What is the pH of ascorbic acid

Experiments:
Experiment: The simultaneous redox and acid / base titration of ascorbic acid
Experiment: Potentiometric titration of ascorbic acid with potassium hexacyanoferrate (III)
Experiment: Potentiometric titration of ascorbic acid with silver nitrate


The less acidic or the more alkaline its solution becomes, the more unstable it is to oxygen. The pH value and redox potential of ascorbic acid are mutually dependent. This is because protons are involved in the redox equilibrium of ascorbic acid. If the protons are removed, the equilibrium shifts in the direction of oxidation.

The standard potential at pH = 7 and 25 C is (depending on the literature) E.0 = +0.06 V to +0.08 V.

The Nernst equation applies here in the following form:

We take apart the fraction in the logarithm.

With the well-known definition pH = - log [H+] becomes from the o. a. Equation:

As the pH value increases, the redox potential of ascorbic acid decreases. That also confirms our experience.

In order to demonstrate the mutual dependence of pH value and redox potential, the redox potential of the solution is measured simultaneously during the acid / base titration of ascorbic acid with sodium hydroxide solution (-> experiment).

Titration of ascorbic acid with sodium hydroxide solution with simultaneous redox measurement


With increasing pH, the reducing power of ascorbic acid increases because the dissociated ascorbic acid can more easily give off electrons from non-binding orbitals than a molecule whose electrons are in the molecular orbital between oxygen and hydrogen atoms hold up.
It is therefore not surprising that in acid / base titration the redox potential shows a course similar to that of the pH value.

The redox curve presented here must not be confused with the classic redox titration curves that are obtained when an oxidizing agent is added! In these, the number of protons in the ascorbic acid system remains constant and the concentration of electrons decreases. Here it is the other way round: the number of protons in the ascorbic acid system decreases, the number of electrons remains constant.
Since the protons released when oxidizing agents are added shift the pH value into acid (-> experiment), the redox potential of the solution naturally also changes continuously in redox titrations in accordance with the Nernst equation. However, this does not interfere with the recognition of the pronounced equivalence point. But if you want to be on the safe side, you work with acid / base buffers of sufficient capacity. However, this is not necessary for the usual redox titrations. We only do this once using the example of titration with hexacyanoferrate (III) (-> experiment).

The pH dependence of the redox potential of ascorbic acid also means that this acid can be used as a pH and redox buffer in the range around its pK values.


Further texts on the topic of ascorbic acid