296
D. Ferri· C. Manfredi· E. Vasca . C. Fontanella· V. Caruso
prelude to the speciation of the solution, i.e. its qualitative and quantitative description in terms of the stoichiometry of the complexes formed and their concentrations. It
follows that a study of chemical speciation of a metal ion does not only provide its
total analytical concentration but also a detailed knowledge of its partition among all
the ligands in the solution. The speciation represents a fundamental requirement for
the comprehension of the chemical and physical behaviour, as well as for the interpretation of the properties of the solution investigated, such as toxicity to living organisms, therapeutic efficiency and biological activity of chemical preparations. It
further allows for understanding the role of the complexes in the genesis of minerals,
for evaluating the possibility that toxic metals be transported in the biosphere, etc:'
We have now introduced a term, speciation, which was practically unknown three
decades ago and is incorrectly and overly used today. However for the sake of clarity,
we are dealing with a particular form of speciation of solutions: that which handles
species coexisting under the condition of chemical equilibrium, defined by the law of
mass action. Now let us attempt to place the few concepts cited above in a broader
framework, without pretending to be exhaustive.
12.2
Equilibrium Analysis Applied to the Modelling of Natural Systems
Natural systems are too complex to be studied directly, thus we divide them into progressively simpler sub-systems, until they ultimately fit with the requisites and the limitations of idealized laboratory experiments, in which, for instance, the reactions of a
single metal ion with one or more ligands, are investigated. The procedure is extended
to all metals and to all ligands, with the aim of producing what we call a Thermodynamic Database. Finally, an approximate picture of the real system is obtained by reassembling the laboratory results, consisting of all the species found and their formation constants. This is modelling of which speciation is the first and essential step.
However speciation and consequently modelling are no better than the database on
which they are founded. The databases are then the limiting parameter of the reliability of
models. This is the crucial point mentioned above, on which a few comments have to be made.
To delimit the boundaries of this theme, let us focus our attention on metals, as the central ions.
Vast collections of data have so far been reported by several authors, starting from
Sillen and Martell's fundamental work: "Stability constants of metal-ion complexes"
(Sillen and Martell 1964). However, these collections are seldom proposed as critical
tables of thermodynamic parameters, and if they are, the criteria of selection are not
clearly indicated. In a few cases, bad data are easily spotted. But generally the selection is not so straightforward. The reluctance of compilers of tables to recommend
"the most reliable data" can be understood. But one must also realize that equilibrium
analysis does not generally produce "fingerprint" evidence of the complexes claimed,
rather it gives "probable" results. This means that the model (the stoichiometry of the
complexes) that we propose to explain the experimental data may be not unique. The
philosophy supporting EA recommends explaining the experimental data with the
minimum number of complexes, which, for instance, produce the lowest value of the
minimized function or the best fit in graphical approaches. Then, most obviously: the
more accurate the experimental data, the more probable the results, in terms of the
composition and formation constants of the complexes chosen.
D. Ferri· C. Manfredi· E. Vasca . C. Fontanella· V. Caruso
prelude to the speciation of the solution, i.e. its qualitative and quantitative description in terms of the stoichiometry of the complexes formed and their concentrations. It
follows that a study of chemical speciation of a metal ion does not only provide its
total analytical concentration but also a detailed knowledge of its partition among all
the ligands in the solution. The speciation represents a fundamental requirement for
the comprehension of the chemical and physical behaviour, as well as for the interpretation of the properties of the solution investigated, such as toxicity to living organisms, therapeutic efficiency and biological activity of chemical preparations. It
further allows for understanding the role of the complexes in the genesis of minerals,
for evaluating the possibility that toxic metals be transported in the biosphere, etc:'
We have now introduced a term, speciation, which was practically unknown three
decades ago and is incorrectly and overly used today. However for the sake of clarity,
we are dealing with a particular form of speciation of solutions: that which handles
species coexisting under the condition of chemical equilibrium, defined by the law of
mass action. Now let us attempt to place the few concepts cited above in a broader
framework, without pretending to be exhaustive.
12.2
Equilibrium Analysis Applied to the Modelling of Natural Systems
Natural systems are too complex to be studied directly, thus we divide them into progressively simpler sub-systems, until they ultimately fit with the requisites and the limitations of idealized laboratory experiments, in which, for instance, the reactions of a
single metal ion with one or more ligands, are investigated. The procedure is extended
to all metals and to all ligands, with the aim of producing what we call a Thermodynamic Database. Finally, an approximate picture of the real system is obtained by reassembling the laboratory results, consisting of all the species found and their formation constants. This is modelling of which speciation is the first and essential step.
However speciation and consequently modelling are no better than the database on
which they are founded. The databases are then the limiting parameter of the reliability of
models. This is the crucial point mentioned above, on which a few comments have to be made.
To delimit the boundaries of this theme, let us focus our attention on metals, as the central ions.
Vast collections of data have so far been reported by several authors, starting from
Sillen and Martell's fundamental work: "Stability constants of metal-ion complexes"
(Sillen and Martell 1964). However, these collections are seldom proposed as critical
tables of thermodynamic parameters, and if they are, the criteria of selection are not
clearly indicated. In a few cases, bad data are easily spotted. But generally the selection is not so straightforward. The reluctance of compilers of tables to recommend
"the most reliable data" can be understood. But one must also realize that equilibrium
analysis does not generally produce "fingerprint" evidence of the complexes claimed,
rather it gives "probable" results. This means that the model (the stoichiometry of the
complexes) that we propose to explain the experimental data may be not unique. The
philosophy supporting EA recommends explaining the experimental data with the
minimum number of complexes, which, for instance, produce the lowest value of the
minimized function or the best fit in graphical approaches. Then, most obviously: the
more accurate the experimental data, the more probable the results, in terms of the
composition and formation constants of the complexes chosen.