When buying the antidote(AgCl) from a chemist you will test purity by:
(a) Exposing to UV light
(b) Exposing to IR light
(c) Giving it on patient
(d) Making sure they don't own cemetery
!!AgCl!! changes color from white to grey due to the decomposition of silver chloride into silver and cholorine by UV light.
!!2AgCl_(s)-> 2Ag_(s) + Cl_2!! (in presense of UV light)