Patricia I. Ortiz

EDTA modified LDHs as Cu2+ scavengers: removal kinetics and sorbent stability.

EDTA modified layered double hydroxides (LDHs) were investigated as potential sorbents to remediate heavy metals pollution. The polidentate ligand was introduced by an exchange method in a Zn-Al-LDH, which takes place with partial erosion of the layers, causing the intercalation of [Zn(EDTA)](2-) complex instead of the ligand. [Cu(H(2)O)(6)](2+) cation was selected as a model cation to study the uptake mechanism, exploring the elimination kinetics from the first minutes up to the steady state. A flow injection analysis system coupled to an amperometric detector (FIA-AM) was applied to perform fast and reliable [Cu(H(2)O)(6)](2+) determinations in monodisperse solid-aqueous solution systems. Furthermore, the sorbent stability was determined as a function of the pH and the nitrate concentration. The [Cu(H(2)O)(6)](2+) elimination is produced by an exchange reaction with [Zn(EDTA)](2-) anions placed either in the solid interlayer or in the aqueous solution, this last being released from the sorbent. Additional [Cu(H(2)O)(6)](2+) removal is produced by Cu(OH)(2) precipitation at high copper concentrations due to the LDHs high pH buffering capacity. The sorbent removes [Cu(H(2)O)(6)](2+) with high affinity in a wide concentration range. The elimination process reaches equilibrium in less than 30 min and leaves metal cation concentrations lower than 0.05 ppm in the supernatants.

Electrochemical behaviour of tungsten in alkaline media: Part I. NaOH solutions

Abstract The electrochemical behaviour of tungsten in alkaline NaOH solutions in the pH range 7–13 was investigated using single and repetitive triangular potential sweeeps and the rotating disc electrode. In the −0.7 to 0.3 V (SHE) potential range the anodic process involves the formation of soluble tungstate species and at higher potentials the electroformation of different tungsten oxides occurs up to the completion of the electrooxidation process to give the WO3 oxide. For potentials more positive than 1 V, the anodic WO3 film grows according to a high-field mechanism as in a typical valve metal. The onset of oxygen evolution is observed at ca. 8.7 V. The electroreduction processes correspond to the formation of a tungsten and hydrogen “bronze” on the electrooxidised surface prior to the hydrogen evolution reaction.

Reflectometry applied to electrochemically generated phenoxy radical adsorption monitoring

Abstract This paper reports on a study by reflectometry of the adsorption process of phenolic compounds electrochemically oxidized on glassy carbon electrodes. This technique provides information about the adsorption rate and the maximum adsorbed amount. The adsorption process depends on several variables like adsorbent concentration, solution pH and working potential, while ionic strength has no significant influence. The best adsorption conditions were obtained at high phenolic concentrations (0.1 M), high pH values (>10) and working potential values higher than 0.600 V. The adsorption process on glassy carbon electrodes showed irreversible behavior, reaching the maximum adsorbed amount at relatively short times (less than 100 s).

BHA and TBHQ Quantification in Cosmetic Samples

An amperometric flow-injection method for the determination of the antioxidants BHA, TBHQ and a mixture of them, using a 4-hydroxybenzaldehyde/formaldehyde polymer modified electrode is proposed. The procedure is based on the electrochemical oxidation in phosphate buffer solution (pH 7.2). Different analytical parameters and electrode stability were analyzed to obtain the best electrode performance, being the optimal conditions: working potentials 0.200 V and 0.700 V and flow rate 0.74 mLmin–1. The proposed method was used for antioxidants quantification in a cosmetic sun tanning lotion, without any previous sample treatment and mean recoveries of 104% were obtained. Interferences from other antioxidants (BHT and PG) were also analyzed and in all the cases, no synergic behavior was observed.

Determination of tert‐Butylhydroxytoluene by Flow Injection Analysis at Polymer Modified Glassy Carbon Electrodes

A flow injection method for the determination of the antioxidant BHT using polymer modified electrodes in connection with amperometric detection is proposed. The procedure is based on the electrochemical oxidation at 0.700 V (vs. Ag/AgCl[3M NaCl]) in phosphate buffer solution (pH 7.2). BHT is determined over the range 0.001–0.020 or 0.005–0.030 gL−1. Other analytical parameters, such as electrode working potential, flow rate, electrodeposited charge and solution pH were optimized. The proposed method was applied to the determination of BHT in electrical transformer and vegetable oils samples. The results were compared with the HPLC-UV detection standard technique.

