Ascorbic acid, titration with iodine

Because of the clinical significance of vitamin C, it is essential to In-able to detect and quantify its presence in various biological materials. Ana lytical methods have been developed to determine the amount of ascorbic acid in foods and in biological fluids such as blood and urine. Ascorbic acid may be assayed by titration with iodine, reaction with 2,4-dinitrophenylhy-drazine, or titration with a redox indicator, 2,6-dichlorophenolindophenol (DCIP) in acid solution. The latter method will be used in this experiment because it is reasonably accurate, rapid, and convenient and can be applied to many different types of samples. 

When 14 was treated with diazomethane, the corresponding methyl ester was obtained.559 In an attempt to prepare 1 from this compound by a one-pot reduction-oxidation-cyclization procedure, the methyl ester was heated in pyridine in the presence of aluminum isopropox-ide. By titration with iodine, it was found that some of this ester had been converted into a material that might have been L-ascorbic acid. Unfortunately, the alleged product was not further characterized, and therefore this report must be considered with some scepticism. 

If ascorbic acid is present, this is stoichiometrically oxidized to dehydroascor-bic acid, which does not interfere with the determination, by titration with iodine solution. 

There is a very simple and quick method that can be used to detect the reducing power of sulphur dioxide, developed in the last century and often called the Ripper titration (Ough, 1988). In this method, sulphur dioxide is titrated against iodine or potassium iodate/potassium iodide solution in the presence of starch. When all the sulphur dioxide has been oxidised, a blue colour is produced by the reaction of free iodine with the starch. This is a very quick method but will give only an estimate of the level of sulphur dioxide as other reducing substances, such as ascorbic acid, will interfere consequently, this method is not particularly appropriate for juices with high ascorbate levels. 

Ascorbic acids reduce Fehling solution in the cold, and react with ferric chloride to produce the violet color typical of enolic compounds. They are readily oxidized reversibly to their primary oxidation products, 2,3-glycodiulosono-1,4-lactones (commonly known as dehydroascorbic acids 148, Scheme 16), by such mild oxidizing agents as aqueous iodine. Titration with such oxidants constitutes a quantitative method for distinguishing ascorbic acids from 2-glyculosonic acids. 

Assay for Ascorbic Acid Dissolve about 400 mg of sample, accurately weighed, in a mixture of 100 mL of water, recently boiled and cooled, and 25 mL of 2 N sulfuric acid. Titrate the solution immediately with 0.1 IV iodine, adding starch TS near the endpoint. Each milliliter of 0.1IV iodine is equivalent to 8.806 mg of C6H806. 

The nature of the active site in beta-amylase is not unambiguously known for enzymes from different sources. Early experiments on purified barley and on malted barley first indicated, from studies of the modification of the enzyme with nitrous acid and ketene, that free tyrosine and sulfhydryl groups are essential for activity, whereas free a-amino groups are not. The importance of the sulfhydryl groups was emphasized by the partial recovery of activity of the modified or oxidized enzyme (that is, treated with nitrous acid, iodine, phenyl mercuribenzoate, ferricyanide, and cupric ions) when it was treated with hydrogen sulfide or cysteine. Barley feeto-amylase (not highly purified) has been reported to contain 12—15 sulfhydryl groups per molecule by titration with p-chloromercuribenzoate, and the loss of free sulfhydryl content by treatment with L-ascorbic acid in the presence of cupric ions was found to be directly related to the loss of activity. 

Specifications for ascorbic acid, sodium ascorbate, calcium ascorbate, and ascorbyl palmitate are found in the UnitedSfates Pharmacopeia/NationalFormu/a (71) and the Food Chemicals Codex (72). The official assay for all four compounds is the iodimetric titration with 0.1 iodine solution and starch as the indicator. 

Typical examples of applications include the titration of ferrous ion with permanganate the titration of arsenic(III) with bromate the determination of ascorbic acid with iodine and the determination of organic compounds such as azo, nitro, and nitroso compounds and quinones with chromous ion. 

The chemical assay of ascorbic acid depends on the marked reducing properties of the vitamin and simple titration with standard iodine solution can be employed for the pure isolated compound. 1 ml 0 01 N... 

Prepare the indicator solution by making a 0 08 per cent solution of 2,6-dichlorophenolindophenol in copper-free water, filter and keep in the dark. The strength diminishes more rapidly in daylight after some time it gives a solution which during titration deposits a bluish precipitate and interferes with the end-point, it must then be discarded. The dye solution should be standardised at least daily and preferably just before use against freshly prepared ascorbic acid solution (50 mg/100 ml in 20 per cent metaphosphoric acid) which in turn has been standardised against 0 01 N iodine. 

A useful method of analysis is to dissolve a known mass of the compound in a known excess of standardised ascorbic acid solution, back-titrating the excess with standardised Mn(III) sulphate solution or other oxidants. Alternatively, a known mass of the compound is treated with an excess of iodide solution, titrating the liberated iodine with standardised thiosulphate. 

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