another part of the bench that contained ethyl acetate from a big chromatography run the night before. Ethyl acetate is a pretty poor substitute for hydrochloric acid, most of the time, when you stop to think about it.
Then the overhead stirrer began to bog down, which takes a mighty thick mixture to achieve. I hadn't added up what had happened at this point, but I knew that things were going wrong in all directions at once. I pulled the glass hood sash down some more, saying "I think you better stand back -" WHOOOOMPH!
And there it went! The whole reaction went up in a big fireball, which filled a good part of the hood and came roaring out of the gap in the front sash. I felt the heat roll over me, yelled something incoherent, and bolted for the safety shower. I didn't have to run up Toxic Jim's back, either: he was making for the door in championship time. Pulling the chain of the shower dumped a hundred gallons of ice water on me immediately, not that I needed any more waking up.
When I opened my eyes and took inventory, things weren't as bad as I thought. Limbs and appendages all present, head and facial hair still attached - though lightly singed and frizzed - skin not even sunburnt, although it (along with my lab coat) was generously splattered with green. That was what remained of the chromium trioxide. It was now the Cr(III) oxide, having given up three oxidation levels by turning the ethyl acetate into carbon dioxide, most likely. There were a few orange-brown spots of the Cr(VI) stuff, but those were mostly confined to the front of the lab coat, in a vivid line that showed where the hood sash had gotten pulled down to.
My hood wasn't looking its best. There was smoke hanging in the air, although that was getting pulled out. There was a huge stain of the green and brown chromium mixture all over the inside, thickest in the directions of the three open necks of the flask. Which was still intact - if I'd been foolish enough to set this up in a closed system, the whole thing would have gone up as Pyrex shrapnel. Even the ceiling had a line of gunk on it, from the thin gap in the hood sash assembly.
While I was taking this in, wondering what the hell had gone wrong, and wondering what I could possibly do to TJ that was worse than what he'd just gone through, the emergency crews arrived. It was a Saturday morning, but Bob across the hall saw the explosion and immediately dialed 911. In came the fire crews, trying to talk through their breathing apparatus: "Mumph heff deff umphh cafulteff. . " "What?" "We hear there's a casualty up here"
I put my hands on my hips, and gave them the full effect of my green spots, frizzed hair, and soaking wet lab coat: "That would be me."