The determination of trace elements in complex matrices by electrochemical techniques

Please read the abstract in the section 00front of this documentDissertation (MSc (Chemistry))--University of Pretoria, 2006.Chemistryunrestricte

Study of bismuth chemistry toward medicinal applications

The use of bismuth in medicinal applications has been limited despite the many promising indications of its effectiveness in treatments for a large number of ailments. This is predominantly due to the lack of understanding of bismuth chemistry, including thermodynamic and kinetic aspects, thus hindering the design of improved drugs. This, in turn, is due to the difficulty in studying the complex chemistry of this element. Bismuth undergoes hydrolysis from below pH 1 and forms precipitates around pH 2 already, thus has to be studied from low pH. The most commonly used technique to determine stability constants, namely glass electrode potentiometry, cannot be employed in very acidic solutions. Complex formation has previously been studied by polarography where potential shifts and changes in current are used to determine solution species and evaluate stability constants. The benefits of employing polarography here are that low bismuth concentrations can be used to postpone precipitation and it can be used across the pH range. However, the diffusion junction potential becomes significant below pH 2 and changes with pH. Protocols to determine the stability of bismuth complexes using polarography were developed in this study. Firstly, the junction potential cannot be measured directly, so a witness metal ion was introduced into the solution to monitor its magnitude with changing pH. For this thallium (I) was used as it does not readily undergo complexation and hence potential shifts observed with changing pH is due to changes in the junction potential. This process was successfully tested on the cadmium(II)-picolinic acid system. Secondly, it was suggested that the reduction of bismuth(III) is quasi-reversible, so mechanisms of determining the reversible reduction potentials were investigated using the copper(II)-picolinic acid system, as copper(II) has a reduction potential almost identical to bismuth(III) and its reduction is also quasi-reversible. However, it was found that bismuth was reversibly reduced under the polarographic conditions employed. Thirdly, the free bismuth(III) potential had to be determined in order to calculate potential shifts due to complex formation. This potential cannot be measured directly either, so procedures were developed to determine this value by accounting for both hydrolysis and complex formation with the background electrolyte anion (nitrate). Three bismuth-ligand systems were studied where the ligands were picolinic acid, dipicolinic acid and quinolinic acid. It was necessary to determine the stability constants for these systems by using a combination of direct polarographic data interpretation and the use of virtual potentiometry

Application of protocols devised to study Bi(III) complex formation by voltammetry : the Bi(III)-picolinic acid system

Bi(III) coordination chemistry has been largely neglected due to the difficulties faced when studying these systems even though Bi(III) is used in various medicinal applications. This study of the Bi(III)– picolinic acid system by voltammetry applies the rigorous methodologies already developed to enable the study of Bi(III) systems starting in very acidic solutions to prevent precipitation. This includes calibrating the glass electrode accurately at these low pHs, compensating for the diffusion junction potential below pH 2 and determining the reduction potential of uncomplexed Bi(III) which cannot be directly measured. The importance of including nitrate from the background electrolyte as a competing species is highlighted, especially for data acquired below pH ∼ 2. From analysis of the voltammetric data, it was not clear whether a ML3OH species formed in solution or whether it was a combination of ML4 and ML4OH. Information from crystal structures and electrospray ionizationmass spectrometry measurements was thus used to propose the most probable species model. The log β values determined were 7.77 ± 0.07 for ML, 13.89 ± 0.07 for ML2, 18.61 ± 0.01 for ML3, 22.7 ± 0.2 for ML4, and 31.4 ± 0.2 for ML4OH. Application of these methodologies thus opens the door to broaden our understanding of Bi(III) complexation.National Research Foundation (NRF) of South Africa and the University of the Witwatersrand.http://pubs.acs.org/journal/jpcbfk2017-12-31hb2017Chemistr

A novel approach to monitoring of the diffusion junction potential in speciation studies by Polarography under very acidic conditions : Part II : The quasi-reversible Cu(II)-picolinic acid system

