Linking solution and solid state studies of bismuth and cadmium complexes
Date
2014-08-01
Authors
Vieira, Vanessa Lourenco
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Abstract
In this project the link between species in solution and the solid state was considered.
This is relevant due to the many applications in life where there is this interchange
between solid and solution state, for example drug design, environmental metal
speciation and the manufacture of materials that are in contact with solution (such as
outdoor surface coatings, containers and so on). Complexation of two metal ions,
namely cadmium(II) and bismuth(III), was studied. With bismuth showing so much
promise in medicinal applications it was pertinent to investigate this interchange since
the intake of medication is generally in the solid form which then converts to solution
species as it dissolves in the body where it becomes active. For cadmium it is mainly
the environmental concerns which we are faced with that call for the examination of
speciation of complexes in solution, as well as their disposition upon precipitation or
crystallization.
A correlation was found between solution species and the complex that was isolated in
the crystalline form with regards to pH for a number of metal-ligand species. We show
how the results from solution experiments (achieved using direct current
polarography) and those of crystalline complexes can complement each other when
using species distribution diagrams as the intermediary. The distribution of species
can be varied by changing the concentration and ligand-to-metal ratio at which the
species distribution diagram is plotted. It is this characteristic which allows the
solution and crystalline complexes – which are achieved using differing experimental
conditions – to be correlated.
The speciation diagram for a metal-ligand system, calculated using formation
constants derived from solutions studies, was used in most instances to target specific
species for their growth in the solid state. In some cases the solid state structure was
used to confirm a suspected solution species, and in others the result was used to
identify minor solution species which cannot be detected by the techniques used in
determining formation constants. Further, we show that doing solution experiments
at a range of temperatures can also aid in elucidating these minor species.
The growth of crystalline species at low pH was important for this work because the pH
titrations used for solution experiments were conducted from below pH 2 where the
diffusion junction potential is large and changing. An in-situ witness ion was
incorporated into the experiment to monitor the shifts due to the diffusion junction
potential so that they could be compensated for. Additionally, for bismuth-ligand
systems, hydrolysis and complexation with nitrates occurs in this same pH region. The
formation constants and the species identified below pH 2 therefore do carry some
uncertainty, so obtaining crystalline complexes of these species provides further
confidence in their prediction in solution.