1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride as a spectrophotometric reagent for the determination of lodate in acetic and perchloric acid media

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with iodate ions in acid medium. The yellow color obtained has been used to proposed Spectrophotometric methods for determination of IO₃^? in the concentration range 1.0–11.0 ppm in acetic acid medium (molar absorptivity 1.08 × 10⁴ liters mol^?1 cm^?1 at a wavelength of 415 nm) and 0.5–8.0 ppm in perchloric acid medium (molar absorptivity 2.05 × 10⁴ liters mol^?1 cm^?1 at a wavelength of 400 nm).     

Cyclopentane-1,3-dione bis(4-methylthiosemicarbazone) monohydrochloride as a spectrophotometric reagent for the determination of chlorate in perchloric acid medium

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with chlorate ions in strongly acid medium. The yellow color obtained has been used to propose a spectrophotometric method of C10₃^? determination in the concentration range 0.5–6.0 ppm (molar absorptivity 1.26 × 10⁴ liters mol^?1 cm^?1 at a wavelength of 397 nm).     

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 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.     

Aqueous polymerization of acrylonitrile by ascorbic acid–peroxodisulfate redox system

The polymerization of acrylonitrile initiated by an ascorbic acid–peroxodisulfate redox system was studied in an aqueous solution at 35°C in the presence of air. Molecular oxygen was found to have no effect on the polymerization reaction. An increase in ionic strength slightly increased the rate. The overall rate of polymerization, R_p, showed a square dependence on [monomer] and a half-order dependence on [peroxodisulfate]. A first-order dependence on [ascorbic acid] at low concentrations (<3.0 × 10^?3 mol L^?1) followed by a decrease in R_p at higher concentrations of ascorbic acid (>3.0 × 10^?3 mol L^?1) was also noted. R_p remained unchanged up to 40°C and showed a decline thereafter. Addition of catalytic amounts of cupric ions decreased the rate whereas ferric ions were found to increase the rate. Added sulfuric acid in the range (6.0?50.0) × 10^?5 mol L^?1 decreased the R_p.     

Polarographic Behaviour and Determination of Stability Constants of Uranyl-Ascorbate Complex

The complex species of UO₂(HA)(H₂A)⁺ and UO₂(HA)₂ were identified in the ascorbic acid solution of uranyl ion at pH<2.1 and pH>2.1, respectively. Polarographic wave was proved to be the simultaneous reduction of UO₂⁺² and UO₂(HA)(H₂A)⁺ at pH <2.1. However, at pH>2.1, the wave is due to the reduction of U0₂(HA)₂ The stability constants of the two complex species were found to be 5.1×10⁺ and 1.0×10⁵, respectively. The hydrolysis constant of uranyl ion in the solution of ascorbic acid was determined.     

Photobiological effects of metal ions. Photosensitizing action of uranyl acetate on Escherichia coli K-12

Abstract— The Uranyl acetate sensitized killing of Escherichia coli K-12 by a light source approximating sunlight in both intensity and wavelength distribution is demonstrated to occur at a concentration of 5.0 × 10O^-4M uranyl acetate (pH 7.0). The photosensitized killing was evident after 150 min and almost complete within 320 min. Auxotrophic mutants were isolated from cultures incubated in both light and dark for 160 min at this concentration of uranyl acetate. Binding of UO₂²⁺ to E. coli is shown to occur with 82% of the UO₂²⁺ ions in a 5 × 10^-4M solution (pH 7.0) being bound to the cell wall. In the dark as well as at other pH values the extent of binding was much less. Most of the binding occurred in a time less than 30 min. The observation of rapid binding but delayed photosensitization is attributed to the necessity for penetration of uranyl ions into the cells' interior to effect photosensitization.     