The use of polarography to accurately determine stability constants of complexes formed under very acidic conditions (below pH 2) is demonstrated. The diffusion junction potentials, which must be accounted for below pH 2, were evaluated by applying protocols developed where Tl(I) is used as an internal reference. The Cu(II)-picolinic acid (2-pyridinecarboxylic acid) system studied was chosen since the CuL+ species only exists in solution below pH 2 under the conditions used and literature data exists to confirm the accuracy of procedure. Additionally, the reduction of Cu(II) was quasi-reversible and procedures to determine the reversible half-wave potentials were investigated. Average log values of 7.75 0.09 for CuL+ and 14.8 0.1 for CuL2 were obtained, which compared well to literature data.National Research Foundation (NRF). University of the Witwatersrand.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1521-41092016-02-28hb201

A novel approach to monitoring of the diffusion junction potential in speciation studies by polarography under very acidic conditions : Part I : The reversible cd(ii)-picolinic acid system

A new polarographic procedure has been developed for the study of metal-ligand equilibria at pH<2 by applying an in situ monitoring of the diffusion junction potential by introducing a noncomplexing witness metal ion, Tl(I). As a case study, this procedure was applied to the Cd(II)-picolinic acid (pyridine-2-carboxylic acid) system; DCTAST polarography was employed at 25 8C and ionic strength of 0.25–0.5 M (H,Na)NO3. Log b values of 4.26 0.03, 7.86 0.11 and 10.47 0.12 were obtained for the ML, ML2 and ML3 complexes, respectively, and compare well to literature values. The first log b (6.27 0.07) value for the MLH species was successfully determined, for which a crystal structure has been reported recently.National Research Foundation (NRF) and the University of the Witwatersrandhttp://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1521-4109hb201

The feasibility of pyrite dissolution in the deep eutectic solvent ethaline: experimental and theoretical study

Environmental concerns about the traditional gold extraction process, and the potential volume of encapsulated gold in sulfidic minerals i.e. pyrite, have motivated researchers to find effective, efficient and ecologically benign ways to expose the enclosed gold for improved extraction. Neoteric deep eutectic solvents (DESs) are an analogue of ionic liquids (ILs) which are gaining more attention as eco-friendly solvents. This study examined the viability of pyrite dissolution in a DES comprised of choline chloride (ChCl) and ethylene glycol (EG), called Ethaline. The pH of the ethaline solvent mixed with hydrogen peroxide oxidant, and different solid-to-liquid ratios were examined. Ethaline solution with pH 8 provided the desired condition at which EG is deprotonated to [C2H4O2]2−, a favourable ligand for Fe complexation and thus pyrite dissolution. A solid-to-liquid ratio of 1/20 was optimal and achieved 23.6% Fe extraction as an indication of pyrite dissolution. Density functional theory (DFT) was applied to determine which of the two ligands provided by ethaline (Cl– and/or [C2H4O2]2−) can form the most stable complex with Fe2+ and/or Fe3+. As a result, the tetrahedral complex [Fe(C2H4O2)2]− with the ligand [C2H4O2]2− through O-donor chelating with Fe3+ was found to be the most probable and stable complex. However, the ethaline solvent did not deliver adequate Fe extraction compared to commonly used reagents/solvents like mineral acids, to fully break down pyrite and expose any encapsulated gold

The kinetics of pyrite dissolution in nitric acid solution

Refractory sulphidic ore with gold captured in pyrite has motivated researchers to find efficient means to break down pyrite to make gold accessible and, ultimately, improve gold extraction. Thus, the dissolution of pyrite was investigated to understand the mechanism and find the corresponding kinetics in a nitric acid solution. To carry this out, the temperature (25 to 85 °C), nitric acid concentration (1 to 4 M), the particle size of pyrite from 53 to 212 µm, and different stirring speeds were examined to observe their effect on pyrite dissolution. An increase in temperature and nitric acid concentration were influential parameters to obtaining a substantial improvement in pyrite dissolution (95% Fe extraction achieved). The new shrinking core equation (1/3ln (1 − X) + [(1 − X)−1/3 − 1)]) = kt) fit the measured rates of dissolution well. Thus, the mixed–controlled kinetics model describing the interfacial transfer and diffusion governed the reaction kinetics of pyrite. The activation energies (Ea) were 145.2 kJ/mol at 25–45 °C and 44.3 kJ/mol at higher temperatures (55–85 °C). A semiempirical expression describing the reaction of pyrite dissolution under the conditions studied was proposed: 1/3ln(1 − X) + [(1 − X)−1/3 − 1)] = 88.3 [HNO3]2.6 r0−1.3 e−44280/RT t. The solid residue was analysed using SEM, XRD, and Raman spectrometry, which all identified sulphur formation as the pyrite dissolved. Interestingly, two sulphur species, i.e., S8 and S6, formed during the dissolution process, which were detected using XRD Rietveld refinement

