Trisodium Phosphate: Phosphoric Acid, Soda, then Caustic Soda

Trisodium phosphate is the strongly alkaline sodium phosphate used to throw down cobalt violet. Thorpe (1913): soda added to phosphoric acid until alkaline gives disodium phosphate; excess caustic soda turns that into the trisodium salt, which crystallises in six-sided prisms. Caustic soda burns skin: gloves and goggles. The picture: trisodium phosphate (Wikimedia Commons).