Vitamin C, determination

Fruits and vegetables are the most common food sources for vitamin C (15-18). In food, vitamin C exists as two vitamers L-ascorbic acid (AA) and its oxidation product, dehydro-L-ascor-bic acid (DHAA) (Fig. 1) (15,17-19). Total Vitamin C is the sum of the AA and the DHAA contents. A stereoisomer, isoascorbic acid (IAA) is often added to food as an antioxidant. Both LAA and its oxidation product, dehydroisoascorbic acid (DHIAA), can interfere in Total Vitamin C determinations. 

Ascorbic acid (1) is most commonly used for testing the performance of electrodes in redox systems. Thus, a Ag-Ag ascorbate selective electrode was constructed with view to use it for vitamin C determination. Its reproducibility and stability was satisfactory and ascorbate ion concentration could be determined in neutral, alkaline and alcoholic media" . A voltametric study was carried out for the evaluation of graphite-epoxy composite (GEC) electrodes for use in the determination of ascorbic acid and hydroquinone. They were compared with mercury and CPE in similar operating conditions of pH and supporting electrolytes. Like all redox electrodes, also GEC electrodes deteriorate on exposure to air or after repeated usage, and the surface had to be renewed for activation. GEC electrodes were found to be adequate for redox system analyses"". The electrocatalytic oxidation of 1 is an amplification method for determination of specific miRNA strands using the An biosensor described in Table 1 . 

Egberg et al. (21) used Norit as the oxidant in a semiautomated total vitamin C determination, using a simultaneous oxidation and extraction step. DHA could be determined by omitting Norit in the extraction. The procedure was successfully used on a variety of foods correlation with manual procedures was excellent. 

There are two standard procedures for vitamin C determination in vegetables and legumes. The sample preparation is common to both and involves macer-ation/dilution of the sample in a stabilizing solution, such as 5% metaphosphoric acid or trichloroacetic acid, followed by filtration. The ascorbic acid can then be determined by titration with 2,6-dichloroin-dophenol, where the ascorbic acid reduces the oxidation/reduction indicator dye to a colorless solution or by fluorimetric detection, in which the ascorbic acid is oxidized to dehydroascorbic acid, which then reacts with o-phenylenediamine (1,2-diamino benzene) to produce a fluorophore. The latter method has the advantages that it is suitable for colored solutions and can be used to measure levels of naturally occurring dehydroascorbic acid as well as ascorbic acid. Many other methods are available for determination of vitamin C with the use of LC techniques, currently the subject of much interest. Reversed-phase LC techniques can be used to determine dehydroascorbic acid, ascorbic acid, and their isomers. 

Vitamin C status can be assessed by measuring plasma levels or urinary excretion. However, due to practical disadvantages, e.g., quantitative sampling of urine and instability of vitamin C, determination of vitamin C in urine has been mainly replaced by determination of plasma levels. Methods include direct determination by FIPLC, or automated assays based on a derivatization of ascorbic acid forming colored or fluorescent derivatives. While a plasma concentration <11.4p.molH is widely used to characterize deficiency, literature data signifying adequate status vary from >17 to 28.4p.moll. ... 

Mice were taken for vitamin C determinations immediately following death from anaphylaxis, and were assayed for total vitamin C as described in the text. 

Between 1910 and 1921, the vitamin was obtained in almost pure form from lemons and some of the physical and chemical properties were determined (10). It was discovered that vitamin C is easily destroyed by oxidation and best protected by reducing agents, and that... 

Two techniques for sorption-spectroscopic determination of ascorbic acid have been proposed. The first one is the recovery by silica modified with tetradecyl ammonium nitrate of blue form of molibdophosphoric HPA in the presence of vitamin C. And the second one is the interaction between the ascorbic acid in solution and immobilized on silica ion associate of molibdophosphoric acid with lucigenine. The detection limits of vitamin C are 0.07 and 2.6 mg respectively. The techniques were successfully applied to the determination of ascorbic acid in fmit juices. 

Ascorbic acid commonly known as vitamin C, is one of the most important water soluble vitamins. Ascorbic acid is involved in many biological processes and it is an essential compound in the human diet [1]. The determination of ascorbic acid has gained increase significance in pharmaceutic, clinical, and food applications. So far, different methods have been developed for determination of ascorbic acid [2, 3]. 

The following polyvitamin prepai ations were analyzed Kal tsid (OAO Comfort Plus , Russia), Asvitol (OAO INC Marbiofarm , Russia), Pikovit (KRKA, d.d. The New Place, Slovenia), Yeast with vitamin C (000 EKKO Plus , Russia). Chromatographic experiment has been carried out using Silufol UV-254 (Kavalier, Czech Republic) and acetone - ethyl acetate - acetic acid - ethanol (3 5 1 1) - CTAB (2T0 M) as a mobile phase mixture. The linearity calibration plot, built in coordinate S = f (IgqAC), is valid in the interval 5-25 p.g. Correctness of the determination has been checked by photometry. The obtained results for the ascorbic acid determination are presented below. 

The isolation of ascorbic acid was first reported by Albert Szent-Gyorgyi (who called it hexuronie add) in 1928. The structure was determined by ITirst and Haworth in 1933, and, simultaneously, Reichstein reported its synthesis. Haworth and Szent-Gyorgyi, who together suggested that the name be changed to L-ascorbic acid to describe its antiscorbutic (antiscnrvy) activity, were awarded the Nobel Prize in 1937 for their studies of vitamin C. 

