Formic acid

The First Doctor and Ian Chesterton found a pool on Vortis with what the Doctor said held similar properties to formic acid. (TV: The Web Planet)

Behind the scenes
Formic acid was one of the several chemicals of which bottles were kept by Steven Moffat in the underground volcano lair beneath 221B Baker Street where he would retreat to write Doctor Who. They were found there by Christel Dee. (NOTVALID: Stealing Series 9 Scripts)