Spectrophotometric Determination of Trace Amounts of Ascorbic Acid

Abstract

Colourless silver-gelatin complex is quantitatively reduced by ascorbic acid to yellow silver sol in water within the pH range 7.5–10.0 at room temperature. The determination of 1–10μg/ml of ascorbic acid is possible at 415 nm in the presence of glycine, alanine, fructose, sucrose, citric, tartaric, oxalic, malic, succinic acids and also in the presence of various reducing agents. The molar absorptivity of ascorbic acid at the δ_max is found to be 21500 lit mol^?1 cm^?1 and the Sandell sensitivity of the sol is 8.18x10^?3 μg ascorbic acid cm^?2 for 0.001 absorbance. The relative standard deviation is ±0.22% and the confidence limit (20 determinations, 95%) being 8.806±0.0093%.     

Electro-Catalytic Oxidation of Ascorbic Acid at a Cobalt-Salen Polymer Modified Electrode and Analytical Applications

ABSTRACT

The preparation and electrochemical characteristics of electrodes modified by cobalt complexes of N, N' - bis(salicylidene)-ethane -1, 2- diamine (salen) are described. A cobalt-salen polymer film modified electrode has strong electro-catalytic effects for the oxidation of ascorbic acid. The anodic peak potential of ascorbic acid shifted negatively for 400 m V. The catalytic reaction rate constant determined by rotating disk experiments is 7.08×10⁵ mol s^?1 cm^3. The catalytic mechanism and the effect of film thickness are discussed. A sensitive voltammetric response for ascorbic acid was obtained covering a linear range from 1.0×10^?6 to 1.0×10^?3 mol-L^?1 The modified electrode showed good stability and reproducibility. The electrode was used to the determination of ascorbic acid in fruit juices and showed promising results compared with conventional methods. The electro-catalytic effect of several metal-salen complexes and a similar Schiff base derivative for ascorbic acid was compared.     

A Chemiluminescence Reaction Between Hydroxyl Radical and Rhodamine 6G and its Applications

Abstract

A novel chemiluminescence (CL) reaction between hydroxyl radical and ascorbic acid is described in this paper. Hydroxyl radical generated on line by the reaction between Fe³⁺ solution and H₂O₂ solution in HCl medium could oxidize rhodamine 6G to produce weak chemiluminescence. It was found that ascorbic acid could enhance the chemiluminescence and the excited rhodamine 6G was the emitter of the chemiluminescence reaction. The possible mechanism of the CL system was also discussed. Ascorbic acid can be determined in the range of 2.0×10^?6?8.0×10^?4 mg/ml with a detection limit of 1×10^?6 mg/ml (3σ). A complete analysis could be done in 1 minute with the relative standard deviation of 3.1% for 5.0×10^?5 mg/ml (n=11). In order to study the chemiluminescence reaction further, the application to the determination of ascorbic acid in food using the chemiluminescence reaction combined with flow injection is investigated.     

A Comparative Study on Spectrophotometric,Conductometric and Potentiometric Determination of Ascorbic Acid

Abstract

A comparison has been made on the selectivity, accuracy and precision of spectrophotometric, conductometric and potentiometric methods for the determination of ascorbic acid in water which is based on the reduction of gold(III) ion by ascorbic acid. Gold(III) ion forms a complex with gelatin in alkaline medium which on reduction with ascorbic acid produces coloured gold sol. The sol shows an absorption maximum at 540 nm with molar absorptivity of 2.3×10^?3litre mol^?1 cm^?1 and the Sandell's sensitivity of 7.6 × 10^?2 μgcm^?2. The relative standard deviation is 0.22% and the confidence limit (20 determinations, 95%) is 2.0 ± 0.009%. To examine the extent of sensitivity of the spectrophotometric method the proposed method is compared with the sensitivities of the conductometric and potentiometric methods.     

New N-aryloxy-phthalimide derivatives. Synthesis,physico-chemical properties,and QSPR studies

Starting from N-hydroxyphthalimide 1 and the reactive fluoro- or chloro-nitroaryl derivatives 2, 3 and 4a-e (2-chloro-3,5-dinitropyridine; 3, NBD-chloride; 4a, 1-fluoro-2,4-dinitrobenzene; 4b, picryl chloride; 4c, 4-chloro-3,5-dinitrobenzotrifluoride; 4d, 2-chloro-3,5- dinitrobenzotrifluoride; 4e, 4-chloro-3,5-dinitrobenzoic acid) the corresponding N-(2-nitroaryloxy)-phthalimide derivatives 5a-e, or 6 and 7 were obtained and characterized by IR, UV-Vis ¹H-NMR and ¹³C-NMR spectroscopy. The TLC behavior and the hydrophobicity of these derivatives have been experimentally evaluated by R_M0 parameters (using RP-TLC). The experimental R_M0 parameters were compared with the calculated partition coefficient, log P. A QSPR study was also performed to establish possible correlations between the structure and physical properties (λ_max and R_M0) of compounds 5a-e, 6, and 7.      