New automatized method with amperometric detection for the determination of azithromycin.

A FIA-amperometric method for azithromycin determination was developed. A working glassy carbon electrode and a Ag/AgCl/NaCl (3M) reference electrode were used. The determination is based on the electrochemical oxidation of the azithromycin at 0.9V in Britton-Robinson buffer solution (pH 8.0). Due to the adsorption of the reaction products on the electrode surface, an effective cleaner cycle was implemented. By using the optimum chemical and FIA conditions, a concentration linear range of 1.0-10.0mgL(-1) and a detection limit (LOD) of 0.76mgL(-1) are obtained. The method was validated and satisfactorily applied to the determination of azithromycin in pharmaceutical formulations.

Electrochemical characterization of glassy carbon electrodes modified by resol mixtures

Abstract The present work aims to obtain more insights into the effects of the electrochemical polymerization of a 4-hydroxybenzaldehyde+formaldehyde (resol) mixture on the electrochemical properties of modified glassy carbon electrodes. Different redox couples, having a permanent positive charge (Fe-phenathroline), a permanent negative charge (K 3 Fe(SNC) 6 ) and either a positive or negative charge, depending on solution pH ( l -Dopa), have been analyzed. Cyclic voltammetry and impedance spectroscopy experiments were performed at different deposited polymer charge and pH values for the proposed redox systems and the comparison with polished electrodes is also analyzed. Electrophoretic mobilities were investigated and acid–base titrations were also carried out in order to prove that the surface is negatively charged. Electrochemical measurements showed an important electrostatic effect between the electrode surface and the couples analyzed. This effect plays an important role on the electrode response when used for analytical purposes.

Electrochemical behaviour of tungsten in alkaline media: Part II. Sodium carbonate solutions

Abstract The electrochemical behaviour of tungsten in alkaline carbonate buffered solutions in the pH range 7.3–12.8 was investigated using single and repetitive triangular potential sweeps and a rotating disc electrode. In the potential range −0.7 to 0.1 V(SHE). tungsten is electrooxidised to tungstate through an overall process controlled by OH − diffusion. At more positive potentials, there is an additional anodic process which corresponds to tungsten electrooxidation to give soluble tungstate species with the participation of the carbonate ion through a chemical reaction preceding the charge transfer reaction. The OH − concentration is regulated by the carbonate equilibrium concentration through the carbonate hydrolysis reaction. In the 0.3 to 1.2 V(SHE) potential range, the anodic reaction can be associated with the electroformation of different tungsten oxides to give the WO 3 oxide. At potentials higher than ca. 1.2 V(SHE), irreversible growth of WO 3 and chemical dissolution of the film take place.

Electrochemical Determination of Ag(I) and Cu (II) Using Activated Carbon Paste Electrodes

Abstract An electroanalytical method for Cu (II) and Ag (I) quantification is described. The determination was performed by cyclic voltammetry on a supporting electrolyte with a previous step of cations accumulation by electrodeposition at constant potential. Linear dependence between the anodic current peaks and the bulk concentration of both cations were obtained: the concentration ranging from 5.0 × 10−8 M to 2.0 × 10−5 M for copper and from 5.0 × 10−9 M to 4.0 × 10−5 M for silver. No interferences of Zn (II), Mn (II), Cr (III), Pb (II) and Ni (II) were observed. The method here proposed offers a reliable, sensitive and practical way to determine copper and silver.

Characterization and application of humic acid modified carbon electrodes

Four routes for the modification of carbon electrodes with humic acids and the determination of three divalent metallic cations were studied. The determination of bound Fe(2+), Cu(2+) and Ni(2+) was performed by cyclic and square wave voltammetry using either a batch or flow analysis system. Using the FIA system and SWV, linear relationships between the oxidation (or reduction) current and the cations concentration were obtained with the modified electrodes, while no signals were obtained for the same conditions for bare carbon electrodes. The system can be used to study the interaction between a wide range of electroactive cations and humic substances; however, the performance as an analytical tool is limited due to the high limits of detection (muM). However, some advantages like simplicity, short analysis time, inexpensive instrumentation needs and miniaturization capabilities are remarkable.