There are a lot of vitamin C serums on the market. The most effective serums contain ascorbic acid. However, ascorbic acid is unstable in water -- it gradually oxidizes and becomes less effective. Because commercial serums require a long shelf-life they use potentially irritating preservatives, less effective ascorbic acid derivatives, or expensive patented processes to slow oxidation.
Supplying the serum as pre-measured powders is my solution to the oxidation problem. If stored properly, the freshly mixed serum is stable for a few months -- long enough to finish the bottle if used regularly.
How do you know if your serum has oxidized? It turns yellow or amber. You can slow oxidation by tightly capping the serum after use and storing it in a cool place away from bright light (or the refrigerator).
Mix it fresh and bypass the skincare-industrial process!