Osmium is the densest of all natural elements and certainly one of the rarest, with worldwide production of about 545 kilograms annually. It’s incredibly expensive stuff, and yet, look at all the varied uses! Osmium is used by itself or as an alloy for fingerprint detection and in fountain pen tips, pacemakers, light filaments, and jewelry. And it’s reacted with oxygen to form osmium tetroxide.
The word osmium actually comes from the Greek word “osme,” or odor, for the unique acrid odor given off by OsO4. Osmium tetroxide is incredibly toxic and has an OSHA permissible exposure limit (PEL) of 0.002 mg/m3. For comparison, elemental mercury vapor has a PEL of 0.1 mg/m3. Osmium tetroxide might even be considered a perfect component of a terrorist “dirty bomb,” but it’s simply too expensive to buy enough to make that practical.
A primary use for OsO4 is for tissue fixation in electron microscopy. Hundreds of hospitals use it in their clinical labs, and when the solution is spent, it needs to be disposed safely. My experience with OsO4 stems primarily from efforts to recycle the spent compound. Ironically, despite its obvious toxicity, OsO4 isn’t regulated as hazardous waste. While it is certainly toxic to humans, it breaks down fairly readily in the environment, (apparently) isn’t toxic to aquatic or marine life, and isn’t mobile enough to be considered a threat to drinking water. That means that theoretically one could take this non-regulated waste and sell it for a handsome sum to a refiner who could recover and resell the metal.
However, here is a lesson in making sure you know the hazardous waste regulations thoroughly! It turns out that one of the several buffers that labs use with OsO4 is cacodylic acid, which has the formula (CH3)2As(O)OH. Therein lies the rub. While EPA decided that osmium isn’t hazardous to the environment, arsenic is. So, any refiner recovering osmium from the spent solution also containing that particular buffer must have a full-blown EPA hazardous waste treatment, storage, and disposal facility permit! Use a different buffer and you’re fine.
It only took me stops at five hospitals to find out the popularity of the cacodylic acid buffer, thus ruining my plans for an early retirement.
For more on osmium, check out this essay on the metal from C&EN’s 80th anniversary special issue on the periodic table and this video from the Periodic Table of Videos: