Potassium ($\ce{K}$) seems to be the transitional alkali metal in terms of safety. Lighter than $\ce{K}$ and we can store under oil with no worries. Heavier than $\ce{K}$ and only ampules will work. $\ce{K}$ is such that even under oil it can absorb enough oxygen to form superoxides that are contact explosives. But this is the theory: does anyone have any experience with extended proper storage of potassium?
I ask for two reasons. First, there are lots of online videos showing $\ce{K}$ under oil that the scientist then proceeds to cut, smash, and otherwise manhandle with no fear of detonation. Second, my $\ce{K}$ arrived tarnished, which I have stored under oil, and then vacuumed sealed in a bag, which has shown zero change in visual oxidation. So while I get that leaving an open jar of $\ce{K}$ under oil is problematic, I'm less clear on well stored $\ce{K}$.
So I'd appreciate any comments from practitioners that use $\ce{K}$ regularly. What situations are genuine dangers? Thanks!
Answer
I have never worked with potassium personally, but have experience with other alkali metals. You are spot on about potassium superoxide. I have direct knowledge of an explosion that occurred when potassium metal was stored for an extended period of time in mineral oil. I would therefore be extremely cautious if you intend to store it in this manner. Factors such as length of storage time, permeability of your container seals and storage temperature can all affect the amount of superoxide that forms. Based on the evidence I have seen, it is not safe to assume that there will be no transport of oxygen through a mineral oil barrier. The only way I would work with this metal is in an inert glovebox, period. That is my own preference, of course, and I always tend to err on the side of caution.
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