Ascorbic acid is a sugar acid with antioxidant properties. Its appearance is white to light-yellow crystals or powder, and it is water-soluble. One form of ascorbic acid is commonly known as vitamin C. The name is derived from a- (meaning no) and scorbuticus (scurvy), the disease caused by a deficiency of vitamin C. In 1937 the Nobel Prize for chemistry was awarded to Walter Haworth for his work in determining the structure of ascorbic acid (shared with Paul Karrer, who received his award for work on vitamins), and the prize for Physiology or Medicine that year went to Albert Szent-Györgyi for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s, it was called hexuronic acid by some researchers.

Acidity

Ascorbic acid, the formula of which is C₆H₈O₆, behaves as a vinylogous carboxylic acid, wherein the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two resonance structures for the deprotonated form, differing in the position of the double bond.

Another way to look at ascorbic acid is to consider it as an enol. The deprotonated form is an enolate, which is usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.

Tautomerism

Ascorbic acid also interconverts into two unstable diketone tautomers by proton transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an enol reaction. There are two possible forms: 1,2-diketone and 1,3-diketone.

Determination

The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving titration with an oxidizing agent.

Iodine

Another method involves using iodine and a starch indicator, wherein iodine reacts with ascorbic acid, and, when all the ascorbic acid has reacted, the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end-point of the titration. As an alternative, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.

Iodate and iodine

The above method involving iodine requires making up and standardising the iodine solution. One way around this is to generate the iodine in the presence of the ascorbic acid by the reaction of iodate and iodide ion in acid solution.

N-Bromosuccinimide

A much-less-common oxidising agent is N-bromosuccinimide, (NBS). In this titration, the NBS oxidises the ascorbic acid (in the presence of potassium iodide and starch). When the NBS is in excess (i.e., the reaction is complete), the NBS liberates the iodine from the potassium iodide, which then forms the blue/black complex with starch, indicating the end-point of the titration.