This work presents an experiment done for chemistry students about the effect of the nature of the electrolyte and the nature of the interface on the hydrogen evolution reaction by electrolysis. Two techniques are suggested: method A - two identical copper electrodes in a direct current circuit and method B - three different electrodes in a dc potentiostatic circuit, with copper electrode (working), platinum electrode (counter) and silver-silver chloride-KCl sat electrode (reference). The hydrogen evolution was observed using the A technique by increasing potentials between the two electrodes and the visual observation of the gas production at the cathode. The results showed “semi-quantitative” effect of the pH on the hydrogen evolution. It was observed a larger potential difference in alkaline medium than in acid medium. The results obtained with the B technique from polarization curves showed quantitatively the thermodynamic effect of the pH in addition to the least catalytic effect of copper presented in alkaline medium when compared to copper in acid medium for hydrogen evolution. The technique A can be used in technical courses, high school and general chemistry class for under graduate students. The technique B is appropriate for under graduate electrochemistry class and graduate students.

Resumo Estudos eletroquímicos, caracterização de superfície e teste de citotoxicidade in vitro foram realizados da liga CoCrW em meios de saliva artificial e NaCl 0,15 mol.L-1, com o objetivo de avaliar a sua aplicação como material de prótese dentária. Foram usadas como técnicas, curvas de polarização anódica, medidas cronoamperométricas, espectroscopia de impedância eletroquímica (EIE), microscopia eletrônica de varredura (MEV), espectroscopia por energia dispersiva de raios X (EDS) e espectroscopia fotoeletrônica de raios X (XPS). O teste de citotoxicidade também foi realizado. O comportamento eletroquímico da liga CoCrW foi comparado nos dois meios estudados desde o potencial de corrosão (Ecorr) até uma sobretensão anódica de 600 mV. Foi observado, a partir de medidas eletroquímicas, que a liga CoCrW se encontra passivada em uma ampla faixa de potencial e que a sobretensões mais elevadas apresenta apenas corrosão generalizada nos dois meios. Análises por MEV e EDS mostraram que a liga apresenta inclusões não metálicas de carbeto de nióbio, de óxidos de silício e de manganês. Os resultados de XPS indicaram que o cobalto não contribui significativamente para a formação do filme passivo. O teste de citotoxicidade mostrou que a liga CoCrW não se apresenta citotóxica. Estes resultados sugerem que a liga estudada pode ser usada como biomaterial a ser aplicado como prótese sobre implantes dentários.

Abstract In order to evaluate its application as a dental prosthesis material, a CoCrW alloy was subjected to in vitro cytotoxicity test, surface characterization and electrochemical studies performed in artificial saliva and 0.15 mol.L-1 NaCl medium. The used techniques were: anodic polarization curves, chronoamperometric measurements, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) analysis and X-ray photoelectron spectroscopy (XPS). Cytotoxicity test was also performed. The electrochemical behavior of CoCrW alloy was compared in both studied media, from corrosion potential (Ecorr) to a 600 mV anodic overvoltage. From the electrochemical measurements it was observed that the CoCrW alloy in both media presents only generalized corrosion. SEM and EDS analysis showed that the alloy presents carbide niobium and silicon and manganese oxides as nonmetallic inclusions. XPS results indicated that cobalt does not significantly contribute to the passivating film formation. Cytotoxicity test showed no cytotoxic character of CoCrW alloy. These results suggest that the CoCrW alloy can be used as biomaterial to be applied as prosthesis in dental implants.

Undergraduate students on the first year of Chemistry Courses are unfamiliar with the representation of acid-base reactions using the ionic equation H+ + OH- → H2O. A chemistry class was proposed about acid-base reactions using theory and experimental evaluation of neutralization heat to discuss the energy involved when water is formed from H+ and OH- ions. The experiment is suggested using different strong acids and strong base pairs. The presentation of the theme within a chemistry class for high school teachers increased the number of individuals that saw the acid-base reaction from this perspective.

Foi estudado o comportamento eletroquímico a 37°C do aço inoxidável ISO 5832-9, em meios de NaCl 0,9 %, de Ringer Lactato e meio mínimo de Eagle (MEM), por voltametria linear e análises da superfície por microscopia eletrônica de varredura (MEV) e por espectroscopia por dispersão de energia (EDS). Foram feitos ensaios mecânicos e testes de toxicidade. O aço ISO 5832-9 se encontra passivado no potencial de corrosão e não apresenta corrosão por pite nos três meios estudados em toda faixa de potencial investigada, desde o potencial de corrosão até 50 mV acima do potencial de transpassivação. Em meio de MEM, no entanto, as análises por MEV e EDS mostraram que o referido aço, nesse valor mais elevado de potencial, apresentou um comportamento diferente, com perda das inclusões de óxido de manganês. Os potenciais de corrosão, Ecorr (potencial de circuito aberto estacionário) bem como os valores de densidade de corrente de passivação, variaram na seguinte ordem: Ecorr, RL < Ecorr, NaCl < Ecorr, MEM. e jMEM << jRL ≅ jNaCl. No ensaio de citotoxicidade, o aço foi caracterizado como não-tóxico.

