The aim of this work is to obtain polymer precursors based on styrene copolymers with distinct degrees of sulfonation, as an alternative material for fuel cell membranes. Acetyl sulfate was used to carry out the sulfonation and the performance of the polyelectrolyte was evaluated based on the content of acid polar groups incorporated into the macromolecular chain. Polymeric films were produced by blending the sulfonated styrene-indene copolymer with poly(vinylidene fluoride). The degree of sulfonation of the polymer was strongly affected by the sulfonation reaction parameters, with a direct impact on the ionic exchange capacity and the ionic conductivity of the sulfonated polymers and the membranes obtained from them. The films produced with the blends showed more suitable mechanical properties, although the conductivity of the membranes was still lower than that of commercially available membranes used in fuel cells.
The corrosion behavior of iron and AISI 304 stainless steel in aqueous tungstate solutions containing chloride ion was investigated by cyclic voltammetry, open circuit measurements and impedance spectroscopy. The obtained results point out that the inhibitive performance of tungstate depends on the presence of dissolved oxygen, being strongly related to the adsorption of this anion on the surface of the electrode. Under anodic polarization, at low sweep rate and electrode rotation the passive film is more stable and the corrosion rate decreases. At the open circuit potential, the EIS data have clearly demonstrated that for longer immersion times the tungstate inhibitive action is considerably improved.
O comportamento do ferro e do aço inoxidável AISI 304 frente à corrosão em solução aquosa de tungstato de sódio em presença de cloreto foi investigado usando as técnicas de voltametria cíclica, medidas de potencial de circuito aberto e impedância eletroquímica. Os resultados mostraram que a ação inibidora do ânion tungstato está relacionada com sua adsorção sobre a superfície eletrodódica e depende marcadamente da presença de oxigênio dissolvido no meio. Filmes formados por polarização anódica a baixas velocidades de varredura de potencial e rotação do eletrodo são mais estáveis permitindo um decréscimo na taxa de corrosão. Medidas de impedância eletroquímica em potencial de circuito aberto mostraram que a ação inibidora do tungstato é consideravelmente favorecida em tempos de imersão prolongados.
The corrosive behavior of Ti and its alloys in fluoride media is well known. Based on electrochemical studies, this paper describes the effect of fluoride ions on the passive behavior of Ti6Al4V alloy in lactated Ringer's serum. The open circuit potential of the alloy in the serum, which lies in the passive region of TiO2, shifts to more negative values as fluoride ions are added. The voltammogram obtained in the serum presents an active-passive behavior close to -0.2 V (SCE) that changes with the presence of fluoride ions, evidencing higher anodic currents. Electrochemical impedance spectroscopy plots obtained at OCP after varying immersion times in the serum display an almost capacitive behavior and the polarization resistance becomes enhanced after 7 days. With the addition of fluoride ions, the film's resistance decreases, but a stable protective layer develops after 7 days of immersion time, indicating the film's repair.
The electrodeposition of Cd2+ ion was investigated in pH 4.8 sulfuric-sulfate solutions in the presence of dissolved O2. In potentiostatic conditions, using reticulated vitreous carbon (RVC) electrodes of 30, 60 and 100 pores per inch (ppi), high removal efficiency values were achieved in the potential range from --0.90 to --1.10 V for solutions containing 5 and 10 mg L-1 of Cd2+ ion. In this potential range, Cd electrodeposition is a mass transport controlled reaction and the concentration of the metallic ion decays exponentially with time following a pseudo-first order kinetics. For the 30 ppi RVC, the current efficiency and removal efficiency values found were, respectively, 45 % and 96 % for a solution containing 10 mg L-1 of Cd2+ ion after 30 minutes electrolysis at --0.90 V while 33 % and 99% were found for the 60 ppi RVC. The concentration decay of Cd2+ ion in the solution was monitored after each experiment by anodic stripping voltammetry at a hanging mercury drop electrode.