Determination of L-Ascorbic Acid by Adsorption Potentiometry with Carbon Paste Electrode

Abstract

This paper reports the potentiometric measurement of ascorbic acid in the solution of 0.10 mol/L NaOH-0.1 mol/L NaCl using carbon paste (the mixture of spectroscopic graphite powder and di-iso-octyl phthalate) electrode(room temperature 15°C), with the linear range 7.0 × 10^?7 × 4.0 × 10^?5mol/L, average response slope 95mV/decade and detection limit 1 × 10^?7mol/L. Phenol, sulfite, Mn²⁺ and so on pose no interference to the measurement of ascorbic acid. This method is characterized by fine selectivity, reproducibility and accuracy. The potential response behavior is caused chiefly by chemisorption of ascorbic acid to the surface of the carbon.

Each year yields a number of papers concerning the determination of ascorbic acid by various methods, including gas chromatographymass spectrometry¹, capillary electrophoresis², spectrophotometry³, voltammetry⁴, titrimetric method⁵, biosensor⁶ and so on, Each method has its merits and defects in analyzing different samples. M. Petersson⁷ worked out the potentiometric sensor for determining ascorbic acid by modifying monolayer of ferrocene upon the surface of half-oxidized platinum electrode with an average response slope 50±8.8mV/decade, but this sensor suffers from inadequate selectivity. In our study, carbon paste electrode (without ionophore) is applied in the determination of ascorbic acid by potentiometry with an average response slope 95mV/decade. This method displays fine selectivity, accuracy, convenience and rapidity of determination.     

Indication Ion Square Wave Voltammetric Determination of Thiamine and Ascorbic Acid

Abstract

Cd²⁺ ion was used as an electrochemical indicator to detect V_B1 or Vc using square ware voltammetry (SWV) at a mercury film‐coated glassy carbon (GC) electrode. At pH=10 NH₃‐NH₄Cl buffer, a new cathodic peak was found at ?0.360 V (vs.SCE) by addition of thiamine, and the peak current of SWV was linear with the concentration of thiamine in the range of 1×10^?6 to 4×10^?3 M. On the other hand, the SWV peak current of Cd²⁺ at ?0.856 V linearly decreased with addition of ascorbic acid in the range of 6×10^?6～10^?3 M. The effects of interference, such as citric acid, DL‐malic acid, and calcium panlothenate, on thiamine or ascorbic acid determination were investigated. This method was successfully applied to the determination of thiamine or in pharmaceutical preparation.     

Flow Injection Analysis Determination of Ascorbic Acid with Spectrofluorimetric Detection

Abstract

A flow injection system for the fluorescence determination of low level of ascorbic acid is proposed. The method is based on the rapid oxidation of ascorbic acid by thallium(I). The fluorescence signal at 419 nm is proportional to the amount of ascorbic acid in the range of (1.4–28.0) × 10^?7 mole. The relative standard deviations for ten replicate measurements of 1.4 × 10^?6 mole of ascorbic acid was 1.3%. The sample rate of 45 ± 5 sample per hour was achieved. The usefulness of the method was tested in the determination of ascorbic acid in fruit juices and vitamin C tablets.     

