Question: why is dpph a stable free radical?
Answer: The molecule of 1,1-diphenyl-2-picryl- hydrazyl (α,α-diphenyl-β-picrylhydrazyl; DPPH: 1) is characterised as a stable free radical by virtue of the delocalisation of the spare electron over the molecule as a whole, so that the molecules do not dimerise, as would be the case with most other free radicals.