How do you seal your vials?

I am a staunch parafilmer, and an avid Twitterer – so much so, that I even made a meme about parafilming. Most chemistry labs have a few rolls of parafilm for wrapping around caps of bottles and vials. It’s great for keeping things tidy, clean and sealed, and I had always assumed it also kept air and moisture out. So imagine my surprise when a colleague saw my meme and told me that they believed parafilm did very little to protect reagents. Rather than argue about it, we discussed how to test our beliefs with an experiment. My resulting Twitter post of photos of vials with fading ketyl attracted a lot of attention. 

If you have not had the pleasure of using ketyl, it is a vividly coloured species that forms when benzophenone reacts with sodium, typically in ethereal solvents. The radical anion formed by a single electron reduction is deep blue, and the dianion is red, so these solutions often take on a deep purple colour. However, ketyl quickly reacts with oxygen and water to form colourless species, making it an excellent way to test if a solvent is dry. In many commercial ‘anhydrous’ solvents the colour of a single drop of ketyl solution added to 10ml of solvent will not persist, but we can usually get our ethereal solvents to stay blue with this test after storing them over molecular sieves. 

I’m a relative newcomer to worrying about water and oxygen in chemistry. My training was in total synthesis, and while water would ruin most reactions, the excesses of reagents and the large scales we worked with typically hid a lot of effects of water incursion. My undergraduate lab did have a beautiful tetrahydrofuran (THF) still, where dry THF is distilled on demand from sodium and benzophenone. Then in my postdoc I learned a new level of care (and a new definition of dryness), since we aspired to low loadings of water-sensitive metathesis catalysts.