Mechanism of catalytic ascorbic acid oxidation system Cu2+–ascorbic acid–O2

Analysis is made of reported results on the kinetics and mechanism of ascorbic acid oxidation with oxygen in the presence of cupric ions. The diversities due to methodological reasons are cleared up. A kinetic study of the mechanism of Cu²⁺ anaerobic reaction with ascorbic acid (DH₂) is carried out. The true kinetic regularities of catalytic ascorbic acid oxidation with oxygen are established at 2.7 ≤ pH < 4, 5 × 10^?4 ≤ [DH₂] ≤ 10^?2M, 10^?4 ≤ [Cu²⁺] ≤ 10^?3M, and 10^?4 ≤ [O₂] ≤ 10^?3M: where??₁ (25°C) = 0.13 ± 0.01 M^?0.5˙sec^?1. The activation energy for this reaction is E₁ = 22 ± 1 kcal/mol. It is found by means of adding Cu⁺ acceptors (acetonitrile and allyl alcohol) that the catalytic process is of a chain nature. The Cu⁺ ion generation at the interaction of the Cu²⁺ ion with ascorbic acid is the initiation step. The rate of the chain initiation at [Cu²⁺] ± 10^?4M, [DH₂] ± 10^?2M, 2.5 < pH < 4, is where??_i,1 (25°C) = (1.8 ± 0.3)M^?1˙sec^?1, E_i,1 = 31 ± 2 kcal/mol. The reaction of the Cu⁺ ion with O₂ is involved in a chain propagation, so that the rate of catalytic ascorbic acid oxidation for the system Cu²⁺? DH₂? O₂ is where??₁ (25°C) = (5 ± 0.5) × 10⁴ M^?1˙sec^?1. The Cu⁺ ion and a species interacting with ascorbate are involved to quadratic chain termination. By means of photochemical and flow electron spin resonance methods we obtained data characteristic of the reactivities of ascorbic acid radicals and ruled out their importance for the catalytic chain process. A new type of chain mechanism of catalytic ascorbic acid oxidation with oxygen is proposed: .     

A Spectrophotometric Determination of Ascorbic Acid

A new, simple and sensitive method for the spectrophotometric indirect determination of ascorbic acid in fruits, beverages, and pharmaceuticals is described. In this method, the ascorbic acid reduces Cu²⁺ to Cu⁺ and reacts with 2,9‐dimethyl‐1,10‐phenanothroline (neucoproine) to form Cu (neucoproine)⁺ complex, and it was extracted with N‐phenylbenzimidoylthiourea (PBITU) in chloroform. The apparent value of molar absorptivity of the complex in terms of ascorbic acid is (3.52) × 10⁴ L mole^?1 cm^?1 at λ_max, 460. The detection limit of ascorbic acid is 40 μg L^?1 and the method obeys Beer's law over the concentration range of 0.1–4.0 μg mL^?1. The proposed method was successfully applied for the determination of ascorbic acid in various samples. The validity of the present method was checked by the flow injection analysis (FIA) method.     

Meldola's Blue Immobilized on a SiO2/SnO2/Phosphate Xerogel,a New Sensor for Determination of Ascorbic Acid in Medicine and Commercial Fruit Juice

A modified carbon paste electrode with SiO₂/SnO₂/Phosphate/Meldola's blue, SSPMelB, was used to study the electrocatalytic oxidation of ascorbic acid by cyclic voltammetry and chronoamperometry. The adsorbed dye mediates ascorbic acid oxidation at an anodic potential of 0.04 V vs. saturated calomel electrode (SCE) at pH 7.0, in 0.5 mol L^?1 solution. The linear range of the sensor is between 4.0×10^?7 and 2.0×10^?3 mol L^?1, with a limit of detection of 4.0×10^?7 mol L^?1. This novel electrode shows good analytical performance for determination of ascorbic acid in medicine and commercial fruit juice.     

Simultaneous Determination of Ascorbic Acid and Uric Acid by Using a PVC/TTF‐TCNQ Composite Electrode as Detector in a FIA System

A PVC/TTF‐TCNQ composite electrode has been employed as detector in a flow injection system. The proposed method allows the simultaneous detection of ascorbic acid (AA) and uric acid (UA) in mixtures by using a FIA system in a simple manner, without pre‐treatment or modified electrode. This method is based on the amperometric determination of (a) ascorbic acid at 0.15 V and (b) both analytes at 0.35 V, being the response linear in the range 1×10^?2–4×10^?4 M for both analytes with detection limits (S/N=3) of 1.2×10^?4 M and 8.1×10^?5 M for AA and UA, respectively.     

Direct potentiometric titration of thiosulfate,thiourea, and ascorbic acid with lodate using an iodide ion-selective electrode

A potentiometric method has been developed for the semi-automatic direct titration of thiourea, thiosulfate, and ascorbic acid with potassium iodate in strongly acidic solutions using an iodide ion-selective electrode to monitor the reaction and locate the endpoint. The method is simple, fast, precise, and accurate. Amounts ranging from 0.15–1.5 mg of thiourea (3.9 × 10^?4–3.9 × 10^?3, M), 0.3–3.0 mg of thiosulfate (5.4 × 10^?4–5.4 × 10^?3, M), and 0.5–5.0 mg of ascorbic acid (5.7 × 10^?4–5.7 × 10^?3, M) have been determined with an average error of about 1%. The method has been applied to the determination of ascorbic acid in tablets. Results checked closely with those obtained with a standard titrimetric method.     

