Voltametria de onda quadrada. Primeira parte: aspectos teóricos

The theoretical aspects of square wave voltammetry were discussed. Reversible, irreversible and quase-reversible electrode reactions were analyzed and the correlations between parameters like frequency, period, square wave potential and amplitude were showed. In this way, diagnostic relationships allow to characterize the electrode process. The analytical applications were discussed in base of the increment in the analytical response (current) due to the characteristics of the developed equations and the unique mode of collecting the electrode response, i.e., the direct and reverse signals. Finally, recent advances in the basic theory, as the applications to the hydrodynamic electrode and the ultramicroelectrode were also analyzed, and the multiple pulses square wave voltammetry was also introduced.

Estudo eletroanalítico do herbicida paraquat em soluções aquosas por voltametria de onda quadrada utilizando ultramicroeletrodos

The electrochemical behavior of paraquat on Pt, Au and carbon fiber ultramicroelectrodes were studied in laboratory samples by square wave voltammetry at high frequencies. The results showed two reversible peaks for paraquat reduction, in agreement to the literature data. The first peak was associated to the reduction of paraquat molecule in solution, with the further adsorption of the intermediate on the electrode surface. This adsorbed species undergoes to electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters showed the best conditions to reduce paraquat as pH 5.0, frequency as high as 1000 s-1, scan increment of 2 mV and square wave amplitude of 50 mV. At such conditions, a variation of paraquat concentrations from 4.3 x 10-6 to 1.66 x 10-4 mol L-1 presented values for the detection limit equal to 3.9, 6.2 and 20.3 ppb on Pt, Au and carbon, respectively, at 1000 s-1. These values are quite below17 the allowed limit of paraquat in drinking water.

Argilas especiais: argilas quimicamente modificadas - uma revisão

The simultaneous use of the specific values of some structural and chemical properties of clay minerals, such as kaolinite, montmorillonite and talc, allows the development of new properties for these materials, especially in relation to the external and internal microcrystal surfaces. These developments are very diversified for montmorillonite, due to the high specific surface area, expansible basal spacings, easy intercalation inside the 2:1 structural layers and a reversible and high cation exchance capacity. The review presents examples of chemical modifications on kaolins, montmorillonites (bentonites) and talcs.

Dinâmica complexa no sistema bromato/hipofosfito/acetona/manganês e ferroína

New chemical systems have been recently designed for the study of complex phenomena such as oscillatory dynamics in the temporal domain and spatiotemporal pattern formation. Systems derived from oscillators based on the chemistry of bromate are the most extensively studied, with the celebrated Belousov-Zhabotinsky (BZ) reaction being the most popular example. Problems such as the formation of bubbles (CO2) and solid precipitate in the course of the reaction and the occurrence of simply short-lived oscillations under batch conditions are very common and, in some cases, compromise the use of some of these systems. It is investigated in this paper the dynamic behavior of the bromate/hypophosphite/acetone/dual catalyst system, which has been sugested as an interesting alternative to circumvent those inconvenients. In this work, manganese and ferroin are employed as catalysts and the complete system (BrO3-/H2PO2-/acetone/Mn(II)-ferroin) is studied under batch conditions. Temporal symmetry breaking was studied in a reactor under agitation by means of simultaneous records of the potential changes of platinum and Ag/AgBr electrodes, both measured versus a reversible hydrogen electrode. Additionally, spatio-temporal formation of target patterns and spiral waves were obtained when the oscillating mixture was placed in a quasi two-dimensional reactor.

Anestésicos locais: interação com membranas biológicas e com o canal de sódio voltagem-dependente

Many theories about the mechanism of action of local anesthetics (LA) are described in the literature. Two types of theories can be distinguished: those that focus on the direct effects of LA on their target protein in the axon membranes, i.e. the voltage-gated sodium channel and the ones that take into account the interaction of anesthetic molecules with the lipid membrane phase for the reversible nerve blockage. Since there is a direct correlation between LA hydrophobicity and potency, it is crucial to take this physico-chemical property into account to understand the mechanism of action of LA, be it on the sodium channel protein, lipid(s), or on the whole membrane phase.

A primeira lei da termodinâmica dos processos homogêneos

Thermodynamics of homogeneous processes, which corresponds to the very special situation in thermodynamics of continuous media, is used to discuss the first law. An important part of this work is the exposition of some typical mathematical errors, frequently found in the traditional presentation of thermodynamics. The concepts of state and process functions are discussed, as well as reverse and reversible processes, temporality and its implications on thermodynamics, energy reservoirs and symmetry. Our proposal is to present the first law by using a time dependent viewpoint coherent with mechanics and the foundations of that viewpoint.

O experimento de Clement-Desormes no século XXI

The Clement-Desormes experiment is reviewed. By reason of a finite difference between the pressure within the system and its surroundings, Bertrand and McDonald have criticized the usual consideration of the adiabatic expansion as reversible. Garland, Nibler and Shoemaker oppose, defining regions through virtual boundaries where the surroundings do not operate. For Holden, the use of virtual boundaries is expendable. Experiments cannot support a hypothesis testing due to experiment's intrinsic uncertainty. The role of polytropy in uncertainty is discussed. Both thermodynamic definitions and kinetic model depict the real processes as irreversible phenomena and the reversible ones as a limiting hypothetical case.

