Kinetic studies on the stripping of thorium (IV) and uranium (IV) from tributylphosphate-kerosene with dilute nitric acid solution

The cold solution of a pyrophosphate kit was prepared and dispensed into 2-ml fractions which were stored at −20°C for one week. The solution was prepared by using two protection methods. The first was nitrogen purging to exclude air and the second one the addition of the well known antioxydant ascorbic acid. The aim was to determine the stability of^99mTc(Sn)-PyP obtained by labeling these fractions prepared under the given experimental conditions. The content of^99mTc-pertechnetate in the unprotected samples raises with time. Nitrogen purging provides some protection but already after a few days the content of pertechnetate exceeds 5%. The addition of ascorbic acid gave good results. The content of pertechnetate was 1–2% after seven days in the presence of 50 μg of acid per ml of the cold kit.     

Effect of experimental conditions on the in vitro stability of99mTc (Sn)-pyrophosphate

The in vitro stability of^99mTc (Sn)-PyP as a function of experimental conditions of the preparation of the kit and time elapsed after labeling has been tested. The preparation was protected by using nitrogen-purged reactant solutions and kit vials and by ascorbic acid. The samples under nitrogen are stable for 6 h when the content of^99mTc-pertechnetate raises to 5%. The best stability was achieved by addition of 5 g of ascorbic acid per ml of the kit (content of^99mTc-pertechnetate about 0.5%). To accelerate the decomposition, exogenous hydrogen peroxide was used. In this case it was found that the presence of 10 g of ascorbic acid inhibits the effect both of oxygen and peroxide (6 g H₂O₂/ml of the kit). Radiochemical purity of^99mTc (Sn)-PyP remains practically unchanged for 6 h (content of^99mTc-pertechnetate about 0.5%).     

Monitoring of vitamin C species in aqueous solution by flow injection analysis coupled with an on-line separation with reversed-phase column

Two vitamin C species of ascorbic acid and dehydroascorbic acid in aqueous solution were monitored by flow injection analysis. Ascorbic acid and dehydroascorbic acid were resolved by a reversed-phase column, and dehydroascorbic acid was reduced to ascorbic acid by an on-line post-column reaction with dithiothreitol. Both natural and reduced ascorbic acids were photometrically detected at 260 nm, and the two vitamin C species were simultaneously determined. The determination range was from 0 to 8 × 10⁻⁵ M with a limit of detection of 1.7 × 10⁻⁶ M. The proposed method was applied to the conversion monitoring of ascorbic acid and dehydroascorbic acid in weakly acidic to weakly alkaline aqueous solutions, as well as to the determination of the vitamin C in some beverage samples.     

Anion-exchange high-performance liquid chromatographic determination of ascorbic acid and hexavalent chromium in rat lung preparations after treatment with sodium chromate in vitro and in vivo

Simultaneous analysis of ascorbic acid and chromium (VI) in soluble fractions and bronchoalveolar lavage fluids of rat lungs treated with sodium chromate in vitro and in vivo was performed by anion-exchange high-performance liquid chromatography coupled to a photodiode-array detector. Absorbances at 265 and 370 nm were used for the determination of ascorbic acid and chromium (VI), respectively. The calibration graphs of standard solutions were linear in the test ranges of ascorbic acid an chromium (VI) (below 10 and 8 ppm, respectively). The detection limits of ascorbic acid and chromium (VI) were 1 and 0.5 ng, respectively. The recovery of ascorbic acid from lung tissues homogenized at pH 7.4 was 99%, and that of chromium (VI) was 96%, when tissues were homogenized under alkaline conditions (pH 11.4). Using this method, ascorbic acid levels in the soluble fractions and lavage fluids of normal rat lungs were determined. In the lung of a rat intratracheally injected with a saline solution of sodium chromate, ascorbic acid decreased to 80% of the normal level, and ca. 90% of the chromium (VI) was reduced within 4 min after injection, indicating that the ascorbic acid-related reduction of chromium (VI) is very rapid. The present method will be useful for studies of the reduction of chromium (VI) by ascorbic acid in biological systems.     

