Thesis (Ph. D.)--Dept. of Chemistry and School of Pharmacy. University of Missouri--Kansas City, 2006. / "A dissertation in chemistry and pharmaceutical sciences." Advisor: J. David Van Horn. Typescript. Vita. Leaves 174-177 inserted between leaves 168 and 169. Description based on contents viewed Jan. 29, 2007; title from "catalog record" of the print edition. Includes bibliographical references (leaves 156-176). Online version of the print edition.

Thesis (M. Sc.)(Applied Sciences)--University of Petoria, 2005. / Abstracts in English and Afrikaans. Includes bibliographical references. Available on the Internet via the World Wide Web.

Made available in DSpace on 2014-06-11T19:32:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2000-12Bitstream added on 2014-06-13T20:03:19Z : No. of bitstreams: 1 silva_gc_dr_araiq.pdf: 2408570 bytes, checksum: 6b882172e55e4b02cccf55295ef87fbb (MD5) / O crômio é o principal elemento de liga, responsável pela alta resistência à corrosão dos aços inoxidáveis. Esta resistência está relacionada com a formação de óxidos e/ou hidróxidos de Cr3+ na superfície. A literatura reporta muitos trabalhos sobre a comportamento eletroquímico do crômio em meio de ácido sulfúrico. Entretanto pouco se conhece sobre o crômio em soluções contendo cloreto. Este conhecimento pode ser muito importante para entender o mecanismo de formação dos filmes e passivação da superfície do crômio e dos aços inoxidáveis. Neste trabalho estudou-se o comportamento eletroquímico do crômio em soluções contento íons cloreto, utilizando-se técnicas eletroquímicas convencionais. Espectroscopia fotoeletrônica e Uv-visível foram também empregadas para analisar, respectivamente, a superfície e espécies de crômio em solução. As curvas de polarização do crômio em soluções mostram três regiões de potenciais: ativa, passiva e transpassiva. Na região ativa verificou-se que o crômio se dissolve como Cr2+ junto com geração de hidrogênio. Para soluções com pH<3 a formação do filme passivo deve ocorrer via dissolução/precipitação enquanto que para soluções com pH>3 o mecanismo deve ser via formação direta do filme. As medidas de espectroscopia de impedância eletroquímica mostraram um circuito equivalente para a interface em solução com pH=0,3 e outro diferente para a interface em soluções com 0,3<pH. As análises por XPS revelaram que o filme é constituído basicamente por Cr2O3. Os resultados das medidas eletroquímicas e de espectroscopia UVvisível mostraram que o crômio e o filme passivante se dissolvem como dicromato na região de transpassivação. / Chromium is the main alloying element for stainless steels responsible for their high corrosion resistance. This is due to the formation of Cr3+ surface oxides and/or hydroxides at the chromium surface. In the literature there are many papers reporting studies on the electrochemical behavior of chromium in sulfuric acid. Nevertheless, there is still a lack of knowledge about electrochemical in chloride ions medium. This knowledge can be useful to understand the film formation and passivation mechanisms of chromium and stainless steels. This work reports the influence of the pH dependence on the electrochemical behavior of chromium in aqueous Cl-—containing solutions using conventional electrochemical techniques. X-ray Photoelectron and UV-vis Spectroscopies were used to analyze the surface composition and chromium species in solution, respectively. The polarization curves of chromium in acid solutions showed three potential regions: active, passive and the transpassive region. In the active region chromium dissolve as Cr2+ and hydrogen evolution was also observed. It seems that for solutions with pH<3 the formation of a passive layer occurs via a dissolution/precipitation process while for pH>3 the mechanism changes to direct film formation. The electrochemical impedance spectroscopy measurements, performed in the passive region agree with a circuit equivalent at pH=0.3 and another for 0.3< pH. XPS analysis revealed that the passive layer is basically constituted by Cr2O3. For the transpassive region the electrochemical results and UV-vis spectroscopy measurements indicated that both chromium and the passivating film dissolved as dichromate ions.

