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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. [1] demonstrated that there is indeed a change in degradation based on pH.

References

  1. 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.
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    $\begingroup$ see pubs.acs.org/doi/abs/10.1021/jf9805404 $\endgroup$ – DavePhD Dec 11 '15 at 19:58
  • $\begingroup$ How does pH affect the electron densities of various reactive centres of the molecule? $\endgroup$ – Eashaan Godbole Jul 17 '19 at 14:15
  • $\begingroup$ If you draw the structures of the ascorbic acid and the ascorbate, and if you analyze, what is easier to oxidate, and what is the oxidation product, it should be obvious, why it is pH dependent, considering AA is a weak acid. $\endgroup$ – Poutnik Dec 14 '19 at 14:35
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Ascorbic Acid is a known antioxidant molecule and its stability has always been an issue especially for pharma industry which often makes use of AA in formulation. Now as much as pH is a culprit in AA degradation, its only one among other factors like solvent, exposure to light/uv, and the nature of solution (viscous/non-viscous). There are quite a lot of studies on stability of Ascorbic Acid like this one

I normally prepare Ascorbic Acid freshly for each experiment, but it would be an interesting experiment to check the stability of AA for an extended time period at different pH values.

Here is an interesting simulation demonstrating degradation of Ascorbic Acid.

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    $\begingroup$ Thank you for your answer, and welcome to the site! Are you sure your answer addresses the question's fundamental inquiry (How is AA degradation affected by pH)? $\endgroup$ – William R. Ebenezer Jul 17 '19 at 8:36
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A comment above cites a publication claiming a role for the hydrogen ion in the decomposition of Vitamin C.

I would agree especially if a low pH fosters a Fenton/Fenton-like chemistry. The latter could proceed in the presence of transition metals impurities, which can engage in Fenton chemistry liberating hydroxyl radicals, directly attacking Vitamin C and consuming H+. The chemistry is further promoted in the presence of light/UV (Photo-Fenton).

Also, a role for oxygen leading to reactive oxygen species (like H2O2 which can feed a Fenton reaction).

Dust particles can be rich in metal oxides and air exposure can also be a source of water vapor. So just frequent opening of a container may accelerate decomposition.

Also, elevated temperatures, in general, could impact redox reaction rates, so cold/dry storage may assist in limiting the impact of a reduction of pH on Vitamin C in the presence of metal impurities and exposure to air/water/dust and possible elevated temperatures.

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