Chapter 10
Equilibrium Analysis, the Ionic Medium Method and
Activity Factors
1. Grenthe
10.1
Introduction
Chemical models are used extensively to describe environmental systems, transport
of pollutants, sorption phenomena, etc. The following chapter describes two different
models for ionic medium/ionic strength corrections of equilibrium constants used in
chemical speciation calculations. Published equilibrium constant data often refer to
ionic media very different from those encountered in nature and in engineering systems, and they have therefore to be recalculated to the conditions relevant for the system under study. In addition, one may be interested to compare experimental data
obtained in different ionic media in order to ascertain their quality. It is important to
notice that models used in science and technology are provisional; they are used as
tools, and it is their "practicality" that often decides which model one selects. The prime
requirement of a model is that it has a scientific basis and that it is internally consistent. Before describing the scientific basis and structure of ion interaction theories,
we will give a short discussion of equilibrium analysis, the method used to obtain precise information of the composition and equilibrium constants, i.e. the data used in
speciation calculations.
10.2
Equilibrium Analysis
Equilibrium analysis is an analytical method used to study the species present in multicomponent solution systems, most commonly aqueous solutions where the different
species are in rapid equilibrium with one another, as exemplified by the following twocomponent system:
pM + nLHMpLn; l~p~P and 1~ n~N
(10.1)
The method has some similarity with the classical elemental analysis; the analytical
data only provide information on constitution and the amounts of the different species, but not on their structure, the presence of isomers, etc. Equilibrium analysis is
the first step in the scientific exploration of problems in solution co-ordination chemistry such as the structure of complexes, chemical bonding and reactivity. The results
of equilibrium analysis is given as equilibrium constants, the numerical values of which
are valid at a certain ionic strength/ionic medium composition and temperature. Data
of this type are useful in many applications, e.g. to determine the chemical speciation
of complex chemical systems, such as surface and ground waters, where the total con-
Equilibrium Analysis, the Ionic Medium Method and
Activity Factors
1. Grenthe
10.1
Introduction
Chemical models are used extensively to describe environmental systems, transport
of pollutants, sorption phenomena, etc. The following chapter describes two different
models for ionic medium/ionic strength corrections of equilibrium constants used in
chemical speciation calculations. Published equilibrium constant data often refer to
ionic media very different from those encountered in nature and in engineering systems, and they have therefore to be recalculated to the conditions relevant for the system under study. In addition, one may be interested to compare experimental data
obtained in different ionic media in order to ascertain their quality. It is important to
notice that models used in science and technology are provisional; they are used as
tools, and it is their "practicality" that often decides which model one selects. The prime
requirement of a model is that it has a scientific basis and that it is internally consistent. Before describing the scientific basis and structure of ion interaction theories,
we will give a short discussion of equilibrium analysis, the method used to obtain precise information of the composition and equilibrium constants, i.e. the data used in
speciation calculations.
10.2
Equilibrium Analysis
Equilibrium analysis is an analytical method used to study the species present in multicomponent solution systems, most commonly aqueous solutions where the different
species are in rapid equilibrium with one another, as exemplified by the following twocomponent system:
pM + nLHMpLn; l~p~P and 1~ n~N
(10.1)
The method has some similarity with the classical elemental analysis; the analytical
data only provide information on constitution and the amounts of the different species, but not on their structure, the presence of isomers, etc. Equilibrium analysis is
the first step in the scientific exploration of problems in solution co-ordination chemistry such as the structure of complexes, chemical bonding and reactivity. The results
of equilibrium analysis is given as equilibrium constants, the numerical values of which
are valid at a certain ionic strength/ionic medium composition and temperature. Data
of this type are useful in many applications, e.g. to determine the chemical speciation
of complex chemical systems, such as surface and ground waters, where the total con-