Structural determination of ascorbic acid 2-o-phosphate formed via acid hydrolysis of an ascorbic acid 3-O-phosphinate

The synthesis and X-ray structural determination of 3-O-[(bis-morpholino)phosphinyl]-5,6-O-isopropylidene-l-ascorbate (9) are described. Acid-catalyzed hydrolysis of 9 afforded the 2-O-phosphate 6. Definitive structural proof of 6 is based on a study of the pH profile of its UV spectra as compared with those of ascorbic esters, 2 and 9 (Figs. 1–3).     

Determination of ascorbic acid with ferricyanide

Ascorbic acid can be determined by titration with potassium ferricyanide in sulphuric, phosphoric or hydrochloric acid media. Twelve indicators, including ferroin and some triphenylmethane and thiazine dyes, have been found suitable. The method has been successfully applied to assay of commercial products for vitamin C. Several organic substances likely to be associated with ascorbic acid in preparations and plant tissues do not interfere.     

Microtitration of ascorbic acid with copper

Summary A new titrimetric method for the microdetermination of ascorbic acid has been developed. It is based on the oxidation of ascorbic acid to dehydroascorbic acid with copper(II) in presence of ammonium thiocyanate; potassium iodide and starch are used as indicator. The method is especially suitable for dilute solutions (<0.05N) and can be used over the pH range 1–7. Glucose, fructose, aspirin, caffeine, rutin and calcium gluconate do not interfere, and neither do citrate, oxalate and tartrate if the titration is done at pH 1-2. Bisulphite can be masked with acetone, but novalgin must be absent. The method is simple, sensitive and accurate.

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Mikrotitration von Askorbinsäure mit Kupfer

Zusammenfassung Eine neue maßanalytische Methode zur Mikrobestimmung von Askorbinsäure wurde ausgearbeitet. Sie beruht auf deren Oxydation zu Dehydroaskorbinsäure mit Kupfer(II) in Gegenwart von Ammoniumrhodanid; Kaliumjodid und Stärke dienen als Indikator. Das Verfahren eignet sich besonders für verdünnte Lösungen (<0,05N) und ist im pH-Gebiet 1-7 anwendbar. Glukose, Fructose, Aspirin, Coffein, Rutin und Calciumglukonat stören nicht, ebenso wenig Zitrat, Oxalat und Tartrat, soferne bei pH 1-2 titriert wird. Bisulfit kann mit Aceton maskiert werden, aber Novalgin darf nicht anwesend sein. Das Verfahren ist einfach, empfindlich und genau.

     

Simultaneous kinetic determination of uric acid ascorbic acid mixtures

A kinetic stopped-flow method is described for the simultaneous determination of uric acid and ascorbic acid with tris (2,2'-bipyridine)iron(III). For the least favourable ratios of uric to ascorbic acid, in a total concentration of 10^-5 M, the error in the determination of uric acid is estimated at ±10%.     

Applications of a simultaneous assay of ascorbic acid, dehydroascorbic acid and ascorbic sulphate in biological materials

A modified spectrophotometric assay for ascorbic acid and its derivatives based on their reaction with 2,4-dinitrophenylhydrazine (DNPH) is described. Using standard ascorbic acid or ascorbic sulphate solutions, together with animal tissue or compound diet extracts, the conditions for ascorbic acid degradation were determined. For the differential measurement of reduced ascorbic acid (AA), dehydroascorbic acid (dAA) and ascorbic sulphate (AS), five series of simultaneous determinations were performed. These included the use of (1) KBrO3 for the hydrolysis of AS, (2) 2,6-dichlorophenolindophenol as an oxidant, (3) DNPH to form a hydrazone derivative with dAA and (4 and 5) two blanks (where ascorbate was degraded) to correct for interfering substances. A variety of vertebrate and invertebrate tissues were examined for their ascorbate content, and the advantages of the modified procedure over currently available assays are discussed. The results suggest that the Artemia cyst is a unique material in which ascorbic sulphate is present in large amounts whereas fish tissues do not contain this form of vitamin C.     

Fluorimetric determination of ascorbic acid with o-phenylenediamine

A simple and sensitive fluorimetric method for the determination of ascorbic acid (AA) is described. The method is based on the condensation reaction between AA and o-phenylenediamine (OPDA) in the absence of the oxidant. The fluorescence intensity is measured at excitation and emission wavelengths of 360 and 430 nm, respectively. Under optimum condition, a linear relationship is obtained between the fluorescence intensity and the concentration of AA in the range of 0.05-40 μg ml⁻¹. The detection limit is 0.006 μg ml⁻¹, which is obviously lower than that of other fluorimetric methods reported.     

