Search

Squaric acid (1,2-dihydroxy-3,4-diketo-cyclobutene) is used in a specific reaction with Fe(III) for the spectrophotometric determination of Fe(III) and total iron content. The optimization of the experimental parameters leads to the establishment of a simple, fast and accurate analytical method. The analytical procedure includes mixing ammonium squarate (40 mM), prepared in a phthalate buffer solution of pH 2.7, with the sample and measuring the absorbance at 515 nm. The molar absorptivity of the colored product is 3.95x10(3) L.mol(-1).cm(-1), at 515 nm. Calibration graphs for Fe(III) are rectilinear for 0.5-20 mgL(-1), with a detection limit of 0.3 mgL(-1) and r.s.d. not exceeding 2.5%, for five replicates of a 3.0 mgL(-1) standard solution. The method has been successfully applied to the determination of iron (III) and the total iron content after quantitative oxidation of iron (II). The results for several analyzed samples when compared with those acquired by using the FAAS technique, were found to be in satisfactory agreement. Microchimica Acta

The on-line incorporation of cloud point extraction (CPE) to flow injection analysis (FIA) is modified to extract and preconcentrate metal species rapidly, avoiding the formation of hydrophobic complexes, using a mixed micellar medium. Coupling the procedure with chemiluminescence (CL) detection based on the catalytic activity of metal species on the luminol-hydrogen peroxide reaction and enhancing the signal with the presence of a micellar carrier containing bromide ions produces a powerful tool for the preconcentration and determination of metal species at ng l(-1) levels. As an analytical demonstration ultratrace concentrations of chromium were conveniently detected and quantified in samples with a complex matrix such as seawater and wastewater. The figures of merit for the determination of chromium were: 0.9-1.6% R.S.D. (n = 5) with detection and quantification limits 0.5 and 2.0 ng l(-1), respectively. The calibration graph was rectilinear from 2 to 200 ng l(-1) (r = 0.9996, n = 6). Several other metal ions were determined in ideal situations, with analogous results. (C) 2002 Elsevier Science B.V. All rights reserved. Analytica Chimica Acta

A selective and sensitive method for the determination of the total amount of vanadium in nutritional and biological substrates is proposed. The method is based on the reaction of vanadium with 2-alpha-pytidylthioquinaldinamide (PTQA) in the presence of H2O2. The product of this reaction emits constant fluorescence, in a sulfuric acid environment, at 490 nm, with the exciting radiation set at 340 run. Various parameters such as acidity, flow rate, solvents, and temperature were studied. The presence of a surface-active agent was also considered in order to increase sensitivity. At the optimal conditions, a calibration curve was constructed, revealing a linear range of 2-100 mug L-1 and a detection limit as low as 0.5 mug L-1 while the RSD ranged in the area of 0.1-1.8%, depending on vanadium concentration. The method was successfully applied to the analysis of a wide variety of food samples, which are known to contribute to the dietary required amount of vanadium and to relevant biological matrixes. Reversing the conditions of the above reaction, the effect of the peroxy group on the vanadium-PTQA system was examined. The formation of a vanadyl complex was revealed which was suitable for the determination of hydrogen peroxide and peroxy acids. linear calibration curves in the range of 0.2-50 muM for H2O2 and 0.1-2 muM for a respective peroxy acid were obtained, yielding detection limits of 0.05 and 0.03 muM, respectively. Anal Chem

A novel, water-insoluble, salt of alpha -Keggin-type silicotungstic heteropolyacid with hexadecylpyridinium cation (SiW(12)Hex) was synthesized. The salt was immobilized onto the surface of spectroscopic graphite electrodes through physical adsorption from acetone solution to produce chemically modified electrodes (SiW(12)Hex-CMEs). Some preliminary electrochemical data of soluble silicotungstate salts as well as of the modified electrode are given. Parameters such as the roughness of the electrode surface, the surface coverage and pH effect were investigated. The modified electrode was employed for the amperometric detection of periodate through its electrocatalytic reduction to iodate at -0.40V (vs. Ag-/AgCl) at pH 4.5 and for the indirect determination of ethylene glycol in antifreezing fluids. Linear plots of cathodic current versus the ethylene glycol concentration in the range of 0.2-5 mM ethylene glycol were obtained. An accuracy better than 2.1% and a reproducibility (relative standard deviation) of 1.9% (n = 10) for a 5 x 10(-4) M standard solution of ethylene glycol were achieved. The recovery was 98-101 %. Electroanalysis

