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Samarium’s unusual oxidation by hydrochloric acid

The Lanthanides (Ln elements, first f-block, Rare Earth elements) are a group of elements with strong chemical similarities but some unusual differences too. While the ‘natural’ oxidation state of most Ln elements is +3, some show +2 and +4 oxidation numbers.

In the case of Samarium (Z = 62) this results in colourful behaviour when the metal is dissolved in strong HCl, as shown in the video below (around 1 minute).

Like the other Ln elements, samarium is sufficiently electropositive to be oxidised to oxidation number +3 by hydronium ions (H3O+), the latter then being reduced to hydrogen and water.

Yet when Sm metal is dissolved in strong HCl that’s not what happens, or at least not at first. Instead and quite spectacularly the samarium is oxidised to the blood red Sm2+ ion (oxidation number +2). Only later into the dissolution process does this ion get further oxidised to Sm3+, pale yellow in colour.

And then there’s a small twist in the tale. I wanted to find out whether the reduction of Sm(III) to Sm(II) would be possible using magnesium or aluminium in quasi-neutral conditions. The Electrochemical Series tells that these reductions are thermodynamically favourable in both cases, with Mg expected to be the best reducing agent of the two.

As far as samarium goes the only material that contains it in my possession (well… not anymore!) was an old 7 g uncoated SmCo5 magnet, bought cheaply on eBay. That contains theoretically 36 w% of Sm and the separation of Sm and Co should have been a doddle. Not quite.

Without going into much detail, my laudable effort to extract a bit of Sm chloride or sulphate from the SmCo5 magnet was thwarted by strong contamination with iron. The iron contamination I could easily prove to have come from the material itself and not from any reagents used. Hence I obtained a very rusty mess, not worth disentangling for a few gram of low grade Sm compound.

Some high grade Sm metal is now finding its way to our premises as I write this. To be continued…

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