INVESTIGATION ON THE RELATIONSHIP BETWEEN INTRINSIC VISCOSITY AND MOLECULAR WEIGHT OF POLY (VINYL CHLORIDE-DIETHYL MALEATE)

A poly (vinyichloride-diethyl maleate) copolymer has been fractionated by repeated precipitation method. All fractions and the unfractionated sample have been characterized by viscometry, dynamic osmometry, Zimm static osmometry, light scattering and gel permeation chromatography. After correction for polydispersity, a [η]～M relationship for monodisperse polymer solutions has been obtained:[η]=1.99×10~(-3)M~(0.87) (ml/g, at 25℃, in cyclohcxanone)For the copolymer solution in THF, the second virial coefficient A_2 decreases as the molecular weight increases. The relationship isA_2=2 slope ((?)_n RT)~(-1/2).     

Formation and reduction of molybdothorophosphoric acid

The stoichiometry of formation and the kinetics of reduction of molybdothorophosphoric acid in nitric acid solutions were studied spectrophotometrically. The Mo:Th:P ratio in the complex is 12:1:1, assuming that Mo(VI) exists in acid solutions as a dimer. The ternary heteropoly acid is more stable and is reduced more slowly to the heteropoly blue than 12-molybdophosphoric acid. The kinetics of the first stages of the reduction were studied with ascorbic acid and p-methylaminophenol as reductants. The experimental rate law is in good agreement with a mechanism involving an equilibrium step in formation of the heteropoly acid followed by the reduction. Under recommended conditions, Beer's law was obeyed up to 40 mg l^?1 thorium.     

Experimental study on equilibrium solubility (at low partial pressures), density, viscosity and corrosion rate of carbon dioxide in aqueous solutions of ascorbic acid

In this paper, ascorbic acid as a new carbon dioxide (CO₂) absorbent was investigated. The equilibrium solubility of CO₂ into 0.5, 1 and 1.5 mol dm⁻³ (M) aqueous ascorbic acid solutions were measured experimentally with a stirred batch reactor at total atmospheric pressure over the CO₂ partial pressure ranging from 0 to 45 kPa and temperatures between 298 and 313 K. The results of the gas solubility are presented as loading capacity (mol CO₂/mol ascorbic acid) as function of partial pressure of CO₂ for all experimental runs. Experimental results showed that solubility of CO₂ increases with increase in molar concentration of ascorbic acid solution at a given temperature and decreases with increase in temperature at a given concentration. The densities and viscosities of the ascorbic acid solutions were measured at the same conditions of the solubility measurement. Some corrosion rate tests were also performed on carbon steel at temperature of 308 K. It was observed that viscosity and corrosion rate increase when the molar concentration of ascorbic acid solution increases.     

Spectrophotometric method for the determination of paraquat in water, grain and plant materials

A sensitive spectrophotometric method for the determination of paraquat using ascorbic acid (an easily available reducing agent) is described. Paraquat is reduced with ascorbic acid in alkaline solution to give a blue radical ion with an absorbance maximum at 600 nm. Beer's law is obeyed in the range 12-96 micrograms of paraquat in 10 ml of the final solution (1.2-9.6 ppm). The important analytical parameters and the optimum reaction conditions were evaluated. The method was applied successfully to the determination of paraquat in water, grain and plant materials.     

Determination of beryllium in urine by graphite-furnace atomic absorption spectrometry

A method has been developed for the determination of beryllium in urine by graphite-furnace atomic absorption spectrometry. Ammonium 12-molybdophosphate and ascorbic acid were employed as a matrix modifier. The tolerable charring temperature for beryllium in both aqueous solution and urine was raised to 1400 °C in the presence of a matrix modifier. The sensitivity for the determination of beryllium was also improved by a factor of 1.5 in comparison with that obtained by using magnesium nitrate as a matrix modifier. The mechanism of the enhancement effect of ammonium molybdophosphate was ascribed to the effectiveness of the formation of gaseous BeO, which is a precursor of free beryllium. Beryllium in urine can be determined simply by dilution with ascorbic acid solution. The relative standard deviation for seven replicate determinations of beryllium was 1.9% for a urine sample containing 0.029 μg ml^?1 of beryllium.     

Application of differential pulse polarography to the assay of ascorbic acid

A polarographic method for the determination of ascorbic acid in pharmaceutical preparations and fruit juices is described. The method is based on the oxidation of ascorbic acid by iodine, and the resulting iodide, after removal of excess iodine, is oxidized with bromine to iodate which is measured polarographically. The method enables analysis of solutions as dilute as 1.7 × 10^?6M ascorbic acid.     

