New spectrophotometric evidence for the existence of HCrO4−

Cruywagen, J. J.; Heyns, J. B. B.; Rohwer, Erich G.

doi:10.1016/s0277-5387(97)00443-9

Cited by 40 publications

(30 citation statements)

References 14 publications

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“…Quantitative determination of Cr͑VI͒ concentration was calculated from UV absorption at the isosbestic point of 340 nm, where the absorption is independent of pH and is linearly related to the concentration of total Cr͑VI͒. 4,6,7 The linear calibration relation of Cr͑VI͒ concentration to absorption measured at 340 nm can be written as ͓Cr͑VI͔͒ ϭ (Abs 340 0.00157)/1062. An absorption of 1 is almost equal to 1 mM of total Cr͑VI͒; hereafter, we express the concentration of total Cr͑VI͒ by absorption.…”

Section: Methodsmentioning

confidence: 99%

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release

Akiyama¹,

Markworth²,

McCoy

et al. 2003

J. Electrochem. Soc.

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The release of chromate ions from chromate conversion coatings ͑CCCs͒ on Al alloys was studied, and the effect of aging of CCCs on the chromate release kinetics was investigated. Chromate release from CCCs into aqueous solutions was monitored by measuring the change in the chromate concentration in solution using UV-visible spectroscopy. Heat-treatment of the CCC greatly reduced the chromate release rate. The chromate release rate also decreased with increasing aging time at room temperature. A diffusion-control model was proposed based on the notion that the CCC in an aqueous solution is a porous, two-phase structure consisting of a solid phase with adsorbed Cr͑VI͒ species that is in local Langmuir-type equilibrium with an interpenetrating solution phase. This model results in a concentration gradient of soluble Cr͑VI͒ in the solution phase of the CCC as chromate is released. The concentration and diffusion coefficients of soluble Cr͑VI͒ in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time, suggesting that the CCC is modified with aging time. For aircraft coating technology, understanding the mechanism of aluminum corrosion and the effect of chromates on corrosion inhibition is important. Chromate conversion coatings ͑CCCs͒ are commonly used on aluminum alloy surfaces to improve the corrosion resistance and to establish an adhesive base for organic coatings. Chromate ions have increased the pitting potential of aluminum alloys in chloride media and reduced the rates of pit nucleation and growth if present in a high enough concentration.1,2 One of the important aspects of CCCs is the dynamic repair of newly created breaks on coated metals, which is called self-healing. For example, a CCC-treated Al alloy sample with a scribe mark through the coating to the metal exhibits minimal corrosion in salt-spray testing.Zhao et al. demonstrated the migration of chromates from a CCC to an uncoated metal surface by detecting Cr on the surface of an initially untreated AA2024-T3 sample immersed in chloride solution in close proximity to a CCC-treated sample. 3 The release of Cr͑VI͒ from CCCs was further studied using UV-Visible spectroscopic analysis of solutions exposed to samples with CCCs. 4 The observations were consistent with a release mechanism similar to Langmuir adsorption and desorption of Cr͑VI͒ from a porous, insoluble Cr͑III͒ matrix. To fully understand the inhibition behavior of CCCs, one must fully understand the chromate release phenomenon. In the present study, the release of Cr͑VI͒ from CCCs is examined for samples under a range of conditions, and a model describing the release kinetics is developed. ExperimentalCCCs were prepared on aluminum alloys AA1100 and AA2024-T3 by immersion in Alodine 1200S ͑Henkel Surface Technologies͒ solutions at room temperature. The Alodine bath consisted of 9.0 g L Ϫ1 of a mixture reported to be 5 50-60% CrO 3 , 20-30% KBF 4 , 10-15% K 3 Fe͑CN͒ 6 , 5-10% K 2 ZrF 6 , and 5-10% NaF by weight, which is approximately 0.065 M in Cr. The pH ...