M.Sc. (Physics) / Please refer to full text to view abstract

M.Sc. / Please refer to full text to view abstract

The present investigation was concerned with a study of the preparation and properties of chromium tetrafluoride and its complexes; magnetic properties and X-ray structures were also studied. Attempts to prepare the pentafluoride of chromium using fluorinating agents other than fluorine were unsuccessful. Chromium tetrafluoride was prepared by the action of fluorine on the heated metal, the products being involatile chromium trifluoride, volatile chromium tetrafluoride, and more volatile fluorides which were condensed at -78°C. The best yields of chromium tetrafluoride were obtained at 350°C and with a moderate flow-rate of fluorine. Chromium tetrafluoride is a glassy solid which forms blue vapours when heated. It does not dissolve in the usual organic solvents and is hydrolysed readily in moist air. It is paramagnetic, the moment corresponding to two unpaired electrons, and has a Weiss Constant of -70°. Chemically, chromium tetrafluoride is surprisingly inert and at room temperature does not react with ammonia, pyridine, sulphur dioxide, sulphur trioxide, iodine pentafluoride, bromine trifluoride, bromine pentafluoride, chlorine trifluoride, or selenium tetrafluoride. However, it reacts instantaneously with water, giving Cr⁺³ and CrO₄¯² ions in solution. When heated with bromine trifluoride for longer periods, reaction occurs to give CrF₃.0.5BrF₃. On heating with a mixture of bromine trifluoride and bromine pentafluoride, a wine-coloured solution was obtained, and the residue after distillation of volatile materials was CrOF₃.0.25BrF₃. The reaction of selenium tetraf luoride with chromium tetrafluoride at 120°C yielded two compounds, CrF₃SeF₄ and CrF₂.SeF₄. Complexes of the type ACrF₅, where A = K, Rb, or Cs, were prepared for the first time, by heating the respective alkali metal chloride and chromium tetrafluoride in a 1:1 molar ratio in bromine trifluoride. The resulting complex compounds contained half a molecule of bromine trifluoride per molecule of the complex, if the removal of excess bromine trifluoride, after the reaction, was carried out at 100°C. However, if removal was carried out at 160°C, the pure complexes were obtained. Complexes of the type A₂CrF₆, where A = K or Cs, were prepared in bromine trifluoride solutions. The presence of extra bromine trifluoride in the molecule of the complex compound affected the structure seriously in potassium complexes but had no effect on cesium complexes. Thus K₂CrF₆0.5BrF₃ was tetragonal and on heating at 160°C in, vacuo yielded the cubic modification of pure K₂CrF₆; Cs₂CrF₆.0.5BrF₃and Cs₂CrF₆ were both cubic. Reactions of chromium trioxide with bromine trifluoride, bromine pentafluoride, and chlorine trifluoride were also investigated. With bromine fluorides, the products obtained were the corresponding adducts of CrOF₃. These are very reactive and sensitive to moist air. The compounds are paramagnetic. The reaction with chlorine trifluoride was very interesting because the appearance of the product depended on/ the experimental conditions. If the reaction was carried out at ordinary temperature, the product was a buff-coloured powdery mass, but if the reaction was performed by passing chlorine trifluoride vapour over heated chromium trioxide (100—120°C), the product was a brick-red substance. Chemical analyses and magnetic measurements indicated that both these products were identical and had the composition CrOF₃.0.25ClF₃. The brick-red product could be converted into the light buff-coloured powdery mass by heating at 70°C in vacuo. Reactions of potassium dichromate with bromine trifluoride, bromine pentafluoride, and chlorine trifluoride were also investigated. Potassium dichromate reacted with these smoothly at room temperature and the product of reaction in each case was KCrOF₄. The compounds investigated in the present research are tabulated below.[ Table omitted ] / Science, Faculty of / Chemistry, Department of / Graduate

I.The luminescence spectra of a number of molecular Cr(III) complexes have been investigated using standard low-temperature spectroscopic techniques. The observed transitions have been assigned, assuming octahedral micro-symmetry, as emission from either the lowest lying doublet state, ²Eg, or the lowest lying quartet state, ⁴T₂g. An empirical classification is proposed which allows one to predict whether a given complex will show phosphorescence or fluorescence at low temperatures. In general, complexes with relatively high ligand field strengths show only phosphorescence because the ²Eg state lies far below the zeroth level of the ⁴T₂g. state. Compounds with relatively weak ligand field strengths have the ⁴T₂g state as their lowest excited state and therefore show only fluorescence. Complex ions with intermediate ligand fields may emit from both levels since the latter lie within a few hundred wavenumbers of each other. The phosphorescence spectra are sharp and show vibrational structure. A line coincident in energy in absorption and emission identifies the origin of the ²Eg ↔ ⁴A₂g transitions. As these spin-forbidden transitions are intra-configurational (in the strong field limit), the vibration structure in emission and absorption is very similar, although a true mirror image relationship is not realized. In many cases the vibrational spacings have been correlated with known infra-red data. The fluorescence emission is broad and shows no vibrational structure. The ⁴T₂g ↔ ⁴A₂g transitions are inter-configurational and the emission maximum is Stokes-shifted by a few thousand wavenumbers from the absorption maximum, as expected. II. The methods that have been used in the past to derive the electron repulsion parameters, B and C, from the spectra of octahedral Cr(III) complexes have been reinvestigated. A number of Cr(III) complexes were chosen for which the spin-forbidden transitions to the three lowest doublet states are known. The complete strong field and weak field matrices have been used to derive the parameters B and C from the known spectral data. A convenient graphical method is given which makes it a simple matter to choose the "best" values of B and C. The assumption that C=4B was discarded because it is incompatible with the nephelauxetic effect. The three parameter (Δ, B, C) crystal field theory reproduces the band positions of the d-d transitions of most Cr(III) complexes to within a few hundred wavenumbers. Because the free ion theory of Condon and Shortley involves about the same uncertainty, it is felt that the results obtained here represent the best that can be obtained from the three parameter crystal field theory. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

The purpose of this research was to determine the potential effect indigenous ions may have on the effectiveness of electrokinetic remediation of chromium contaminated soils. A synthetic soil matrix was tested in conjunction with four common ion pairs found in soils and ground water. The results of this study indicate that the interaction of the indigenous ions is sufficient to inhibit the effectiveness of the electrokinetic remediation process. The mobility of chromium through the soil was altered in the presence of high concentrations of the indigenous ions. Further study in this area will be required to address the effect of site specific conditions for full-scale electrokinetic remediation of soils.

No description available.