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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11938 |
| Date | 12 September 2012 |
| Creators | Billing, Caren |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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