I know that vitamin C degrades over time (as I understand through oxidation). However, I am having trouble understanding how changing the pH of the solution in which the vitamin C is affects the rate at which the vitamin C degrades.
Among others, Roig et al.  demonstrated that there is indeed a change in degradation based on pH.
- Roig, M. G.; Rivera, Z. S.; Kennedy, J. F. A Model Study on Rate of Degradation of L-Ascorbic Acid during Processing Using Home-Produced Juice Concentrates. International Journal of Food Sciences and Nutrition 1995, 46 (2), 107–115. https://doi.org/10/fgwc5c.