# What, if any, are the differences between radical scavenging and radical quenching?

Hindered amine light stabilizers (HALS), introduced in the 1970s, function mainly as free radical scavengers, although they also may act as quenchers or peroxide decomposers. (Polypropylene: The Definitive User's Guide and Databook)

Anthocyanins, as other flavonoids, have been shown to act as scavengers of various oxidizing species, that is, superoxide anion, hydroxyl radical, or peroxy radicals. They may act as quenchers of singlet oxygen or they may react with metal ions and thereby indirectly decrease hydroxyl radical production.(Comprehensive Natural Products II Volume 3, Chemistry of Flavonoid-Based Colors in Plants)

Sources like these (all emphasis mine own) have led me to question whether radical quenching and scavenging are indeed the same thing.

The article "Radical Scavenging and Singlet Oxygen Quenching Activity of Marine Carotenoid Fucoxanthin and Its Metabolites" appears to use different measurements for scavenging and quenching, the effective concentration for 50% scavenging (EC50) and quenching rate constants ($$k_\mathrm{Q}$$, $$\pu{×10^10 M−1 s−1}$$). While it is clear that one quantifies rate while the other measures concentration, it is unclear why both quantities couldn't be termed similarly.

From what I can see, it appears that "quenching" is generally more commonly used for singlet oxygen, an example being the second quoted passage. One other example would be: "Lidocaine: a hydroxyl radical scavenger and singlet oxygen quencher".

However, in a number of examples, antioxidants that react with singlet oxygen are also termed as scavengers, such as in: "Tocopherol as singlet oxygen scavenger in photosystem 2" and "Ascorbic acid as a scavenger of singlet oxygen".

Are "scavenger" and "quencher" used differently because of a significant difference in how singlet oxygen and "other" ROS (such as superoxide anions, hydroxyl radicals, or peroxy radicals) are neutralised, which makes one more suitable than the other in either case? Is termination of all radicals not fundamentally similar? If so, elaboration on the reaction mechanisms in both cases would be appreciated.

• I do comment because I am not sure, and in particular my answer comes from a semantic pov, and considering that there is not a quencher in my language. Scavenger emphasises the property of removal of something (in our case free radicals) while quencher emphasises that a process is put to a stop. A subtle nuance is that quenching should result in a stable enough specie, while generic scavenging can be even capturing a free radical and forming another still reactive one. – Alchimista Apr 24 at 7:09
• To clarify a clean use of the terms would be "adding a radical scavenger such as X quenches the reaction. .". – Alchimista Apr 24 at 7:19