Uses of L-ascorbic acid

Food additive

The traditional method for analyzing ascorbic acid content is titration with oxidizing agents,and various methods have been developed.The popular iodometric method uses iodine in the presence of a starch indicator.Iodine is reduced by ascorbic acid, and when all the ascorbic acid has reacted, iodine is in excess, forming a blue-black complex with the starch indicator.This indicates the endpoint of the titration.

• As an alternative, ascorbic acid can be treated with an excess of iodine and then back-titrated with sodium thiosulfate using starch as an indicator.

• This iodometric method has been modified to take advantage of the reaction of ascorbic acid with iodate and iodide in acid solution.Electrolysis of potassium iodide solution produces iodine,which reacts with ascorbic acid.The end of the process is determined by potentiometric titration in a manner similar to Karl Fischer titration.The amount of ascorbic acid can be calculated by Faraday's law.

• Another alternative is to use N-bromosuccinimide (NBS) as an oxidizing agent in the presence of potassium iodide and starch. NBS first oxidizes ascorbic acid; when the latter is depleted, NBS releases iodine from potassium iodide, which then forms a blue-black complex with starch.High-dose intravenous ascorbic acid is used as a chemotherapy and biological response modifier,is still in clinical trials.

• Sometimes used as a urine acidifier to enhance the bactericidal effect of methenamine.

Synthesis 

Industrial preparation

Eighty percent of the world's ascorbic acid supply is produced in China.Ascorbic acid is prepared industrially from glucose by a method based on the historic Reichstein process.In the first step of the five-step process, glucose is catalytically hydrogenated to sorbitol, which is then oxidized to sorbose by the microorganism Acetobacter suboxydans.Only one of the six hydroxyl groups is oxidized by this enzymatic reaction.From this point, there are two routes to choose from.Treatment of the product with acetone in the presence of an acid catalyst converts four of the remaining hydroxyl groups to acetals.Unprotected hydroxyl groups are oxidized to carboxylic acids by reaction with the catalytic oxidant TEMPO (regenerated by sodium hypochlorite-bleach solution).Historically, industrial preparation by the Reichstein process used potassium permanganate as the bleaching solution.The acid-catalyzed hydrolysis of this product has the dual function of removal of two acetal groups and ring-closing lactonization.This step produces ascorbic acid.Each of the five steps yielded greater than 90% yield.A more biotechnological process, first developed in China in the 1960s but further developed in the 1990s, bypassing the use of the acetone protecting group.A second genetically modified microbial species, such as Erwinia mutabilis, oxidizes sorbose to 2-ketogluconate (2-KGA), followed by dehydration for closed-loop lactonization.This method is used in the main process of the ascorbic acid industry in China, which supplies 80% of the world's ascorbic acid.Researchers in the US and China are racing to design a mutant that can undergo direct one-pot fermentation from glucose to 2-KGA, bypassing the need for a second fermentation and reduction of glucose to sorbitol.There is a type of D-ascorbic acid, which does not exist in nature, but can be synthesized artificially. Specifically, L-ascorbic acid is known to participate in many specific enzymatic reactions that require the correct enantiomer (L-ascorbic acid instead of D-ascorbic acid).The optical rotation of L-ascorbic acid .D = +23°.

Determination

The traditional method for analyzing ascorbic acid content is titration with oxidizing agents, and various methods have been developed.

The popular iodometric method uses iodine in the presence of a starch indicator. Iodine is reduced by ascorbic acid, and when all the ascorbic acid has reacted, iodine is in excess, forming a blue-black complex with the starch indicator. This indicates the endpoint of the titration.As an alternative, ascorbic acid can be treated with an excess of iodine and then back-titrated with sodium thiosulfate using starch as an indicator.This iodometric method has been modified to take advantage of the reaction of ascorbic acid with iodate and iodide in acid solution.Electrolysis of potassium iodide solution produces iodine, which reacts with ascorbic acid.The end of the process is determined by potentiometric titration in a manner similar to Karl Fischer titration.The amount of ascorbic acid can be calculated by Faraday's law.Another alternative is to use N-bromosuccinimide (NBS) as an oxidizing agent in the presence of potassium iodide and starch. NBS first oxidizes ascorbic acid; when the latter is depleted, NBS releases iodine from potassium iodide, which then forms a blue-black complex with starch.