A eletrodeposição do íon Cd2+ foi investigada em soluções aeradas de ácido sulfúrico-sulfato de potássio em pH 4,8. Sob condições potenciostáticas, usando eletrodos de carbono vítreo reticulado (CVR) de 30, 60 e 100 poros por polegada (ppp), uma elevada eficiência de remoção foi obtida para soluções contendo 5 e 10 mg L-1 de íon Cd2+, na faixa de potenciais entre --0,90 e --1,10 V. Neste intervalo, a eletrodeposição do cádmio é controlada por transporte de massa e a concentração de íon Cd+2 varia exponencialmente com o tempo, seguindo uma cinética de pseudo-primeira ordem. Para a concentração 10 mg L-1 de íon Cd2+ e usando o eletrodo de 30 ppp, as eficiências de corrente e de remoção determinadas foram, respectivamente, 45% e 96%, após 30 minutos de eletrólise a --0,90 V, enquanto para 60 ppp foram encontrados 33% e 99%, respectivamente. A voltametria de redissolução anódica com eletrodo de gota pendente de mercúrio foi empregada para monitorar a concentração do íon Cd2+ após cada experimento de eletrodeposição.
The development of simple methods for removal of heavy metals from aqueous samples is a relevant field of research. In this connection, the electrodeposition of the Cd+2 ion, one of the most toxic species for animals and human beings, was investigated in aerated pH 4.8 sulfuric-sulfate solutions. In potentiostatic conditions, the maximum rate of cadmium deposition at a neutral polypyrrole (PPy0) coated reticulated vitreous carbon (RVC) working electrode occurs at -3.0 V vs. the saturated calomel reference electrode (SCE). Moreover, the conversion rate depends both on the applied potential and on the mass transport regime, and, for solutions containing 10 mg L-1 of Cd2+, the highest removal efficiency achieved is 84% after 90 min of electrolysis. The concentration decay of the Cd+2 ion in the solution was monitored by anodic stripping voltammetry (ASV) at a hanging mercury drop electrode. Besides, metallic cadmium deposited onto the polypyrrole modified RVC electrode was evidenced by Scanning Electron Microscopy (SEM) analysis using the backscattered electron image (BEI) technique and by Energy Dispersive Spectrometry (EDS).
The eletrodissolution of Cu-Zn alloys with different chemical compositions (Cu37Zn and Cu39Zn3Pb) in acetate buffer pH 5 was studied by means of corrosion potential measurements and cyclic voltammetry, under static and dynamic conditions. The lower corrosion resistance of the brass in comparison to pure Cu, was attributed to the anodic layer structure consisting of a complex ZnO.x H2O /Cu2O-CuO layer, which is less protective than the Cu2O-CuO passive layer produced onto Cu. No selective dissolution of Zn was detected at both alloys, being their electrodissolution rate higher than that of pure Cu and smaller than that of pure Zn.
A eletrodissolução de ligas de Cu-Zn, de diferentes composição química (CuZn37 e CuZn39Pb3) foi estudada em solução tampão de acetato, pH 5, através de medidas de potencial de circuito aberto e voltametria cíclica, sob condições estáticas e dinâmicas. A menor resistência à corrosão detectada nestas ligas, comparativamente ao Cu puro, foi atribuída à estrutura da camada anódica formada ZnO.x H 2O /Cu2O-CuO, a qual apresenta menor proteção que a camada Cu2O-CuO, formada sobre o eletrodo de Cu puro. Não foi observada a dissolução seletiva do Zn, verificando-se a dissolução simultânea dos constituintes das ligas. A velocidade de eletrodissolução das ligas neste meio, mostrou-se superior à do Cu puro e inferior à do Zn puro.
The results presented in this paper clearly indicate that precocene and azadirachtin are effective inhibitors of moulting and reproduction in the hemipteran Rhodnius prolixus. The time of application is important and only applications of these substances early in the intermoulting period cause their effects in nymphs. The inhibition of moulting is fully reversed by ecdysone therapy. Precocene and azadirachtin also affected drastically the oogenesis and egg deposition in this insect. Precocene-induced sterilization is reversed by application of juvenile hormone III. However, this hormone is unable to reverse the effect of azadirachtin on reproduction. Ecdysteroid titers in nymphs and adult females are decreased by these treatments. In vitro analysis suggest that precocene and azadirachtin may act directly on the prothoracic glands and ovaries producing ecdysteroids. Based on these and other findings the possible mode of action of these compounds on the development and reproduction of Rhodnius prolixus is discussed.