Flow-injection spectrophotometric determination of ascorbic acid by reduction of vanadotungstophosphoric acid

Ascorbic acid may be determined spectrophotometrically at 360 nm based on reduction of vanadotungstophosphoric acid using flow-injection analysis. The carrier stream was distilled water and the reagent streams were buffer solution (pH 3.0), 1.735 × 10^?3 M dodecatungstophosphoric acid and 1.735 × 10^?3 M sodium vanadate. The injection rate was 80 h^?1. The calibration graph was linear up to 80 μg ml^?1 ascorbic acid and the relative standard deviation for the determination of 20 μg ml^?1 ascorbic acid was 1.5% (n=10). The detection limit was 1.0 μg ml^?1 ascorbic acid, based on an injection volume of 250 μl. The system was applied to the determination of ascorbic acid in vitamin C tablets.     

Synthesis and characterization of activated carbon–ethylenediamine–cobalt(II) tetracarboxyphthalocyanine conjugate for catalytic oxidation of ascorbic acid

We report on the synthesis and characterization of activated carbon–ethylenediamine–cobalt(II) tetracarboxyphthalocyanine conjugate (AC–CONHCH₂CH₂NH₂–CoPc) and its electrocatalytic behavior for oxidation of ascorbic acid. Ultraviolet–visible (UV–Vis), Fourier-transform infrared (FTIR), and electrochemical impedance spectroscopies, and cyclic and square-wave voltammetry were used to characterize the electrode modifiers and modified glassy carbon electrode. The limit of detection was found to be 0.26 µm using 3δ notation. The linear dynamic range was from 1.5 × 10^?4 to 1 × 10^?2 M with electrode sensitivity of 0.01 A mol^?1 L cm^?2. A Tafel slope of 200.8 mV decade^?1 was found. The concentration of ascorbic acid in the tablet was 0.034 M. Oxalic acid showed no interference in ascorbic acid determination.     

The Reductions of Bis(terpyridyl) and Ethylenediaminetetraacetato Complexes of Cobalt(III) by Ascorbic Acid

The reductions of Co(terpy)₂³⁺ and Co(edta)^? complexes by ascorbic acid have been subjected to a detailed kinetic study in the range of pH =1–10.9. For each complex the rate law of the reaction is interpreted as a rate determining reaction between Co(III) complex and the ascorbic acid in the form of HA^? (k₁) and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Co(III) complex. With given Ka₁ and Ka₂, the rate constants are k₁ = 0.25 and 9.87 × 10^?5 M^?1s^?1, k₂ = 1.28 × 10⁶ and 18.7 M^?1s^?1 for Co(terpy₎2³⁺and Co(edta)^? complexes, respectively, at T = 25 °C and μ = 0.50M (terpy)and 1.0 M (edta) HClO₄/LiClO₄. The mechanism of the reaction is discussed on the basis of Marcus theory for outer sphere electron transfer process. Spin change and charge effect, duly considered, account for the non‐adiabatic behavior in the reduction of Co(edta)^? complex.     

Studies on the Interaction of Some Biologically Significant Radical Scavengers on Metal‐Ion Catalyzed Phloroglucinol‐Based Chemical Oscillator

The response of the Ce(III)‐catalyzed phloroglucinol (1,3,5‐trihydroxybenzene)‐based Belousov Zhabotinsky system to the addition of various antioxidants (ascorbic acid, glutathione, inosine, N‐acetylcysteine) is monitored at 30°C under stirred batch conditions. This method is convenient and has good sensitivity for the determination of these antioxidants. The addition of these antioxidants to the BZ mixture influences the oscillatory parameter (number of oscillations) to an extent that depends on the concentration of the antioxidant. The experimental results have shown that the number of oscillations decrease on increasing the concentration of antioxidant. The calibration plots show a linear relationship (R² = 0.98 for ascorbic acid and glutathione and R² = 0.99 for inosine and N‐acetylcysteine) between the number of oscillations and the [antioxidant] over the concentration range of 0.0125‐0.5, 0.05‐0.2, 0.025‐0.1, and 0.1‐0.5 mol L^?1 with detection limits 6.9 × 10^?5, 2.762 × 10^?4, 1.381 × 10^?4, and 5.524 × 10^?4 mol L^?1 for ascorbic acid, glutathione, inosine, and N‐acetylcysteine, respectively. Some aspects of the mechanism of these antioxidants on the BZ system have been discussed.     

