Eldrad's acid

A specialised acid was developed by Eldrad that neutralised the molecular bond in silicon-based lifeforms.

When Eldrad was restored from her hand and returned to Kastria, she triggered a booby trap and a dart of the acid struck her. Knowing there to be no antidote, Eldrad had the Fourth Doctor and Sarah Jane Smith conduct her to the regenerator chamber where she returned to her "true" male form. (TV: The Hand of Fear)

The executioner Mulkris attempted to use a dart of the acid against Charlie Gibbs, who was under the influence of Eldrad through the Watcher. However, he turned the point back on her and she was killed. On Kastria, the Eldrad grown from the hand killed that from the Watcher using the acid. Through her genetic ring and the Doctor's TARDIS, Mulkris was restored and she killed the Eldrad restored from the hand with the same dart he used. (AUDIO: Eldrad Must Die!)