The standard go-to solution is ferric chloride. It isn’t too tricky to ... solution was too slow and the cupric acid needs a heated tank, a way to oxygenate the solution, and strict pH controls.

For this, [Jarzębski] is using a small 1.6 Liter aquarium and four aquarium heaters for dunking 120mm x 120mm PCBs in the tank ... is using – ferric chloride, cupric chloride, or otherwise ...

Compounds with a phenol group will form a blue, violet, purple, green, or red-brown color upon addition of aqueous ferric chloride. This reaction can be used as a test for phenol groups. Mix several ...