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Section: Methodsmentioning

confidence: 99%

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release

Akiyama¹,

Markworth²,

McCoy

et al. 2003

J. Electrochem. Soc.

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“…Em ambos os casos, o número de fatores selecionados para o modelo foi igual a dois, os quais foram atribuídos as espécies bicromato (HCrO 4 -) e cromato (CrO 4 2-), respectivamente. As atribuições foram feitas com base nos perfis de pH estimados e nos espectros das espécies obtidos em seguida, através do método da matriz k. As estimativas da distribuição das espécies em função do pH, para os conjuntos de Os perfis de pH obtidos para a distribuição bicromato-cromato estão em boa concordância com a literatura 8,[16][17][18][20][21][22]49,52 , levando em conta que, na concentração de Cr(VI) analisada, menor que 3,3 x 10 -4 mol L -1 , a presença da espécie dicromato pode ser considerada insignificante 8,[19][20][21][22] . A constante de dissociação para o íon bicromato pode ser estimada através do cruzamento das linhas na 18 .…”

Section: Análise De Fatores Do Tipo Q De Imbrie Seguida De Rotação Vunclassified

“…Estudos recentes, como o de Poulopoulou et al 11 , analisando espectros no UV/visível, continuam questionando a presença da espécie HCrO 4 -em soluções aquosas. Entretanto, muitos trabalhos contradizem esta hipótese e reafirmam a existência e a importância das espécies protonadas de Cr(VI) [12][13][14][15][16][17][18] . Outro aspecto que causa polêmica diz respeito às faixas de predominância das espécies em função da concentração total de Cr(VI) e do pH [19][20][21][22] .…”

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“…Outro aspecto que causa polêmica diz respeito às faixas de predominância das espécies em função da concentração total de Cr(VI) e do pH [19][20][21][22] . Valores de constante de equilíbrio para as equações (1) e (2) encontrados na literatura chegam a diferir em mais de uma ordem de grandeza 12,13,[16][17][18][21][22][23][24][25] . De toda esta discussão, é possível propor algumas linhas gerais para a existên-cia e predominância das espécies de Cr(VI) 17,20 : H 2 CrO 4 só deve existir em soluções de pH abaixo de 0; HCrO 4 -deve predominar na faixa de pH entre 1 e 6 e, acima de uma certa concentração (em torno de 10 -3 mol L -1 ), deve coexistir com Cr 2 O 7 2-; em valores de pH acima de 8, deve existir somente CrO 4 2-.…”

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Aplicação de métodos quimiométricos na especiação de Cr(VI) em solução aquosa

Sena¹,

Collins²,

Scarmínio³

2001

Quím. Nova

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Recebido em 31/5/00; aceito em 21/9/00 CHEMOMETRIC METHODS APPLIED TO THE SPECIATION OF AQUEOUS Cr(VI) SOLUTIONS. The equilibria, the spectra and the identities of the species of Cr(VI) that are present in aqueous solution have long been an active subject of discussion in the literature. In this paper, three different chemometric methodologies are applied to sets of UV/Visible spectra of aqueous Cr(VI) solutions, in order to solve a chemical system where there is no available information concerning the composition of the samples nor spectral information about the pure species. Imbrie Q-mode factor analysis, followed by varimax rotation and Imbrie oblique projection, were used to estimate the composition of Cr(VI) equilibrium solutions and, by combining these results with the k-matrix method, to obtain the pure spectra of the species. Evolving factor analysis and self modeling curve resolution were used to confirm the number of the species and the resolution of the system, respectively. Sets of 3.3×10 -4 and 3.3×10 -5 mol L -1 Cr(VI) solutions, respectively, were analyzed in the pH range from 1 to 12. Two factors were identified, which were related to the chromate ion (CrO 4 2-) and bichromate ion (HCrO 4 -). The pK of the equilibrium was estimated as 5.8.