Electrocatalytic Determination of Ascorbic Acid at the Surface of 2,7‐Bis(ferrocenyl ethyl)fluoren‐9‐one Modified Carbon Paste Electrode

A chemically modified carbon paste electrode with 2,7‐bis(ferrocenyl ethyl)fluoren‐9‐one (2,7‐BFEFMCPE) was employed to study the electrocatalytic oxidation of ascorbic acid in aqueous solution using cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The diffusion coefficient (D=1.89×10^?5 cm² s^?1), and the kinetic parameter such as the electron transfer coefficient, α (=0.42) of ascorbic acid oxidation at the surface of 2,7‐BFEFMCPE was determined using electrochemical approaches. It has been found that under an optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 300 mV less positive than that of an unmodified carbon paste electrode. The catalytic oxidation peak currents show a linear dependence on the ascorbic acid concentration and linear analytical curves were obtained in the ranges of 8.0×10^?5 M–2.0×10^?3 M and 3.1×10^?5 M–3.3×10^?3 M of ascorbic acid with correlation coefficients of 0.9980 and 0.9976 in cyclic voltammetry and differential pulse voltammetry, respectively. The detection limits (2δ) were determined to be 2.9×10^?5 M and 9.0×10^?6 M with cyclic voltammetry and differential pulse voltammetry, respectively. This method was also examined for determination of ascorbic acid in pharmaceutical preparations.     

Kinetic Spectrophotometric Determination of Ascorbic Acid in Pharmaceutical Samples by Oxidation of Ponceau 4R by Hydrogen Peroxide

A new sensitive and simple kinetic method is developed for determination of traces of ascorbic acid based on its activated effect on oxidation of trisodium‐2‐hydroxy‐1‐(4‐sulphonato‐1‐naphthylazo)naphthalene‐6,8‐disulphonato (red artificial color Ponceau 4R) by hydrogen peroxide, in the presence of Cu(II) as catalyst, in borate buffer. The reaction is followed spectrophotometrically by tracing the oxidation product at 478.4 nm within 1 min after addition of H₂O₂. The optimum reaction conditions are: borate buffer (pH = 11.00), Ponceau 4R (9.6·10^?6 mol/L), H₂O₂ (2·10^?2 mol/L), Cu(II) (8·10^?7 mol/L) at 22 °C. Following this procedure, ascorbic acid can be determined with a linear calibration graph up to 1.76 ng/mL and a detection limit of 0.28, based on 3S criterion. The relative error ranges between 6.77‐1.66% for the concentration interval of ascorbic acid 1.76‐17.61 ng/mL. The effects of certain foreign ions upon the reaction rate were determined for an assessment of the selectivity of the method. The method was applied for determination of ascorbic acid in pharmaceutical samples, and spectrophotometric method was used like an comparative method.     

Studies of redox polymerization. I. Aqueous polymerization of acrylamide by an ascorbic acid–peroxydisulfate system

The polymerization of acrylamide initiated by an ascorbic acid–peroxydisulfate redox system was studied in aqueous solution at 35 ± 0.2°C in the presence of air. The concentrations studied were [monomer] = (2.0–15.0) × 10^?2 mole/liter; [peroxydisulfate] = (1.5–10.0) × 10^?3 mole/liter; and [ascorbic acid] = (2.84–28.4) × 10^?4 mole/liter; temperatures were between 25–50°C. Within these ranges the initial rate showed a half-order dependence on peroxydisulfate, a first-order dependence on an initial monomer concentration, and a first-order dependence on a low concentration of ascorbic acid [(2.84–8.54) × 10^?4 mole/liter]. At higher concentrations of ascorbic acid the rate remained constant in the concentration range (8.54–22.72) × 10^?4 mole/liter, then varied as an inverse halfpower at still higher concentrations of ascorbic acid [(22.72–28.4) × 10^?4 mole/liter]. The initial rate increased with an increase in polymerization temperature. The overall energy of activation was 12.203 kcal/mole in a temperature range of 25–50°C. Water-miscible organic solvents depressed the initial rate and the limiting conversion. The viscometric average molecular weight increased with an increase in temperature and initial monomer concentration but decreased with increasing concentration of peroxydisulfate and an additive, dimethyl formamide (DMF).