Like all other animals, your body needs ascorbic acid (vitamin C) to make an important protein called collagen. Ascorbic acid is a reducing agent that can donate electrons to or "reduce" another substance during certain chemical reactions. Its ability to act as a reducing agent is measured in terms of its oxidation-reduction, or "redox," potential.
You can find a picture of the structure of ascorbic acid at the first link under the Resources section. Basically, it has a five-membered ring containing one oxygen and four carbons, with four substituents attached to this ring: a double-bonded oxygen (a ketone), two -OH (or hydroxyl groups) connected to a pair of double-bonded carbons and a two-carbon substituent with two hydroxyl groups attached to it. There are two stereocenters (i.e., asymmetric carbon atoms) in this molecule; the one in the two-carbon substituent has the S-configuration, while the one in the ring has the R-configuration.
The oxidised form of ascorbic acid is dehydroascorbate. In this form, the two hydroyl groups attached to the five-membered ring have been oxidised to ketones (double-bonded oxygens), and the double bond that hitherto linked the two carbons to which they were attached has been broken. The two electrons lost by ascorbic acid during this process have been used to reduce the iron atom found in the enzyme prolyl 4-hydroxylase. This reaction is very important for collagen production.
Redox potential measures a compound's tendency to give up electrons in a chemical reaction. All redox potentials are voltages measured by comparison with a standard hydrogen electrode, which provides a kind of benchmark. The redox potential for ascorbic acid at pH 7 is 0.06 volts. Ascorbic acid has the ability to reduce species with a higher or more positive redox potential, while it has the ability to oxidise species with a lower or more negative redox potential. Just because a species has a higher or lower redox potential does not mean it will necessarily react with ascorbic acid, of course, but you can nonetheless use the redox potential to make predictions about reactions that occur.
Free Energy Changes
The redox potential of ascorbic acid is also useful for determining the change in free energy (the maximum amount of energy available for non-mechanical work) associated with a redox reaction. If you take the redox potential of ascorbic acid and that of another species it oxidises or reduces and find the difference, you can use that number to calculate the free-energy change. The equation is as follows:
free energy change = - n F E,
where E is the difference in redox potentials, F is 96.5 kJ/Volts moles and n is the number of electrons lost/gained during the reaction.