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“…Nitrogenase catalyzed substrate reduction reactions require the transient association of the two nitrogenase component proteins, the iron (Fe) protein and the molybdenum iron (MoFe) protein, with the transfer of one electron between the proteins being coupled to the hydrolysis of MgATP. 15,16 Previous studies in the literature [17][18][19][20][21] have shown how complex an equilibria involving molybdenum can be. In very acidic solutions the equilibria contain mainly the species of Mo(VI) protonated (Mo(OH) 6 MoO 4 , the latter being the octahedral ion MoO 4 2-at the stage of second protonation.…”

Section: Introductionmentioning

confidence: 99%

Molybdenum (VI) binded to humic and nitrohumic acid models in aqueous solutions: phthalic, 3- and 4-nitrophthalic acids, catechol and 4-nitrocathecol, part 1

Mercê¹,

Greboge²,

Mendes³

et al. 2005

J. Braz. Chem. Soc.

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Inúmeros modelos matemáticos já foram testados na literatura para o estudo de como os ácidos húmicos (AH) se complexam a íons metálicos. Os AH são compostos de produtos naturais degradados contendo C, N, P e S, contendo grupos hidroxila e carboxilatos aromáticos em sua estrutura principal. A presença de íons metálicos ligados a esses sítios de complexação promove a fertilidade do solo, ao mesmo tempo que permite a retenção desses íons para serem liberados lentamente quando da mineralização dos solos. As substâncias nitrohúmicas (SNH) são artefatos de laboratório contendo maior quantidade de N que os AH, contendo também grupamentos -NO 2 , que são elétron-atraentes. No entanto esses artefatos ainda possuem a estrutura principal dos AH, contendo os grupamentos salicílicos, catecolatos e ftalatos, todos prontos a reagirem com íons metálicos dependendo das condições do ambiente. Esse trabalho se propôs a estudar a capacidade de complexação de alguns modelos do AH possuindo diferentes sítios básicos de Lewis na presença do íon metálico Mo(VI), em solução aquosa, conforme a variação de pH usando algumas técnicas analíticas. As constantes de formação dos complexos do ácido ftálico, 3-e 4-nitroftálico, catecol e 4-nitrocatecol e Mo(VI) bem como a especiação desses complexos de acordo com a variação de pH foram determinadas. Titulações potenciométricas e ciclo-voltamétricas foram utilizadas para calcular as constantes de formação e para monitorar o aparecimento e a decomposição das espécies complexadas.Many mathematical models have been tested in the literature in the search of how humic acids (HA) from many natural sources complex to metal ions. HA are composed of natural degradation sources of C, N, P and S, bearing hydroxyl and carboxyl aromatic units in their inner structure. The presence of metal ions binded to these basic sites promotes fertility to the soil as well as can hold metal ions to be slowly released as the mineralization of the soil occurs. Nitrohumic substances are a laboratory artifact with higher N content then humic acids with an electron withdrawing group -NO 2 . However they still bear the main HA constituent chemical groups such as salycilate, catecholate and phthalate derivatives, all prone to bind to metal ions depending on the chemical conditions of the environment. This work intended to study the complexing behaviour of some HA models having very different Lewis basic binding sites in the presence of molybdenum (VI) ions, in aqueous systems, with varying pH values using some analytical tools. The formation constants of phthalic acid, 3-and 4-nitrophthalic acids, catechol and 4-nitrocatechol with Mo(VI) as well as the speciation of the complex species according to varying pH values were determined. Potentiometric and cyclic voltammetric titrations were employed to calculate the formation constants and to monitor the formation and decomposition of some complexed species. The results showed that although there is complexation between phthalic derived acids and molybdenum, the speciation favours i...

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New spectrophotometric evidence for the existence of HCrO4−

Cited by 40 publications

References 14 publications

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release

Aplicação de métodos quimiométricos na especiação de Cr(VI) em solução aquosa

Molybdenum (VI) binded to humic and nitrohumic acid models in aqueous solutions: phthalic, 3- and 4-nitrophthalic acids, catechol and 4-nitrocathecol, part 1

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