Spectroscopic and tensiometric considerations on anionic surfactants (SDS) and ascorbic acid/ascorbates interactions

The cosmetics and personal care products market is developing dynamically and the formulations of products are subject to numerous modifications due to consumer expectations and market trends. An increase in demand for products containing ascorbic acid or its more stable forms has been observed for several years. In this study, the correlation between SDS and ascorbic acid or its derivatives in aqueous solutions was assessed using spectroscopic and tensiometric methods. The influence of SDS interactions on the total antioxidant capacity of ascorbic acid/ascorbic acid derivatives was analyzed. The highest FRAP values of SDS-AA2G and SDS-EAC systems (0.17 and 2.71 mmol Fe²⁺/g, respectively) were at SDS concentration of 0.40 mM and were much higher than the ones of pure ascorbic acid derivatives. Based on the surface tension isotherms and Szyszkowski's equation, it was found that for the tested SDS/ascorbic acid, ascorbyl glucoside and 3-O-ethyl ascorbic acid systems, the CMC values were: 3.85, 2.54 and 3.31 mmol/dm³. There was also an influence on the values of the adsorption parameters, i.e. the surface excess at the saturated interface, the minimum molecular area in the adsorption layer at the saturated interface and the Gibbs free energy of adsorption. Moreover, in most of the analyzed cases, the tested systems had better wetting properties, which resulted in lower values of the contact angle than in the case of pure SDS solution.     

Fast spectrophotometric determination of titanium in rocks with 3,4-dihydroxybenzoic acid

A fast Spectrophotometric method has been developed for titanium determination in geological matrices, based on the mixture of the sample solution with an exact volume of a single chromogenic solution containing acetate buffer, ascorbic acid and 3,4-dihydroxybenzoic acid DHB, which forms with titanium(IV) ions a yellow complex with absorption maximum at 380 nm. The following parameters were studied: complex stability, pH effect, amount of DHB, amount of acetate buffer, obedience to Beer's law, amount of ascorbic acid and iron masking. The results demonstrated that titanium can be determined in the pH range 4.0–5.0, with a molar absorptivity of 1.43 × 10⁴ 1·mol^–1 cm^–1 and a limit of detection of 2.3 ng/ml. The methodology that allows analysis of 30 samples per hour. Common anions and cations do not interfere, even when present in large amounts. Iron(III) interference can be easily eliminated by reduction to iron(II) using ascorbic acid. Analytical characteristics of the proposed procedure, such as calibration sensitivity, analytical sensitivity, limit of detection and coefficient of variation, were determined. The procedure was applied for titanium determination in various standard geological matrices, with results of satisfactory accuracy and precision (RSD<1%).     

Analysis of Th, U, Pu, and Am in radioactive metal waste using extraction chromatography

In order to analyze actinide elements in radioactive metal waste, the dissolution and chemical separation conditions were optimized. The surfaces of a type 304 stainless steel plate and of pipe waste sampled from the prototype advanced thermal reactor (Fugen) were dissolved in mixed acid solution (HNO₃:HCl:H₂O = 1:1:4). The resulting solution was evaporated to dryness and dissolved with 2 mol/dm³ of HNO₃ to prepare sample solutions. In order to analyze trivalent actinide elements in the sample solution containing a large amount of Fe(III) (>0.1 g) using TRU resin, the effect of Fe(III) concentration on the recovery of Am(III) and reduction effect of Fe(III) to Fe(II) with ascorbic acid were studied. On the basis of results of this study, chemical separation scheme was constructed and Pu and Am in the sample solutions were separated. Thorium and U in the sample solutions were separated with UTEVA resin. High recoveries for all experimented elements were obtained from the analysis of spiked sample solutions, the effectiveness of the method was confirmed.     

Electrothermal atomic absorption spectrometric method for the determination of vanadium in diesel and asphaltene prepared as detergentless microemulsions

A method based on electrothermal atomic absorption spectrometry for the determination of vanadium in diesel and asphalthene fractions is proposed. In order to avoid analyte losses observed at the microgram per liter range for metal traces in organic solutions, diesel samples were stabilized as detergentless microemulsions by mixing with propan-1-ol and nitric acid solution. The solid asphaltene oil fraction was separated and dissolved in dichloromethane before mixing these solution with propan-1-ol and nitric acid solution. Wall atomization as well as no modifier was used. For diesel, aqueous analytical solutions could be used for calibration. For asphaltene, calibration was performed with analytical solutions prepared at the dichloromethane+propan-1-ol+nitric acid medium, spiked with inorganic standard solution. Linear ranges up to 200 μg l⁻¹ were observed, as well as limit of detection of 5 μg l⁻¹ and 4 μg g⁻¹ for diesel and asphaltene, respectively. Good recoveries were obtained for V-cyclohexanebutyrates spiked diesel samples, as well as coherent results for the asphaltene fraction of the NIST 1634c (trace elements in fuel oil) certified reference material.     

