Sunday, 6 February 2011

Lab Mishaps No. 3 - The Old Switcheroo

            Strangely, no-one did anything in the lab this week which one would class as either stupid or remotely dangerous. I was however regaled with a tale by a new postdoc we have working in the lab which he was witness to at his previous institution. At first I did not believe a word of it but he swears blind this is true.

            I’ll set the scene. Nearly every reaction a chemist does requires a work-up of some sort. This will involve washing your reaction mixture (usually) with water, acid or base, and saturated brine. This removes any inorganic crud from your reaction mixture and makes purification of your required product a bit easier. The final step is then adding solid magnesium sulfate to your solvent to remove any water which is let in your organic solvent. A simple filtration should then give a dry solution of your product (or, more commonly, mixture of products). MgSO4 is an innocuous white powder which reacts with very little whatsoever. To all intents and purposes, it is inert.

            On the other hand, should one want to remove an acidic proton from a compound, sodium hydride is a commonly used strong base. It’s a white or slightly grey powder which is pyrophoric and in no way innocuous whatsoever. Any hint of water anywhere and NaH will give off hydrogen gas, set on fire, and then ignite the hydrogen gas. As such, it is used under inert conditions and is often provided as a dispersion of the powder in inert oil to prevent unwanted fires. However, this leaves grease in your reaction and many people (myself included) prefer to use the 95% pure stuff.

            Now, on to the unbelievable piece of lab stupidity. Apparently, a particularly foolhardy PhD student at my colleague’s previous place of work used a lot of NaH in his work. It is usually sent out in 50 gram batches in a plastic bag encased in what looks like a paint can. It is really irritating to get a desired amount of the powder from the bottom of said bag without getting it everywhere. In my lab, it is dispensed into glass vials in 2 gram portions and kept in a desiccator. What this imbecile did was find a decent sized tub (which happened to contain MgSO4 in a previous like) and put his NaH in there. Now this is all well and good, but what he also did was fail to change the label on the tub.

            I had my head in my hands as the story reached this point. What followed was an unfortunate soul who wished to dry his flask full of wet, albeit highly flammable solvent. A few shouts and the use of a fire blanket later, everyone fortunately came out unscathed, apart possibly from the original culprit. I’d have caved the guy’s skull in if it had happened in my lab!

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