The catalytic effect of vanadium on the pyronine B-H2O2 system is examined. Enhancement of the catalytic reaction rate along with the efficiency and selectivity against vanadium is achieved in a formic acid environment in the presence of a nonionic surfactant (Triton X-114). Elimination of drastic interference caused by inorganic acids and aqueous matrix along with a 50-fold preconcentration of vanadium are facilitated through cloud point extraction of its neutral complex with 8-quinolinol in an acidic solution. Subsequent flow injection analysis (FIA) with fluorometric detection renders the proposed method ideal for selective and cost-effective determination of as little as 0.020 mug L-1 vanadium in environmental, biological, and food substrates. The preconcentration step can be applied simultaneously to multiple samples, allowing for massive preparation prior to analysis, compensating, thus, for the time-consuming procedure. Anal Chem

In this work a kinetic spectrophotometric method for,I the determination of trace amounts of vanadium is presented. The method is based on the catalytic effect of the vanadium(V) on the reaction between diphenylamine (DPhA) and hydrogen peroxide in a concentrated solution of formic acid. The formation of the deep-blue oxidation product is followed by a filter spectrophotometer, equipped with an optical fiber assembly, on line with a PC provided with the suitable software. The measurements were taken at 583 nm, with an immersed type cell of 1 cm light path length. The optimization of the operating conditions regarding concentrations of the reagents, temperature and interferences are also investigated. The working curve is linear over the concentration range 0.40-4.0 mug/ml vanadium(Ti). The relative standard deviation for a standard solution of 0.6 mug/ml of vanadium is 0.5% (n = 5). The proposed method proved highly sensitive, selective and relatively rapid for the assay of vanadium, at low level of 0.40 mug/ml without any pre-concentration step. The method was applied to alloys and cosmetics samples. The results were compared to those received with a reference method. Good agreement was attained, with a mean error of 0.5%. Mikrochimica Acta

A relatively simple, sensitive, selective, automatic fluorimetric method for the simultaneous determination of chromium(III) and chromium(vr) by flow injection analysis (FIA) was developed, The method is based on the selective oxidation of the non-fluorescing reagent 2-(alpha-pyridyl)thioquinaldinamide (PTQA), which with Cr-VI yields an intensely fluorescent product (lambda(ex) = 360 nm; lambda(em) = 300 nm), Chromium(III) is oxidized on-line to Cr-VI with sodium metaperiodate and the Cr-VI is subsequently treated with PTQA, Fluorescence due to the sum of Cr-III and Cr-VI is measured and Cr-III is determined from the difference in fluorescence values. The effects of various analytical parameters, such as acidity, flow rate, sample volume, temperature, reagent concentration and interfering species, were studied. Kinetic studies using both the stopped-flow technique and the FIA procedure were utilized in order to study and optimise the oxidation conditions for Cr-III On th, basis of its oxidation efficiency. The calibration graphs were rectilinear in the ranges 0.1-10 mu g ml(-1) for Cr-VI and 0.1-1.0 mu g ml(-1) for Cr-III, The method was successfully verified by performing recovery experiments of Cr in several standard reference materials (peach leaves, sediments and tea), and it was applied to the speciation analysis of Cr-III-Cr-VI in environmental waters (mineral, tap and distilled water), a food sample (tomato juice) and synthetic mixtures. Up to 30 samples per hour can be analyzed with a relative standard deviation of about 0.1-2%. Analyst