The Influence of REE β-Diketone Complexes on the Corrosion Behaviour of Mild Steel and 304 SS in 3.5% NaCl Solution

In the present investigation, four REE β-diketone complexes, namely cerium acetylacetone, cerium hexafluoroacetylacetone, lanthanum acetylacetone, and lanthanum hexafluoroacetylacetone, were investigated as potential corrosion inhibitors for mild steel and 304 stainless steel in 3.5% NaCl solution. The corrosion-inhibition effects of the REE β-diketone complexes were investigated using weight-loss measurements and potentiodynamic polarisation scans. Surface analyses using optical microscopy and scanning electron microscopy (SEM) were used to investigate the morphology of the mild steel and 304 stainless steel after the weight-loss and potentiodynamic tests in 3.5% NaCl solution containing 0.5% mass per volume (m/v) concentration of the tested inhibitor. Fourier transform infrared spectroscopy and Raman spectroscopy were further used to probe the type of corrosion product film that forms on the surface of the tested samples. The obtained results revealed that the four REE β-diketone complexes are very effective inhibitors against corrosion of mild steel and 304 stainless steel in a 3.5% NaCl in a temperature range of 20–60 °C

Glass electrode calibration for use in the voltammetric determination of stability constants under extreme acidic conditions

A glass electrode (GE) can be successfully employed to measure pH in the study of metal-ligand equilibria by voltammetry at extremely low pH (between 0 and 2); two consecutive strong acid–strong base titrations involving different base concentrations (recommended to avoid corrosion of the GE in very basic solutions) are best suited to establish the response parameters of a GE. A novel approach of using a combined linear and binomial GE calibration was developed; this procedure allows measurements between pH 0 and 2 with uncertainty better than ±0.01 pH unit. From an extensive error analysis, it has been established that the uncertainties of about ±0.5 mV in the response slope and ±1.3 mV in E°’ might result in an absolute error in pH of about 0.02 which should not generate errors larger than 0.3%in optimized stability constants (as logKvalues) determined by voltammetry at extremely low pH. A test of GE suitability for the study of metal complexes by voltammetry is also proposed; it should be implemented only for suspect electrodes that show response parameters outside the limits recommended in this work

Application of Protocols Devised to Study Bi(III) Complex Formation by Voltammetry: The Bi(III)–Picolinic Acid System

Bi­(III) coordination chemistry has been largely neglected due to the difficulties faced when studying these systems even though Bi­(III) is used in various medicinal applications. This study of the Bi­(III)–picolinic acid system by voltammetry applies the rigorous methodologies already developed to enable the study of Bi­(III) systems starting in very acidic solutions to prevent precipitation. This includes calibrating the glass electrode accurately at these low pHs, compensating for the diffusion junction potential below pH 2 and determining the reduction potential of uncomplexed Bi­(III) which cannot be directly measured. The importance of including nitrate from the background electrolyte as a competing species is highlighted, especially for data acquired below pH ∼ 2. From analysis of the voltammetric data, it was not clear whether a ML₃OH species formed in solution or whether it was a combination of ML₄ and ML₄OH. Information from crystal structures and electrospray ionization-mass spectrometry measurements was thus used to propose the most probable species model. The log β values determined were 7.77 ± 0.07 for ML, 13.89 ± 0.07 for ML₂, 18.61 ± 0.01 for ML₃, 22.7 ± 0.2 for ML₄, and 31.4 ± 0.2 for ML₄OH. Application of these methodologies thus opens the door to broaden our understanding of Bi­(III) complexation