Vitamin C formulations with a pH greater than 3.5 are not able to penetrate skin. They are unstable and when exposed to light, become oxidized. The oxidation turns them into a new form of vitamin C called Dehydro Ascorbic Acid – and this turns the formulation yellow. By maintaining a pH of less than 3.5, the stability of Vitamin C is maintained. At this pH, the ionic charge is removed from the molecule and it is able to penetrate the outer layer (stratum corneum) of the skin. Learn more here: https://masterpieceskinrestoration.com/blog/c-e-ferulic/