We have determined that the empirical formula of vitamin C is C3II403. However, the empirical formula tells us only that the C, H, and O atoms are present in the sample in the ratio 3 4 3, not the number of each type of atom in a molecule. The molecular formula could be C3H4O3, CbH806, C9H1209, or any other whole-numher multiple of the empirical formula. 

L.18 A tablet of vitamin C was analyzed to determine whether it did in fact contain, as the manufacturer claimed, 1.0 g of the vitamin. One tablet was dissolved in water to form 100.00 mL of solution, and 10.0-mL of that solution was titrated with iodine (as potassium triiodide). It required 10.1 mL of 0.0521 M I3-(aq) to reach the stoichiometric point in the titration. Given that 1 mol L,- reacts with 1 mol vitamin C in the reaction, is the manufacturer s claim correct The molar mass of vitamin C is 176 g-mol-1. 

Dedicated plants predominate in the bulk chemicals industry. They suit the manufacture of well-defined products using a determined technology. Any change of the product or the production process usually produces problems, which illustrates the inflexibility of a dedicated plant. A batch plant may also be operated as a dedicated plant to produce a single chemical. Some fermentation plants (with reactors of up to 200 m volume) are examples of dedicated batch plants for the production of a family of similar products. So-called bulk fine chemicals, i.e. compounds that are produced in larger quantities, are also manufactured in dedicated plants, e.g. vitamin C and aspirin (see Fig. 7.1-1). The va.st majority of batch plants, however, produce several chemicals. 

Routine antioxidant vitamin supplementation, e.g. with vitamins C and/or E, of the diabetic diet should be considered. Vitamin C depletion is present in all diabetics irrespective of the presence of vascular disease. A recent study demonstrated no significant difference between the dietary intake of vitamin C (the main determinant of plasma ascorbate) in patients with diabetes and age-matched controls, confirming the view that ascorbate depletion is secondary to the diabetic process and su esting that diabetic patients require additional intakes of the vitamin to maintain optimal levels (Sinclair et /., 1994). Antioxidant supplementation may have additive beneficial effects on a wide variety of processes involved in diabetic vascular damage including blood pressure, immune function, inflammatory reactions. 

Yang et al. [12] determined the ionization constants of primaquine by a titrimetric method and studied its coordination ratio with vitamin C. The ionization constants of primaquine in 50% (v/v) ethanol in water determined at 25 °C in the ionic strength range of 5 x 10 to 5 x 10-2 mol/L are given. The coordination ratio of primaquine to vitamin C is determined by continuous variation and mole ratio methods based on pH and conductance measurements to be 1 1, indicating that the coordination compound formed in the solution is mainly a 1 1 compound. 

Important intrinsic quality criteria currently determining the market potential of new apple cultivars are related to the sensory quality such as fruit firmness (crispness) and the sugar and acidity contents. On the other hand, the nutritional composition (e.g. the vitamin or antioxidant contents) is currently not used as a criterion in the choice of cultivars, neither in conventional nor in organic fruit production. The difference in the content of such components between fruit species is in most cases more relevant than between cultivars of the same species (e.g. vitamin C content of oranges versus apples). 

More than one hundred enzymes, and many of their coenzymes, have been recognized in animal mitochondria. Whether or not such an organization of enzymes is present in such a cell unit for performing a sequence of reactions with carbohydrates remains to be determined (see the Section on vitamin C). 

George S, Brat P, Alter P and Amiot MJ. 2005. Rapid determination of polyphenols and vitamin C in plant-derived products. J Agric Food Chem 53(5) 1370-1373. 

The recognition of their structure permits the determination of vitamins by the tools of analytical chemistry, but while such methods are widely used in industrial production, the minute quantities in body fluids and tissues limit the purely chemical approach to a few members of this group present in relatively high concentration, e.g., vitamin C (K5). Microchemical methods are in use for the determination of thiamine, riboflavin, and some of the fat-soluble vitamins, based on the most sensitive colorimetric and, in particular, fluorometric techniques. Vitamin D, on the other hand, is determined by animal assay. 

Colorimetric Determination of Vitamin C Using Fe(II)-5-Chloro-7-Iodo-8-Hydroxyquinoline Complex... 

The last of the fat-soluble vitamins to be identified was vitamin K, found by Dam to be an anti-hemorrhagic factor for young chicks, distinct from vitamin C. Its structure was determined by Dam in collaboration with Karrer. Interest in the vitamin was intensified when it was discovered (Link, 1941) that dicoumarol, present in spoiled sweet clover, was the agent producing hypothrombinemia (giving prolonged blood-clotting time) in cattle. Since vitamin K is structurally similar to dicoumarol, the vitamin was presumptively implicated in thrombin formation. This has been fully substantiated by recent work on the role of vitamin K in the synthesis of prothrombin in the liver. 

One way to determine the vitamin C content of a sample is to titrate it with an iodine/iodide solution. The diagram below shows the reaction involved. 

O Design a titration that uses an iodine solution to determine and compare the concentration of vitamin C in a variety of fresh fruit juices. You may find it helpful to research appropriate procedures on the Internet. Include a method for standardizing the iodine solution. (See Challenge question 1.) Make sure that you list all the safety precautions you will use in your titration. Check your procedure with your teacher, and then carry out your titration. 

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