130
Chemical Oceanography, 4th Edition
We shall now discuss the ionic interactions that can affect the state or structure of metal
ions in seawater. Whether the thermodynamic activities can be directly related to biological
activities remains to be seen. It must also be kept in mind that the most probable thermodynamic state may not be the state found in the marine environment because of the slow
kinetics of the formation and degradation of metal complexes and various redox pairs.
To understand ionic interactions in aqueous electrolyte solutions, various chemical
models have been developed. The development of a chemical model for natural waters
has been strongly influenced by the coupling of physical chemical solution theory and
marine chemical, analytical, and experimental data. The major trends of the development
of a chemical model for seawater have resulted from the application of methods developed
on the physical chemistry of electrolyte solutions. In the past, it has taken a long time for
the physical chemical trends to influence marine chemistry. For example, it took 40 years
before the Arrhenius theory was accepted in oceanography and 30 years before the quantitative aspects of the Debye–Hückel (1923) and Bjerrum (1926) theories were applied to
seawater by Garrels and Thompson (1962). Although Wirth (1940) introduced the oceanographic community to the theoretically derived concentration behavior for densities in
1940, it has only been recently used to represent the equation of state of seawater.
In recent years, this trend has changed; it is hoped marine chemistry will accept and
apply these new theories and models to geochemical and biochemical problems. The
application of chemical models to examine ionic interactions in natural waters has been
confined to two major areas: (a) those that are concerned with the bulk thermodynamic
and transport properties of marine waters and (b) those that are concerned with the effect
of marine waters on the activity of dissolved solutes. This is discussed in more detail
elsewhere in this chapter. To understand the state and structure of an ion in seawater, one
must take a number of steps. We examine the state of an ion in seawater by considering
two processes: (a) the ion–water interactions that occur when an ion is transferred from
the ideal gas state to an infinitely large reservoir of water (i.e., where ions cannot interact
with one another),
M + (ideal gas) → M + (infinite dilute solution)
(4.1)
and (b) the ion–ion interactions that occur when an ion is transferred from infinite dilution
to seawater (where interactions of all of the ions in the mixture affect the state (i.e., plus–
plus, plus–minus, and minus–minus interactions):
M + (infinite dilute solution) → M + (seawater)
(4.2)
Before these two processes are discussed, we examine the unique properties of water as a
solvent system.
4.2 Water, the Unique Solvent
Although water makes up 96.5% of ocean waters and accounts for many of the unique
physical and chemical properties of seawater, its importance has been neglected by many
oceanographers. Horne (1972) attempted to stress the importance of understanding the
Chemical Oceanography, 4th Edition
We shall now discuss the ionic interactions that can affect the state or structure of metal
ions in seawater. Whether the thermodynamic activities can be directly related to biological
activities remains to be seen. It must also be kept in mind that the most probable thermodynamic state may not be the state found in the marine environment because of the slow
kinetics of the formation and degradation of metal complexes and various redox pairs.
To understand ionic interactions in aqueous electrolyte solutions, various chemical
models have been developed. The development of a chemical model for natural waters
has been strongly influenced by the coupling of physical chemical solution theory and
marine chemical, analytical, and experimental data. The major trends of the development
of a chemical model for seawater have resulted from the application of methods developed
on the physical chemistry of electrolyte solutions. In the past, it has taken a long time for
the physical chemical trends to influence marine chemistry. For example, it took 40 years
before the Arrhenius theory was accepted in oceanography and 30 years before the quantitative aspects of the Debye–Hückel (1923) and Bjerrum (1926) theories were applied to
seawater by Garrels and Thompson (1962). Although Wirth (1940) introduced the oceanographic community to the theoretically derived concentration behavior for densities in
1940, it has only been recently used to represent the equation of state of seawater.
In recent years, this trend has changed; it is hoped marine chemistry will accept and
apply these new theories and models to geochemical and biochemical problems. The
application of chemical models to examine ionic interactions in natural waters has been
confined to two major areas: (a) those that are concerned with the bulk thermodynamic
and transport properties of marine waters and (b) those that are concerned with the effect
of marine waters on the activity of dissolved solutes. This is discussed in more detail
elsewhere in this chapter. To understand the state and structure of an ion in seawater, one
must take a number of steps. We examine the state of an ion in seawater by considering
two processes: (a) the ion–water interactions that occur when an ion is transferred from
the ideal gas state to an infinitely large reservoir of water (i.e., where ions cannot interact
with one another),
M + (ideal gas) → M + (infinite dilute solution)
(4.1)
and (b) the ion–ion interactions that occur when an ion is transferred from infinite dilution
to seawater (where interactions of all of the ions in the mixture affect the state (i.e., plus–
plus, plus–minus, and minus–minus interactions):
M + (infinite dilute solution) → M + (seawater)
(4.2)
Before these two processes are discussed, we examine the unique properties of water as a
solvent system.
4.2 Water, the Unique Solvent
Although water makes up 96.5% of ocean waters and accounts for many of the unique
physical and chemical properties of seawater, its importance has been neglected by many
oceanographers. Horne (1972) attempted to stress the importance of understanding the