Luis E. León

Electrochemically active films of negatively charged molecules, surfactant and synthetic clays

Abstract The electrochemical behavior of anionic molecules such as hexacyanoferrate and vitamin B 12 hexacarboxilate intercalated into clay–surfactant films is investigated. The results indicate that composites of surfactant cetyltrymethylammonium bromide (CTAB) and synthetic clay facilitate the permeation of the charged molecules through the film. By heating the modified electrode at 120°C for 1 h, the charged molecules, depending on hydrophobic and coulombic forces, are able to accommodate inside the film. Coulombic forces retain the anionic molecules in the clay–CTAB composite. The films are stable with good electroactivity.

Determination of Organic Nitrogen by the Kjeldahl Method Using Microwave Acid Digestion

Abstract Essential aminoacids, vegetables, milk products, seafood, and canned meat were analyzed by the Kjeldahl method to determine their organic nitrogen content. The samples were brought into solution in two different ways, by conventional and by microwave wet acid digestion. The main goal of the study was to assessthe efficiency of the microwave method of sample treatment in regard to the speed of the dissolution process and to the precision and accuracy of the nitrogen determination. The experiments indicate that, in most cases, the precision of the results obtained using the microwave treatment is better than the one obtained by the conventional procedure; the accuracy of both methods of analysis is comparable and the time employed for the sample digestions using microwave heating is shorter.

Comparison of conventional and microwave wet acid digestion procedures for the determination of iron, nickel and vanadium in coal by electrothermal atomisation atomic absorption spectrometry

Certified coal standards and Venezuelan coal samples were submitted to conventional and microwave wet acid digestion procedures to assess and compare the efficiency of both decomposition methods. Several acid mixtures and heating power and time settings were investigated in order to achieve the optimum sample treatment conditions. The resulting solutions were analysed by electrothermal atomisation atomic absorption spectrometry to determine their Fe, Ni and V content. The results obtained showed total recovery of the analytes and good accuracy and precision of the measurements. Sample treatment time was reduced from 11–12 h per sample using conventional heating, to 8–10 min per sample using the microwave-heating source.

Indirect Flow-Injection Analysis of Ascorbic Acid by Photochemical Reduction of Methylene Blue

Abstract Ascorbic acid was determined indirectly by spectrophotometry at 666 nm based on the photochemical reduction of methylene blue using flow-injection analysis. The carrier stream was 12.7 μgml−1 methylene blue in phthalate-HCl buffer solution at pH 3.2. The reactor was irradiated with a 500-Watt halogen lamp to facilitate the development of the photochemical reaction. The system allows determination of ascorbic acid in the range 0.18–6.12 μgml−1 with relative standard deviations of 2.09 and 0.31% for 1.97 and 4.92 μgml−1 samples, respectively, and a sampling frequency of 50–55 h−1. The proposed method was applied successfully to the determination of ascorbic acid in vitamin C tablets.

Amperometric flow-injection method for the assay of l-ascorbic acid based on the photochemical reduction of Methylene Blue

Ascorbic acid (AA) is determined by amperometric detection based on the photochemical reduction of Methylene Blue (MB(+)) in 0.1 M phthalate buffer at pH 3.8. In this medium, MB(+) using flow-injection analysis. The carrier stream is 1 mM MB(+) is reduced quasi-reversibly at a glassy carbon electrode at -0.34 V vs. Ag/AgCl, while AA is oxidized irreversibly at about 0.3 V. The reactor is irradiated with a 500 W halogen lamp to facilitate the development of the photochemical reaction. A laboratory-built wall-jet electrode system was used. The Leucomethylene Blue formed in the reaction is detected at +0.050 V. At 2.2 ml min(-1) and using a sample loop of 43 mul, the method allows the determination of AA in the range 5.0-90.0 mug ml(-1), with a relative standard deviation of 1.3-4.8%, a detection limit of 1.9 mug ml(-1) and a sampling frequency of 45-50 h(-1).

Cyclic Voltammetric Study of the R-Nitroso Salt Application to the Indirect Determination of Iron(II)

Abstract The electrochemical behaviour of the R-nitroso salt in 0.5 M HAc/NaAc at pH 4.7 is studied by cyclic voltammetry. The compound shows a very sharp reduction peak at -0.37 V vs SCE giving the corresponding amine derivative. The height of the peak is shown to be inversely proportional to the iron (II) concentration of the solution. A linear calibration plot is obtained between 0 and 56 ppm of iron (II) using, initially, 4.0 mM solution of the salt. A simple and fast procedure was developed for the analysis of standard samples of ferrous iron.

Indirect determination of iron in a flow-injection system with amperometric detection

Abstract The change in peak current resulting from the reaction of Fe(II) with nitroso-R salt in a flow-injection system is used to quantify Fe(II) with either single- or dual-electrode amperometric detectors. The current change varies linearly with Fe(II) concentration from 0 to 200 mg 1−1. The relative standard deviation was about 5% with the single-electrode detector and about 10% with the dual-electrode detector. The method is evaluated for the determination of iron in dietary supplements.

THE GENERALIZED STANDARD ADDITION METHOD IN A CLOSED-LOOP FLOW SYSTEM

A new approach using flow-injection configuration is proposed as an alternative to apply the generalized standard addition method. The flow system contains a calibrated closed loop in which a spectrophotometric flow-cell is placed. The sample is entrapped into the closed loop by a suitable valve and it is allowed repetitive passages through the flow-cell; the addition of the standard solutions is made through an additional sample valve with a relatively small sample volume. After the addition, transient signals are obtained due to the repetitive passage of the added standard through the detector until it is completely dispersed into the sample by a mixing chamber. The proposed method resembles the manual, conventional procedure; a very small volume of standard solution is added to a relatively large volume of sample in such a way that the matrix of the latter is not significantly modified. Mathematical derivations that relate the concentration of the sample with that of the standard and the number of additions are performed with the analysis of the pH indicator methylorange. The analysis of iron in milk shows that good reproducibility is obtained using only one standard and one addition.

Direct Analysis of Solid Samples by Flameless Atomic Absorption Spectrometry. Application to Some Siliceous Materials

Abstract Fundamental operational parameters like temperature, weight of samples, and time of drying, charring, and atomization are optimized for direct determination of calcium, magnesium, and iron in siliceous materials (feldspar and lime). The use of low intensity absorption lines together with the dispersion of the samples in a graphite matrix, allows the technique to be applied to materials containing up to 2.1% CaO, 0.36% MgO, and 0.75% Fe2O3. The results are compared with those obtained by flame and graphite furnace atomic absorption spectrometry using standard solutions. The correlation found between the three techniques and the reported composition of NBS samples are highly satisfactory.

Indirect Voltammetric Determination of Iron at Parts-Per-Million Levels Using Either Ferron or BPDS

Abstract The electrochemical behavior of ferron and BPDS (bathophenanthrolinedisulfonic acid) in 0.5 M HAc/NaAc at pH 4.7 is studied by cyclic voltammetry at a glassy carbon electrode. Ferron shows a very sharp oxidation peak at + 0.74 V vs SCE while BPDS shows a sharp reduction peak at - 1.03 V. The heights of the peaks are shown to be inversely proportional to the iron (III) and iron (II) concentration of the solution, respectively. Linear calibration plots are obtained between 5 and 60 ppm of iron (III) for ferron and 0 to 60 ppm of iron (II) for BPDS using, initially, 4.00 mM solution of each reagent. A simple and fast procedure was developed for the analysis of iron content in pharmaceutical samples. The results are comparable with those found by spectrophotometry.