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