Every vitamin C serum on the shelf leads with its concentration. Ten percent. Fifteen. Twenty. The number is large, it is easy to print, and it invites you to read the category as a straight line where more is better.
It is not a straight line. Above roughly twenty percent, absorption plateaus and irritation does not, so the highest number on the shelf is frequently the worst product on it. But that is the smaller problem.
The larger one is that L-ascorbic acid is chemically unstable, and a concentration figure describes the day it was blended, not the day you use it.
L-ascorbic acid oxidises on contact with light, heat and air. As it oxidises it converts to dehydroascorbic acid and then further, and the end products are not merely inert. They turn the serum amber, then brown, and they can contribute to the free radical load the serum was purchased to reduce. A twenty percent serum that has gone brown is not a weak antioxidant. It has changed sides.
So the question is not how much is in there. It is how much is still in there, and how long it will stay.
Two ingredients answer that, and both appear in far smaller print. Tocopherol, which is vitamin E, is fat-soluble where ascorbic acid is water-soluble, so between them they cover both compartments of the skin, and vitamin E regenerates oxidised vitamin C back to its active form rather than letting it degrade one way. Ferulic acid stabilises the pair and lowers the formula pH into the narrow band where ascorbic acid actually penetrates.
The combination has been studied for two decades. Fifteen percent L-ascorbic acid with one percent tocopherol and half a percent ferulic acid is not a marketing ratio. It is the arrangement the stability data kept pointing at, and it roughly doubles the photoprotection either antioxidant delivers alone.
The Cygnus Vitamin C and E Serum is formulated to that ratio, because a clear bottle on a sunlit shelf undoes the chemistry regardless of what the label claims.
Practical version. Read past the headline number to the two beside it. Store it cool and dark, use it in the morning under sunscreen, and if it has gone the colour of weak tea, replace it. The number on the front stopped being true some time ago.
