264
1. Grenthe
centrations of the components are known. In addition, one must have access to a chemical database that contains equilibrium constants for the predominant chemical reactions
in the system. The chemical speciation describes the composition and the concentrations
of the species as a function of one or more master variables such as the free hydrogen
ion concentration, the free ligand concentration and the redox potential of the system.
When using an analytical tool, it is necessary to ascertain its range of validity, as
well as its precision and accuracy; this is true also for equilibrium analysis. The following list indicates some of the factors necessary to consider when making equilibrium analytical experiments and when assessing the quality of equilibrium data reported in the scientific literature.
• Is the chemical system studied at equilibrium, or not? It is fairly easy to decide this
in most experimental studies. It is much more difficult to do so in complex systems
in engineering or in nature. Some equilibria may be rapidly attained, e.g. many complex formation reactions, while others may take a very long time, e.g. redox reactions involving the transfer of more than one electron and for which there are major
changes in composition between the reduced and oxidized forms of a certain component. A typical example is the redox chemistry of sulphur, where most equilibrium reactions are extremely sluggish at room temperature.
• The equilibrium analysis provides information on the concentration of the species
formed from experimental values of the total concentrations of the different components and the equilibrium concentration of one or more species, often the free
metal ion or ligand concentration. The free concentration is often measured with an
electrode that responds to the activity, and we must therefore be able to transform
this quantity to concentrations. This is done using the following relationship:
aj= [Ad}';
(10.2)
where aj and}'; are the activity and the activity coefficient, respectively of species i.
How can we design our experiments so that the activity and concentrations are
equal, or related by a constant factor? This is where the ionic medium method enters the stage.
• The activity of the species present in solution varies with the total concentration of
the components. It is well-known that:
limaj= [Ad; [Ad ~ 0
(10.3)
From Eq. 10.3, one must not draw the conclusion that an equilibrium analysis
should be performed in very dilute solutions. In order to analyse even very simple
equilibrium systems it is necessary to vary the concentrations of the components
over a large concentration range, hence we cannot approach the limiting value of
an extremely dilute solution. Instead, we add a strong electrolyte to the chemical
system at a concentration that is much higher than that of the reactants. The strong
electrolyte must not be a reactant, i.e. it must not interfere chemically in any significant way with the reaction studied. The activity coefficients of the reactants in
these systems will then mainly be determined by the ionic medium electrolyte, as
explained by the ion-ion interaction models described in the following text and in
1. Grenthe
centrations of the components are known. In addition, one must have access to a chemical database that contains equilibrium constants for the predominant chemical reactions
in the system. The chemical speciation describes the composition and the concentrations
of the species as a function of one or more master variables such as the free hydrogen
ion concentration, the free ligand concentration and the redox potential of the system.
When using an analytical tool, it is necessary to ascertain its range of validity, as
well as its precision and accuracy; this is true also for equilibrium analysis. The following list indicates some of the factors necessary to consider when making equilibrium analytical experiments and when assessing the quality of equilibrium data reported in the scientific literature.
• Is the chemical system studied at equilibrium, or not? It is fairly easy to decide this
in most experimental studies. It is much more difficult to do so in complex systems
in engineering or in nature. Some equilibria may be rapidly attained, e.g. many complex formation reactions, while others may take a very long time, e.g. redox reactions involving the transfer of more than one electron and for which there are major
changes in composition between the reduced and oxidized forms of a certain component. A typical example is the redox chemistry of sulphur, where most equilibrium reactions are extremely sluggish at room temperature.
• The equilibrium analysis provides information on the concentration of the species
formed from experimental values of the total concentrations of the different components and the equilibrium concentration of one or more species, often the free
metal ion or ligand concentration. The free concentration is often measured with an
electrode that responds to the activity, and we must therefore be able to transform
this quantity to concentrations. This is done using the following relationship:
aj= [Ad}';
(10.2)
where aj and}'; are the activity and the activity coefficient, respectively of species i.
How can we design our experiments so that the activity and concentrations are
equal, or related by a constant factor? This is where the ionic medium method enters the stage.
• The activity of the species present in solution varies with the total concentration of
the components. It is well-known that:
limaj= [Ad; [Ad ~ 0
(10.3)
From Eq. 10.3, one must not draw the conclusion that an equilibrium analysis
should be performed in very dilute solutions. In order to analyse even very simple
equilibrium systems it is necessary to vary the concentrations of the components
over a large concentration range, hence we cannot approach the limiting value of
an extremely dilute solution. Instead, we add a strong electrolyte to the chemical
system at a concentration that is much higher than that of the reactants. The strong
electrolyte must not be a reactant, i.e. it must not interfere chemically in any significant way with the reaction studied. The activity coefficients of the reactants in
these systems will then mainly be determined by the ionic medium electrolyte, as
explained by the ion-ion interaction models described in the following text and in