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I understand that indicator solutions can change color depending on pH, but the following statements in the Phys.org article Super cheap earth element to advance new battery tech to the industry has me baffled.

Most of today's batteries are made up of rare lithium mined from the mountains of South America. If the world depletes this source, then battery production could stagnate.

Sodium is a very cheap and earth-abundant alternative to using lithium-ion batteries that is also known to turn purple and combust if exposed to water—even just water in the air.

Later it says:

They minimized sodium's exposure to the moisture that would make it combust by making the sodium powder in a glovebox filled with the gas argon. To make the powder, they used an ultrasound—the same tool used for monitoring the development a fetus — to melt sodium chunks into a milky purple liquid. The liquid then cooled into a powder, and was suspended in a hexane solution to evenly disperse the powder particles.

Phys.org news items are generally of very high quality, so I'm wondering if this is just a mistake (perhaps a reference to the use of a pH indicator) or if there is some reaction between sodium, water and perhaps air that involves a purple color.

The "purple" statement is echoed on Purdue University's web page Super cheap earth element to advance new battery tech to the industry as well.

So far I've only found the video Sodium gas... omg it's purple!.

The paper in question is Ultrasound-assisted synthesis of sodium powder as electrode additive to improve cycling performance of sodium-ion batteries. Tang, Kye and Pol, Journal of Power Sources, 396, pp 476-482, https://doi.org/10.1016/j.jpowsour.2018.06.067

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I found the paper in Researchgate.

I'm going to leave this tentative answer in place but hopefully I or someone else will be able to investigate further and post a better answer.

These are particles of molten sodium (>98C) dispersed in mineral oil. The linked references suggest that the ultrasonic dispersal technique can be used to make quantum dots, so if these particles are similar size, then the color could be some quantum mechanical effect for the conduction electrons in the liquid sodium nanoparticles.

Since that is a size-dependent QM effect, one would not see it in experiments on bulk sodium.

This doesn't completely rule out Rayleigh scattering effects, but since the color changes as the particles cool (and presumably solidify) then that suggests it's the properties of the particles themselves, rather than there dispersal in oil.

Sodium Powder Preparation: Sodium powder was prepared via ul- trasonic heating, melting, and subsequent fragmentation of solid sodium chunks in an organic solvent. A similar technique was utilized in previous studies involving Sn nanoparticles 32 and dispersed sodium 33. Ultrasonication was generated from a Sonics VCX500 probe equipped with a stepped microtip. In a typical synthesis, 15 mL of mineral oil was first degassed by ultrasonication for 15min inside a 150 mL cone-shape sonochemical reactions vessel; then, about 100 mg of fresh metallic sodium chunks were added into the oil. A continuous argon flow was fed to the vessel to maintain an air/moisture-free environment. The Na/oil mixture was then exposed to ultrasonic irradiation at 40% amplitude. Pulsed ultrasonic irradiation was applied in 4 cycles of 59s-sonication and 30s-rest intervals. The formation of sodium powder took place in two stages. At the first stage, solid Na chunks were melted into liquid sodium when the mixture temperature rose above the melting temperature of sodium (mp = 98 °C). The temperature rise is a result of ultrasound induced heating to the oil. At the second stage, the molten Na particles were dispersed into mineral oil via ultrasound to form a homogenous mixture with purple coloration. Once cooled, the solution color changed to grey. A simplified synthesis method is illustrated in Fig. 1a (A video of the sodium powder synthesis can be found in Movie S1 in the supporting information). The mixture was then washed with anhydrous hexane and centrifuged three times to obtain a dispersion of Na powder in clean hexane. All washing procedure except the centrifugation were completed inside an Ar-filled glovebox to minimize air exposure.

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