Editing Chemistry of ascorbic acid (section)

===Determination===
The traditional way to analyze the ascorbic acid content is by [[titration]] with an [[oxidizing agent]], and several procedures have been developed.

The popular [[iodometry]] approach uses [[iodine]] in the presence of a [[starch indicator]]. Iodine is reduced by ascorbic acid, and when all the ascorbic acid has reacted, the iodine is 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 treated with iodine in excess, followed by back titration with sodium thiosulfate using starch as an indicator.<ref>{{cite journal|url=http://www.saps.org.uk/attachments/article/556/simple_test_for_vitamin_c.pdf |title=A Simple Test for Vitamin C |journal=School Science Review |year=2002 |volume=83 |issue=305 |page=131 |archive-url=https://web.archive.org/web/20160704170133/http://www.saps.org.uk/attachments/article/556/simple_test_for_vitamin_c.pdf |archive-date=July 4, 2016}}</ref>

This iodometric method has been revised to exploit the reaction of ascorbic acid with [[iodate]] and [[iodide]] in [[acid]] solution. Electrolyzing the potassium iodide solution produces iodine, which reacts with ascorbic acid. The end of the process is determined by [[potentiometric titration]] like [[Karl Fischer titration]]. The amount of ascorbic acid can be calculated by [[Faraday's laws of electrolysis|Faraday's law]].

Another alternative uses [[N-Bromosuccinimide|''N''-bromosuccinimide]] (NBS) as the oxidizing agent in the presence of [[potassium iodide]] and starch. The NBS first oxidizes the ascorbic acid; when the latter is exhausted, the NBS liberates the iodine from the potassium iodide, which then forms the blue-black complex with starch.