Electrochemical study, on mercury, of a Meta-nitroarylamine derivative of nor-β-lapachone, an antitumor and trypanocidal compound

The electrochemistry of 2,2-dimethyl-(3H)-3-(N-3'-nitrophenylamino)naphtho[1,2- b]furan-4,5-dione ([Q]-PhNO2), on mercury was investigated. The first peak is consistent with a quasi-reversible one-electron reduction of the ortho-quinone, forming [Q•-]-PhNO2, while the second one, bielectronic, corresponds to the simultaneous reduction of the latter radical to a dianion and the nitro group to a nitro radical anion. The second order rate constant, k disp, for the decay of [Q•-]-PhNO2 is 15.188 x 10³ ± 827 mol"1 L s"1 and the t1/2 equals 0.06 s. E¹7Ic values for [Q]-PhNO2 and its precursor, nor-β-lapachone, are similar. The ease of semiquinone generation and its stability are parameters statistically relevant in the correlation biochemical/theoretical aspects.

Condições de equilíbrio termodinâmico: a função disponibilidade

The thermodynamic equilibrium is a state defined by conditions which depend upon some characteristics of the system. It requires thermal, mechanical, chemical and phase equilibrium. Continuum thermodynamics, its radical restriction usually called homogeneous processes thermodynamics, as well as the classical thermodynamic science of reversible processes, each of them defines equilibrium in a differing way. But these definitions lead to the same physical contents.

Influência do eletrólito na eletrodeposição de nanopartículas de Cu sobre eletrodo de diamante dopado com boro

This paper presents the electrolyte influence on deposition and dissolution processes of Cu nanoparticles on boron doped diamond electrodes (BDD). Morphological, structural and electrochemical analysis showed BDD films with good reproducibility, quality and reversible in a specific redox system. Electrodeposition of Cu nanoparticles on DDB electrodes in three different solutions was influenced by pH and ionic strength of the electrolytic medium. Analyzing the process as function of the scan rate, it was verified a better efficiency in 0,5 mol L-1 Na2SO4 solution. Under the influence of the pH and ionic strength, Cu nanoparticles on DDB may be obtained with different morphologies and it was important for defining the desired properties.

Voltammetric determination of sibutramine in beverages and in pharmaceutical formulations

A simple and sensitive method has been proposed for the determination of sibutramine-HCl in energy drinks, green tea and pharmaceutical formulations using differential pulse voltammetry performed on a hanging mercury drop electrode. In the chosen experimental condition (Mcllvaine pH 4.0 buffer, 50 mV pulse amplitude and 40 mV s-1 scan velocity), sibutramine-HCl presented a reversible behavior and a peak maximum at -80 mV. Detection limit was 0.4 mg L-1 and the working linear range extended up to 33.3 mg L-1 (r = 0.99). Analysis of real and fortified samples enabled recoveries between 91 and 102%. The electroanalytical method was compared with a HPLC method which indicated it accuracy.

Construção de eletrodo de grafite retirado de pilha comum: aplicações didáticas

This paper presents a proposal for using recycled graphite electrodes obtained from exhausted commercial 1.5 V batteries and its application in electroanalysis. The electrode could be prepared by the students and applied in the simple didactic experiments suggested, such as determination of active electrode area, cyclic voltammetry and useful potential range (also called "potential window"), demonstration and effect of scan rate on cyclic voltammograms. The possibility of using the graphite electrode in quantitative analysis was also demonstrated using the ferricyanide/ferrocyanide reversible redox couple ([Fe(CN)6]3-/[Fe(CN)6]4-) as an electrochemical probe by the dependence of peak current with the analyte concentration and flow injection analysis with amperometric detection.

ALCALOIDES ACRIDÔNICOS INIBEM CATEPSINA L E V

Cathepsins represent a class of enzymes that has the primary function of randomly degrading proteins in the lysosomes, although are also involved in different pathologies. The aim of this paper was to evaluate the capacity of acridone alkaloids isolated from Swinglea glutinosa (Rutaceae) to inhibit cathepsin L in vitro . The IC50 values found were in the 0.8-57 µM range and the most promising compounds were alkaloids 1 and 2, with IC50 of 0.9 and 0.8 µM, respectively. Enzyme kinetics revealed that they are reversible competitive inhibitors with respect to the substrate Z-FR-MCA. This small series of acridone alkaloids showed low selectivity for both cathepsins, but represent promising lead candidates for the further development of competitive cathepsin L and V inhibitors.

COMPREENSÃO MATEMÁTICA DA DIFUSÃO NO CONTEXTO ELETROQUÍMICO

The aim of this study was to explain in detail the mathematical methods used to deal with diffusion equations, mainly for students and researchers interested in electrochemistry and related areas. Emphasis was placed on the deduction and resolution of diffusion equations, as well as addressing cartesian, spherical and cylindrical coordinates. Different aspects of mass transfer processes were discussed including the importance of the resolution of Fick's laws equations to understand and derive parameters of the electroactive species (e.g., diffusion coefficients, formal electrode potentials) from the electrochemical techniques. As an example, the resolution of diffusion equations for a reversible reduction process of soluble oxidized species was presented for the chronopotentiometry technique. This study is envisaged to broaden the understanding of these frequently used methods, in which mathematical deductions are not always completely understood.

Stability constants and thermodynamic parameters of cadmium complexes with sulfonamides and cephapirin

Stability constant (log beta) and thermodynamic parameters of Cd2+ complexes with sulfonamide and cephapirin were determined by Polarographic technique at pH = 7.30 ± 0.01 and µ = 1.0 M KNO3 at 250°C. The sulfonamides were sulfadiazine, sulfisoxazole, sulfamethaxazole, sulfamethazine, sulfathiazole, sulfacetamide and sulfanilamide used as primary ligands and cephapirin as secondary ligand. Cd2+ formed 1:1:1, 1:2:1 and 1:1:2 complexes. The nature of electrode processes were reversible and diffusion controlled. The stability constants and thermodynamic parameters (deltaG, deltaH and deltaS) were determined. The formation of the metal complexes has been found to be spontaneous, exothermic in nature, and entropically unfavourable at higher temperature.