Uranyl ion-sensitized polymerization of acrylamide and methacrylamide in aqueous solution

The kinetics of polymerization of the vinyl monomers, acrylamide and methacrylamide, photosensitized by uranyl ions in homogeneous aqueous acid medium was studied systematically. Monochromatic radiation of wavelengths 365, 405, and 436 mμ was used for irradiation. Uranyl perchlorate in aqueous perchloric acid (pH = 0–2) was used as the sensitizer to ensure that only uncomplexed UO₂²⁺ ions existed in the solution. Polymerization was found to proceed without any induction period, the steady state being attained in about 10–20 min., and was followed by the measurement of the rate of monomer disappearance by bromine addition method. The chain lengths of the polymers were determined by viscometry. It was observed that there was no change in the initiator concentration, [UO₂²⁺], during polymerization. The dependence of the rate of polymerization on variables like light intensity, light absorption fraction by the active species, wavelength, monomer concentration, hydrogen ion concentration, temperature, nature of the acid used (HClO₄ and H₂SO₄), viscosity of the medium etc., were studied. A kinetic reaction scheme is proposed and discussed in the light of the experimental results. Certain rate parameters were calculated. The mechanism of photosensitization by uranyl ions with specific reference to primary photochemical act, initiation of polymerization etc., are discussed.     

Flexibility of Na,K-ATPase secondary structure upon drug-protein interaction

The enzyme Na⁺, K⁺-ATPase is an integral membrane protein which transports sodium and potassium cations against an electrochemical gradient. The transport of Na⁺ and K⁺ ions is connected to an oscillation of the enzyme between the two conformational states, the E₁ (Na⁺) and the E₂ (K⁺) conformations. The enzymatic activity of ATPase is largley affected by different ligands complexation. This review reports the effects of several drugs such as AZT (anti-AIDS), cis-Pt (antitumor), aspirin (anti-inflammatory) and vitamin C (antioxidant) on the stability and secondary structure of Na,K-ATPase in vitro. Drug-enzyme binding is mainly through H-bonding to the polypeptide C=O and C-N groups with two binding constants K_1(AZT) = 5.30 × 10⁵ M^?1 and K_2(AZT) = 9.80 × 10³ M^?1 for AZT and one binding constant for K_cis-Pt = 1.93 × 10⁴ M^?1, K_aspirin = 6.45 × 10³ M^?1 and K_ascorbate = 1.04 × 10⁴ M^?1 for cis-Pt, aspirin and ascorbic acid. The enzyme secondary structure was altered from that of α-helix 19.8% (free protein) to almost 22–26% and the β-sheet from 25.6% to 18–22%, upon drug complexation with the order of induced stability AZT > cis-Pt > ascorbate > aspirin.     

Enhanced chromatographic separation of cobalt and uranyl ions using tin (IV) antimonate column from aqueous solution

The effect of electric field on tin (IV) antimonate column bed to separate cobalt and uranium was investigated. Separation was carried out from nitrate solution and ionic strength of 0.6. Variation of applied potential, time and pH were investigated. Ion mobilities at pH 1 are calculated and given to be 5.5 × 10^?4 and 2.73 × 10^?4 cm² V^?1 s^?1 for cobalt and uranyl ion respectively. Number of theoretical plate heights were calculated from the breakthrough curve and given to be 354 and 210 for cobalt and uranyl ions, respectively. Diffusion coefficient were calculated according to Nernst equation and found to be of 7.6 × 10^?6 and 3.5 × 10^?6 cm² s^?1 for cobalt and uranyl ions, respectively. Also, breakthrough capacities were calculated and found to be 0.7 mmol g^?1 for cobalt ion and 0.4 mmol g^?1 for uranyl ion, respectively.     

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.     