Free-radical-mediated oxidation of ascorbic acid by peroxomonosulphate

The kinetics of the oxidation of ascorbic acid (AH₂) by peroxomonosulphate (PMS) were determined in aqueous medium in acidic (pH 4.4), neutral (pH 7.0) and alkaline (pH 9.0) conditions over the temperature range 13-28°C. The reactions were found to obey total second-order kinetics, first-order each with respect to peroxomonosulphate and ascorbic acid concentration, obeying-d[AH₂]/dt = k ₂[PMS][AH₂]. Dehydroascorbic acid was detected as the product of the reaction. The stoichiometry of the reaction, [peroxomonosulphate]/[ascorbicacid] = 1 : 1, indicated the absence of self-decomposition of peroxomonosulphate. The influence of neutral salt (NaClO₄) was found to increase the reaction rate. Evidence for the formation of radical intermediates was obtained. A mechanism involving the formation of hydroxyl, sulphate and ascorbate free radicals as intermediates is proposed. The rate and activation parameters were evaluated to substantiate the mechanism proposed. A comparison is made with the corresponding reactions of the similar peroxides, S₂O₈ ^2- and P₂O₈ ^4-.     

Anionic polymer hydrogel degradation by ascorbic acid

Polymer hydrogel based on sodium 2-acrylamido-2-methylpropansulfonate, covalently crosslinked with N,N′-methylenebisacrylamide has been shown to degrade in aqueous ascorbic acid. The hydrogel degradation is induced by chemical interaction of the polymer crosslink and an ascorbic acid oxidation intermediate.     

PDMS-based gold electrode for sensing ascorbic acid

Electrode with optical shapes is appreciated in microfluidics. In this article, we reported a flexible poly(dimethylsiloxane) (PDMS)-based gold electrode for ascorbic acid detection. Gold nanoparticles were chemically deposited on PDMS and the composite film was applied as working electrode. The electrode could undergo deformation and display good response performance without damage. This biosensor could give quick response to ascorbic acid (AA) (<5s) and the currents were linear with concentrations of AA in range of 0.023-7.00 mM and 30-100 mM, respectively. Limit of detection was 0.008 mM (S/N=3). This biosensor has been applied to determine ascorbic acid content in vitamin C tablets and the results were consistent with traditional iodometric method.     

Flow injection determination methods of ascorbic acid

A review with 100 references is presented to show the advantages involved in the use of flow injection analysis (FIA) for the determination of ascorbic acid. The FI methods proposed for the determination of ascorbic acid are described and compared according to the used detection technique. Analytical figures of merit and interferences are also discussed.     

Determination of ascorbic acid by potassium iodate

Summary Results are given for a two stage oxidation in the ascorbic acid-KIO₃ reaction under controlled conditions of acidity. The first step corresponds to the Landolt reaction and the second end point is obtained by the classical Andrews method. The reproducibility and accuracy of the results calculated on the basis of the two successive end points constitute an interesting new feature of this redox reaction involving the use of KIO₃ as a primary standard for the determination of ascorbic acid.Our sincere thanks are due to Professor S. S. Joshi for kind interest in the work and to the National Institute of Sciences of India for award of a Research Fellowship to one of us (G. S. D.).     

Printed electrochemical sensor for ascorbic acid determination

Simple, strip-type sensors based on 7,7,8,8-tetracyanoquinodimethane-modified graphite were prepared using screen printing techniques. The electrochemical strips operated at low potentials [50 mV at pH 7.0 or 100 mV at pH 4.8 vs. Ag/AgCl (printed)] and had a sensitivity of 3.5–7.1 μA 1 mmol^?1L-ascorbic acid. Determination of ascorbic acid concentration was achieved in 30 s and required samples of ca. 30 μl. The current output of the electrodes was found to be relatively insensitive to variations in pH over the range 5.0–8.5. Between 15 and 35 °C, the temperature coefficient was 2.7% °C^?1. The printed electrodes were suitable for single determinations but demonstrated adequate stability for periodic re-use. The ascorbic acid concentration in the juice of fresh fruit was determined using the electrochemical printed electrodes and a commercially available enzymatic test kit. Close agreement was observed between the two methods [r=0.9997 (n=12),slope=0.9798]. The limit of detection using the printed sensor for real samples was calculated as 4mg l^?1(22 μM).     

An ab initio study of ascorbic acid

The conformations of ascorbic acid and its singly ionized analog are found to agree with the X-ray structures. The calculations are discussed in terms of the known chemistry of ascorbic acid and its metabolites.     

Ab initio study of ascorbic acid conformations

The STO-3G optimized structures of nine different staggered conformers of ascorbic acid are presented. The largest energy difference between the nine local minima is 5.1 kcal/mol. Comparison of the relative energies of the fully optimized structures of ascorbic acid conformers with those of nonoptimized conformers shows that full optimization is essential to obtain meaningful results. However, optimization of the ring structure is almost independent of optimization of the side-chain structure. One of the STO-3G optimized gas phase conformers is very close to the X-ray structure of the crystal.