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.     

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 simple method for fabrication of sole composition nickel hexacyanoferrate modified electrode and its application

Nickel hexacyanoferrate film modified gold electrode was prepared by a simple chemical deposition procedure from a fresh prepared solution containing ferricyanide, Ni²⁺, and sodium nitrate. The resultant films have solo composition and are significantly stable as compared to the electrochemically deposited NiHCF films. For different concentrations of Na⁺ in the solution, the formal potential values of NiHCF shift according to the Nernstian behavior with a slope of 48 mV in the range of 10⁻⁴ to 1.0 M. The NiHCF film was also used for the electrocatalytic oxidation of ascorbic acid. The anodic peak current observed in cyclic voltammetry increased with the ascorbic acid concentration. At a fixed potential under hydrodynamic conditions, the calibration plot was linear over the ascorbic acid concentration range 0.1-12 mM.     

Measurement of catecholamines, their precursor and metabolites in human urine and plasma by solid-phase extraction followed by high-performance liquid chromatography with fluorescence derivatization

A high-performance liquid chromatographic method is described for the determination in human urine and plasma of catecholamines, their precursor and metabolites [amino compounds (norepinephrine, epinephrine, dopamine, normetanephrine, metanephrine, 3-methoxytyramine and L-DOPA), acidic compounds (3,4-dihydroxymandelic acid, 3,4-dihydroxyphenylacetic acid, vanillylmandelic acid and homovanillic acid) and alcoholic compounds (3,4-dihydroxyphenylethyleneglycol and 4-hydroxy-3-methoxyphenylethyleneglycol)]. Urine (0.5 ml) containing 3,4-dihydroxybenzylamine and 4-hydroxy-3-methoxycinnamic acid (internal standards) is deproteinized with perchloric acid, and the resulting solution is fractionated by solid-phase extraction on a strong cation-exchange resin cartridge (Toyopak IC-SP S) into two fractions (amine fraction and acid-alcohol fraction), which include 3,4-dihydroxybenzylamine and 4-hydroxy-3-methoxycinnamic acid, respectively. Plasma (0.7 ml) is deproteinized in the presence of 3,4-dihydroxybenzylamine (internal standard) in the same manner, and the resulting solution is directly used as an acid-alcohol fraction, while an amine fraction is obtained as for urine. Each fraction is subjected to the previously established ion-pair reversed-phase chromatography with post-column derivatization involving coulometric oxidation followed by fluorescence reaction with 1,2-diphenylethylenediamine. The detection limits, at a signal-to-noise ratio of 5, of the compounds measured in urine are 300 pmol/ml for the two mandelic acids, 2-7 pmol/ml for the other acidic and alcoholic compounds, 12 pmol/ml for L-DOPA and 0.6-2 pmol/ml for the other amino compounds; the corresponding values for plasma samples are 80, 0.5-3, 10 and 0.6-3 pmol/ml, respectively.     

Preparation and evaluation of two 188Re-radiopharmaceuticals for endovascular irradiation

Procedures for complexing DTPA with ¹⁸⁸Re from a ready kit and by conventional manipulation were elaborated and the study of the stability and biodistribution of ¹⁸⁸Re perrhenate and ¹⁸⁸Re-DTPA were performed. Best labeling was achieved using DTPA (38 mM) with 2 mg/ml of SnCl₂ ·2H₂O. The radiochemical purity was 95.9±2.9%. The complex was stable for 24 hours when ascorbic acid was used. Greatest uptake of ¹⁸⁸Re-DTPA was by kidneys (19.3±2.1%ID/g) and for ¹⁸⁸Re-perrhenate by stomach (21.3±2.8%ID/g). In conclusion a kit of freeze-dried DTPA was developed. Organ damage is unlikely by virtue of its rapid urinary excretion. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.