The electrochemical behavior of ISO 5832-9 stainless steel at 37 oC in 0.9% NaCl, Ringer Lactate and minimum essential medium (MEM) has been studied, using linear voltammetry, and surface analysis by SEM and EDS. Mechanical and toxicity tests were made. ISO 5832-9 is passivated at corrosion potential (Ecorr) and it does not present pitting corrosion on the media studied from Ecorr to 50 mV above the transpassivation potential (Ei). SEM and EDS analysis have shown that the sample previously immersed in MEM presents a different behavior at 50 mV above Ei: the manganese oxide inclusions are absent in the surface. Ecorr values and passivation current density values jpass changed according to the following. Ecorr, RL < Ecorr,NaCl < Ecorr, MEM and jMEM << jRL ≅ jNaCl. The stainless steel was characterized as non toxic in the cytotoxicity assay.

O aço inoxidável P558, isento de níquel, vem sendo estudado pelo grupo com o objetivo de verificar a viabilidade do seu emprego em implantes ortopédicos. Em trabalhos anteriores, foi mostrado, por técnicas eletroquímicas estacionárias, que ele apresenta, em meio de cloreto de sódio 0,11 mol L-1, comportamento semelhante ao do aço ISO 5832-9 e bem superior ao do aço ISO 5832-1, mais conhecido como F138. Nesse trabalho, foi estudada a evolução temporal da interfase aço inoxidável P558/NaCl 0,11 mol L-1 empregando a técnica de espectroscopia de impedância eletroquímica, no potencial de circuito aberto. Os intervalos de tempo observados foram de 10 min, 1 dia, 2 dias e 5 dias. Observou-se que o filme passivo se torna mais protetor, com o decorrer do tempo. Foi feita simulação dos resultados experimentais através de circuitos elétricos equivalentes, que permitiram sugerir a existência, no início da observação, de apenas um filme passivante menos protetor e, quando atingida a condição estacionária, a existência de um segundo filme mais compacto.

P558 stainless steel, nickel free, has been studied by this group with the purpose of verifying its viability for use in orthopedic implants. In previous works, it has been shown by stationary electrochemical techniques that this steel presents a similar behavior of that presented by ISO 5832-9 with a corrosion resistance significantly higher when compared to ISO 5832-1 stainless steel, usually known as F138 SS. In this work the temporal evolution of the interfase P558/NaCl 0.11 mol L-1 was studied by electrochemical impedance spectroscopy, at the open circuit potential. The time intervals observed were 10 min. 1 day, 2 days and 5 days. It was observed that the passive film became more protective with the increase of time. The experimental data was modeled using the equivalent electrical circuit approach. This approach suggested an initial existence of a slightly protective passive film, which then permitted the formation of a more compact secondary film upon stabilization of the conditions.

UNS S31254 SS electrodes have been built to substitute platinum in conductimetric titrations. The electrodes were tested in both acid-basic titration (chloridric acid and sodium hydroxide) and precipitation titration (sodium chloride and argentum nitrate as titrant). The practical application was exemplified from conductimetric tritations of HF <FONT FACE=Symbol>¾</FONT> HNO3 mixtures used in metalurgical industry to passivate stainless steels. The results were compared with those obtained using commercial platinum electrodes. The equivalent volumes obtained were comparable within 3% experimental error. Its application depends on the nature of electrolyte. These results have shown that stainless steel, less expensive than platinum (about three order of magnitude), can substitute platinum electrodes in routine analyses and didactic laboratories.

The susceptibility to stress corrosion cracking (SCC) of solubilized and sensitized 254SMO SS was studied in sodium chloride, and sodium fluoride solutions at 80 °C and sulfuric acid solutions in presence of sodium chloride at 25 °C. The influence of salt concentration, pH values and the addition of thiosulfate was examined. The susceptibility to SCC was evaluated by Slow Strain Rate Tests (SSRT), at 1.5 x 10-6 s-1 strain rate. The behavior of 254SMO was compared to those of AISI 316L SS and Hastelloy C276. 254SMO showed an excellent resistance to SCC in all conditions, except in the more acidic solutions (pH <= 1) where, in the sensitized conditions, intergranular stress corrosion cracking occurred.