Practical safety of storing potassium

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!

• @venture: Please undelete your answer. There is nothing wrong with it. – user467 Jan 9 '15 at 2:37
• You should have sufficient reputation to cast an undelete vote. If I remember correctly, it needs 3, mine is already on it. – Martin - マーチン Jan 9 '15 at 8:21
• I actually don't have enough reputation--need 4k. – user467 Jan 11 '15 at 19:28

3 Answers

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.

• Right, all of this is what I know. And yet, every demonstration I can document seems to only be storing under oil, whereas I am doing that and using inert gas and then sealing the bottle in a vacuum bag (poor man's glove box). So it either suggests that poorly stored $\ce{K}$ is a bit more resistant to manhandling that we might think, or that lots of what we see on YouTube is rather freshly prepared. The source I bought my $\ce{K}$ from said they had stored for an extended period with no ill effects, which suggests 1). But what you say reads true. – user467 Jan 9 '15 at 2:43
• I would suspect that the risk increases with the amount of superoxide that forms on the metal surface and at some point reaches a critical level that, if ignited, imparts sufficient energy to start the metallic substrate to ignite. Cutting could be a trigger, and I've read accounts of self ignition. I have personally worked with $K$ but have never experienced autoignition or explosions. – docscience Jan 9 '15 at 5:00

The following excerpt regarding the management of alkali metals is from:

Management of time-sensitive chemicals (I): Misconceptions leading to incidents by Jim Bailey, David Blair, Lydia Boada-Clista, Dan Marsick, David Quigley, Fred Simmons and Helena Whyte

"Slow reactions of alkali metals and their alloys with oxygen to form oxides and superoxides have been well documented*. Even when these metals are stored under mineral oil, oxygen can dissolve in the mineral oil and react. Potassium, under these circumstances, forms yellow or orange coating that can explode or catch fire upon cutting. NaK, a eutectic mixture of potassium and sodium, can undergo a similar reaction. Lithium stored under dry nitrogen can react with the nitrogen to form a nitride. Formation of the nitride is autocatalytic and can eventually lead to autoignition of the lithium. Fires of alkali metals are extremely dangerous and difficult to extinguish."

• Management of time-sensitive chemicals (II): Their identification, chemistry and management,Jim Bailey,David Blair,Lydia Boada-Clista,Dan Marsick,David Quigley,Fred Simmons,Helena Whyte: Chemical Health and Safety, Volume 11, Issue 6, November–December 2004, Pages 17–24

I've come to the realization that if you plan to keep chemicals around you need to know all the risks and provide effective management including a plan for periodic inspection.

I have used potassium metal in glass containers stored in mineral oil for 30 years. You are correct that there is residual oxygen in mineral oil that will "rust (oxidize)" the metal, but I have never seen the yellow/orange superoxide layer that some have described. If you need it unreacted, then it needs to be stored so as to not come in contact with oxygen. If you are using it to remove residual oxygen from a glove box, then you probably don't want to even open it until you get ready to use it and then open it in the glove box. If you are using it to demonstrate periodic reactivity, then storing it in mineral oil in a sealed glass container has worked well for me over hundreds of demos for three decades with no issues, even with potassium that has been stored this way for three or more years.