Denise Lowinsohn

Determination of sulphite in wine by coulometric titration

A method for the determination of both free and bound S(IV) in white wine samples by coulometric titration with electrogenerated iodine is described, the S(IV) speciation being performed by extracting SO2 from samples acidified with hydrochloric acid. Titrations of samples treated with NaOH led to the total S(IV) concentration, the results being in agreement with the ones obtained by the distillation procedure as well as by using the classical Monier-Williams method. An average recovery of 97% was obtained for samples spiked with S(IV) in the range 0.4–1.2 mM, which demonstrated the validity of the proposed procedure. The repeatability was 1.3% and the limit of detection was estimated as 0.01 mM. Results from direct titrations of wine added to the electrochemical cell are reported, which give indications of the stability of adducts towards the reaction with iodine.

Sensores eletroquímicos: considerações sobre mecanismos de funcionamento e aplicações no monitoramento de espécies químicas em ambientes microscópicos

Electrochemical sensors have attracted considerable attention in recent years because they provide data about the chemical state of our surroundings and the dynamics of the chemical transformations in the form a spatially resolved image. Particular interest has been directed to measurements in restricted-volume samples as new technologies enable the fabrication of miniaturized versions of sensors with reproducible characteristics. Taking these aspects into consideration, this review focuses on the use of electrodes of micrometer dimensions to acquire chemical information in microdomains in which concentrations may not be spatially homogeneous. This is possible because microelectrodes allow fast-response measurements with micrometer resolution to be performed. On the other hand, the use of microelectrodes as amperometric sensors presents an inherent drawback owing to the insufficient specificity toward the substrate of interest. Hence, some comments on strategies to enhance the selectivity of amperometric sensors are also made. Finally, recent applications of structurally microscopic electrodes as in vivo sensors are shown, as well as a prospect of the future trend in this field.

Sulfite induced autoxidation of Ni(II) and Co(II) tetraglycine complexes. Spectrophotometric and rotating ring-disc voltammetric studies

The oxidation of Ni(II) and Co(II) tetraglycine complexes in borate buffer aqueous solution, by dissolved oxygen, is strongly accelerated by sulfite. The formation of Ni(III) and Co(III) complexes with maximum absorbances at 327 and 265 nm, respectively, was followed by spectrophotometric measurements. Ni(III) formation was also characterized by voltammetry at low temperatures, whose anodic and cathodic components were observed in the recorded voltammograms. Spectra and rotating ring-disc voltammograms, recorded at various rotation speed values, showed that the Ni(III) species decomposes. The electrochemical process related to the couple Co(II)/Co(III), in a medium containing tetraglycine, was not reversible. In both Ni(II) and Co(II) complexes the metal ion oxidation in the presence of oxygen and sulfite involves the reduction of some initial Ni(III) or Co(III) by sulfite to produce the SO(3).- radical, which rapidly reacts with dissolved oxygen to produce SO(5).-, which then oxidizes Ni(II) or Co(II).

Comparative Studies on the Electrochemical Behavior of Mo(VI) at Mercury and Glassy Carbon Electrodes

The electrochemistry of Mo(VI) in sulfuric acid solutions has been investigated at mercury and glassy carbon surfaces, the adsorption of Mo(VI) and electrogenerated products being evident at mercury electrodes. The adsorption phenomenon severely complicates the electrochemical behavior and depressed voltammograms and distorted chronoamperometric curves are obtained at relatively high Mo(VI) concentrations (>1 mM). The influence of experimental parameters on the adsorption of molybdenum species onto the mercury surface such as electrode potential, acidity and time of adsorption was studied by measuring the amount of molybdenum by atomic absorption spectrophotometry. The catalytic process observed when nitrate or chlorate are added to the supporting electrolyte containing Mo(VI) does not appear when glassy carbon is used as working electrode. A dimeric Mo(V) species is formed when potentiostatic coulometric experiments are performed using mercury as a substrate. On the other hand, a black solid constituted by a nonstoichiometric molybdenum compound is deposited at the glassy carbon surface in similar experiments. The concomitant predissociation of molecular hydrogen during the cathodic reduction of Mo(VI) at glassy carbon electrodes, followed by migration of H atoms into the MoO3 lattice, is responsible for the deposit formation.

A biosensor based on immobilization of lactate oxidase in a PB-CTAB film for FIA determination of lactate in beer samples

An amperometric lactate biosensor with lactate oxidase immobilized into a Prussian Blue (PB) modified electrode was fabricated. The advantage of using cetyltrimethylammonium bromide (CTAB) in the electrodeposition step of PB films onto glassy carbon surfaces was confirmed taking into account both the stability and sensitivity of the measurements. The biosensor was used in the development of a FIA amperometric method for the determination of lactate. Under optimal operating conditions (pH = 6.9, E = -0.1 V), the linear response of the method was extended up to 0.28 µmol L-1 lactate with a limit of detection of 0.84 mmol L-1. The repeatability of the method for injections of a 0.28 mmol L-1 lactate solution was 2.2 % (n = 18). The usefulness of the method was demonstrated by determining lactate in beer samples and the results were in good agreement with those obtained by using a reference spectrophotometric enzyme method.

Autoxidation of Ni(II) and Co(II) tetra, penta and hexaglycine complexes accelerated by oxy sulfur radicals

The autoxidation of Ni(II) and Co(II) complexes with tetra, penta and hexaglycine, in borate medium, is accelerated by sulfur(IV) species (H2SO3, HSO3- and SO32-). The formation of Ni(III) and Co(III) complexes was followed spectrophotometrically at 325 and 265 nm, respectively. Electrochemical techniques were also employed to characterize the generation of these complexes. The autoxidation rate increases with S(IV) concentration and is maximum at pH @ 8.5. The process is autocatalytic with Ni(III) or Co(III) acting as initiators, formed by spontaneous oxidation by oxygen. The dependence of the pseudo first order rate constant with sulfite concentration showed evidences of back or parallel reactions with formation of mixed ligand complex prior to the oxidation step.