Effect of anion concentration on the stability of the uranyl-ascorbate complex

The effect of anion concentration and the dependence of uranyl ascorbate on the nature of anion present is systematically studied for nine different anions over the concentration range (0.2–2.0) × 10⁻² M. These anions, commonly encountered in pharmaceutical preparations with ascorbic acid (vitamin C) are nitrate, sulfate, chloride, bromide, fluoride, phosphate, citrate, oxalate, and tartrate. Based on the absorbance data, and on the value of the replacement constant K calculated, the studied anions may be arranged according to their complexing power on uranium as follows: citrate > tartrate > phosphate > oxalate > fluoride > sulfate > nitrate > chloride > bromide.This order is substantiated by the calculated values of the side reaction coefficients α_M of the uranyl ligand complex or the conditional stability constant of uranyl-ascorbate calculated at different ligand concentrations.     

New Methylene Blue (NMB) Encapsulated in Mesoporous AlMCM‐41 Material and Its Application for Amperometric Determination of Ascorbic Acid in Real Samples

New methylene blue (NMB) dye incorporated into AlMCM‐41 surfactant‐free and hybrid surfactant‐AlMCM‐41 mesophase. UV‐vis evidence shows that new methylene blue dye protonated in both cases of zeolites. New methylene blue is electroactive in zeolites and their electrochemical activity has been studied by cyclic voltammetry and compared to that of NMB in aqueous solutions. New methylene blue molecules are not released to the solution during CV measurements and are accessible to H₃O⁺ ions. The presence of surfactant affects the kinetics of the redox process through proton ions diffusion. The midpoint potentials (E_m) values show that new methylene blue dye incorporated into AlMCM‐41 can be reduced easily with respect to solution new methylene blue. New methylene blue interacting with surfactant polar heads and residual Br^? ions as a results, it shows a couple of peaks in high potential with respect to new methylene blue solution. The electrode made with methylene blue‐AlMCM‐41 without surfactant was used for the mediated oxidation of ascorbic acid. The anodic peak current observed in cyclic voltammetry was linearly dependent on the ascorbic acid concentration. The calibration plot was linear over the ascorbic acid concentration range 1.0×10^?5 to 5.0×10^?4 M. The detection limit of the method is 1.0×10^?5 M, low enough for trace ascorbic acid determination in various real samples.     

Spectrophotometric Method for the Estimation of Vitamin C in Drug Formulations

Abstract

The use of 1-chloro-2, 4-dinitrobenzene is described for spectrophotometric estimation of ascorbic acid. The procedure is based on the interaction of ascorbic acid with 1-chloro-2, 4-dinitrobenzene in alkaline medium. The product absorbs maximally at 380 nm and has the molar absorptivity 0.14 × 10⁴1 mole^?1cm^?1. Beer's law is obeyed in the concentration range 0.12–0.6 mg/10ml of ascorbic acid.     

Evaluation of the stability constants of uranyl association complexes with chloride, fluoride, bromide, and sulfate anions in solutions of constant ionic strength

Use of ascorbic acid as an indicator for the determination of the classical formation constants of uranyl association complexes with some ligand anions is studied in solutions of 0.05 ionic strength. The results enable the establishment of a sequence of complexation for the anions investigated vis., fluoride, chloride, bromide, and sulfate. This is as follows in order of decreasing complexing power on uranium: F⁻ > SO₄²⁻ > Cl⁻ > Br⁻. The validity of using the indirect ascorbate method for the determination of classical equilibrium constants is established for the uranyl sulfate complex by comparison of the K values with those obtained by the direct spectrophotometric procedure using the continuous variation method. Both procedures indicate the formation of a 1:1 uranyl sulfate complex.     

