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I have an element collection and noticed that the lanthanide (rare earth) element samples seem to be reacting with the mineral oil I've used to help prevent oxidation.

When I originally placed the samples I researched the lanthanides and found that the lower atomic numbers indeed tend to oxidize in air. Even though I've stored them in jars, I also filled the jars with mineral oil, similar to how I've stored the alkali metals. And at least for the alkali metals, the mineral oil has served as a good barrier to keep out oxygen.

So why would the lanthanides, which are a lot less reactive to oxidation, corrode so quickly? Is there another, perhaps organic reaction going on, between the lanthanides and the mineral oil?

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  • $\begingroup$ Someone ... posted another answer to this question, and I voted it up. Today I caught a glimpse of a message I believe from that someone: "Why did you delete my post ...?" Now I can find no trace, no evidence of the post or the comment. I assure you - I did not delete the post, and it's a mystery to me who would/could delete it. $\endgroup$
    – docscience
    Commented Dec 19, 2017 at 1:30

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David Hamric did a four year Rare-Earth Metal Long Term Air Exposure Test

and has a wonderful set of time-lapsed photographs on his site.

Europium is the most reactive to oxidation.

Several sources, such as Avalon Advanced Materials: Europium make statements like:

Samples of the metal, even when coated with a protective layer of mineral oil, are rarely shiny and it readily ignites in air above 150°C.

So rather than reacting with the mineral oil, I would say that europium and possibly others are reacting with oxygen despite being covered with the mineral oil.

THE SOLUBILITY OF O2, CO, AND N2 IN MINERAL OIL AND THE TRANSFER OF CARBON DIOXIDE FROM OIL TO AIR. The Journal of Biological Chemistry (1927) vol. 72, pages 545-548 gives quantitative values for the solubility of oxygen in mineral oil, but I can't understand what units are being used. It says solubility is 0.134 +/- 0.004 at 24 degrees C. I think this means 0.134 volumes of gas phase oxygen at standard temperature and pressure per volume of oil. Whatever the units are, it does show that significant oxygen can be present in mineral oil.

Also, I'll quote your own answer to another question:

Even when these [alkali] metals are stored under mineral oil, oxygen can dissolve in the mineral oil and react.

Original Source: Management of time-sensitive chemicals (II): Their identification, chemistry and management

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    $\begingroup$ Your answer - almost complete. It's clear the lanthanides indeed oxidize. I knew that. It's a fact that mineral oil is not a perfect barrier to O2; I knew that. What I don't understand is why given that for example potassium and europium, each under mineral oil, the europium appears to oxidize at a much greater rate. I've not run the experiment, but expect in open air, the opposite would be true. Is it that the potassium forms a somewhat protective skin, but europium does not? $\endgroup$
    – docscience
    Commented May 15, 2016 at 15:37
  • $\begingroup$ @docscience Sorry if my answer wasn't helpful. Because the question said "reacting with the mineral oil" it made me think you believed the metals were reacting with the mineral oil rather than oxygen. Maybe the oxides that form on the europium are more oil-soluble or more easily penetrated by the oil than those of potassium. $\endgroup$
    – DavePhD
    Commented May 16, 2016 at 14:30

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