Seawater constant ionic media

Because of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

The chemistry of the carbonic acid system in seawater has been one of the more intensely studied areas of carbonate geochemistry. This is because a very precise and detailed knowledge of this system is necessary to understand carbon dioxide cycling and the deposition of carbonate sediments in the marine environment. A major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a constant ionic medium. This concept is based on the observation that the salt in seawater has almost constant composition, i.e., the ratios of the major ions are the same from place to place in the ocean (Marcet s principle). Possible exceptions can include seawater in evaporative lagoons, pores of marine sediments, and near river mouths. Consequently, the major ion composition of seawater can generally be determined from its salinity. It has been possible, therefore, to develop equations in which the influence of seawater composition on carbonate equilibria is described simply in terms of salinity. 

Using seawater as a constant ionic medium we can define pH by the relationship... 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

Implicit in the recommendations of Pytkowicz and co-workers for pH measurements in seawater is the belief that the liquid junction potential between seawater of a given salinity and a saturated solution of potassium chloride is independent of the nature and concentration of solutes present at low concentrations in the seawater solvent. This would indeed be the case if seawater is a true constant ionic medium. Hawley and Pytkowicz (5) have estimated that this potential difiFerence amounts to 3.2 mV. As already indicated, this constancy of the liquid junction potential is essential for establishing an experimental scale of pmn. 

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

Activity coefficients defined within the infinite dilution activity scale cannot be formulated theoretically for the ionic medium of seawater. Since the oceans contain an ionic medium of practically constant composition, the ionic medium activity scale might be used advantageously in studying acid-base and other equilibria in seawater (see also Appendix 6.2 in Chapter 6). 

In more concentrated solutions and in seawater, our calculations are usually based on the ionic medium scale that is, we use for the evaluation of the concentration of coexistent species at equilibrium apparent equilibrium constants (valid for the medium under consideration) expressed in concentration terms for example, for seawater. 

Another example is shown in Figure 10.6 where it can be seen that that oil (C14) droplets are less sticky in seawater than in pure water, although an attraction of about lOOk T, at very close approach, can still be considered a rather strong attraction. In many cases where we have particles dispersed in an aqueous medium, we have a significant ionic content (e.g. ionic surfactants, electrolytes or ions associated with the particle surface due to surface charge), which influences the value of a given Hamaker constant. 

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