Construction of a new solid-state U(VI) ion-selective electrode based on polypyrrole conducting polymer

In this study, a potentiometric sensor based on a pencil graphite electrode (PGE) coated with polypyrrole doped with uranyl zinc acetate (termed PGE/PPy/U) have been prepared for potentiometric determination of uranyl in aqueous solutions. Electropolymerization reaction for preparing of U(VI) sensor electrode was carried via applying a constant current of 1.0 mA on PGA working electrode in a solution containing 8.0 mM pyrrole and 0.8 mM ZnUO₂(CH₃COO)₄ salt. The constructed electrode displayed a linear and near Nernstian response (22.60 ± 0.40 mV/decade) to U(VI) ions in the concentration range of 1.0 × 10^?6–1.0 × 10^?2 M. A detection limit of 6.30 × 10^?7 M and a fast response time (≤12 s) was observed during measurements. The working pH range of the electrode was 4.0–8.0 and lifetime of the sensor was at least 60 days. The electrode revealed good selectivity with respect to many cations including alkali, alkaline earth, transition and heavy metal ions. The introduced uranyl electrode was used for measurement of U(VI) ion in real samples without any serious inferences from other ions.     

Electrochemical Behavior of Chloranil Chemically Modified Carbon Paste Electrode. Application to the Electrocatalytic Determination of Ascorbic Acid

A p‐chloranil modified carbon paste electrode was constructed and the electrochemical behavior of this electrode was studied in the aqueous solution with different pH. From the E_1/2–pH diagram for this compound the values of formal potential E^0' and pK_a of some different redox and acid‐base couples depending on the solution pH were estimated. The diffusion coefficient, D, value for p‐chloranil was estimated 1.5×10^?7 cm² s^?1. It has been shown by direct current cyclic voltammetry and double potential step chronoamperometry, that this p‐chloranil incorporated carbon paste electrode, can catalyze the oxidation of ascorbic acid in the aqueous buffered solution. Under the optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 325 mV less positive than that at an unmodified carbon past electrode. The catalytic oxidation peak currents was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 7×10^?5 M–4×10^?3 M of ascorbic acid with a correlation coefficient of 0.9998. The limit of detection (3σ) was determined as 3.5×10 ^?5 M. This method was used as simple, selective and precise voltammetric method for determination of ascorbic acid in pharmaceutical preparations.     

Kinetic studies of the reactions of pentacyanoferrate(III) complexes with L‐ascorbic acid

The reduction of Fe(CN)₅L^2? (L = pyridine, isonicotinamide, 4,4′‐bipyridine) complexes by ascorbic acid has been subjected to a detailed kinetic study in the range of pH 1–7.5. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the ascorbic acid in the form of H₂A(k₀), HA^?(k₁), and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Fe(III) complex. With given Ka₁ and Ka₂, the rate constants are k₀ = 1.8, 7.0, and 4.4 M^?1 s^?1; k₁ = 2.4 × 10³, 5.8 × 10³, and 5.3 × 10³ M^?1 s^?1; k₂ = 6.5 × 10⁸, 8.8 × 10⁸, and 7.9 × 10⁸ M^?1 s^?1 for L = py, isn, and bpy, respectively, at μ = 0.10 M HClO₄/LiClO₄, T = 25°C. The kinetic results are compatible with the Marcus prediction. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 126–133, 2005     

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.     

Electrochemistry and electrocatalytic activity of polypyrrole/ferrocyanide films on a glassy carbon electrode

Functionalized polypyrrole films were prepared by incorporation of Fe(CN)₆ ³⁻ as doping anion during the electropolymerization of pyrrole at a glassy carbon electrode from aqueous solution. The electrochemical behavior of the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry. An obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole film with ferrocyanide incorporated was demonstrated by oxidation of ascorbic acid at the optimized pH of 4 in a glycine buffer. The catalytic effect for mediated oxidation of ascorbic acid was 300 mV and the bimolecular rate constant determined for surface coverage of 4.5 × 10⁻⁸ M cm⁻² using rotating disk electrode voltammetry was 86 M⁻¹ s⁻¹. Furthermore, the catalytic oxidation current was linearly dependent on ascorbic acid concentration in the range 5 × 10⁻⁴–1.6 × 10⁻² M with a correlation coefficient of 0.996. The plot of i _p versus v ^1/2 confirms the diffusion nature of the peak current i _p. Received: 12 April 1999 / Accepted: 25 May 1999