What oxidation actually does to vitamin C
Ascorbic acid, the active form of vitamin C in most serums, is chemically unstable. When it reacts with oxygen, it converts first into dehydroascorbic acid and then into diketogulonic acid, a compound with no brightening activity, no collagen-stimulating effect, and no antioxidant function. The serum still looks like a serum. It still smells faintly of citrus. But the ingredient doing the work is gone.
