Activity coefficients in seawater

Tables Comparison of measured and calculated ion activity coefficients in seawater at 25 °C and 35 %o salinity, referenced to yci = 0.666 (Millero, 2001).

For some strongly associated species, ion association can be measured directly, for example, by spectroscopic means (Byrne and Millero, 1985). Because there is no method by which unequivocal structures of the species present may be obtained, ion association is a phenomenological concept. Ion-pair formation is involved in explaining deviations from normal behavior. However, the impossibility of knowing unambiguously the relevant activity coefficients in seawater implies that the concept of normal behavior is not clearly defined. 

Constituent Free Activity Coefficient Total Activity Coefficient in Seawater ... 

The effects of this specific reaction would not he represented in MSM calculations. In fact, it is assumed that no ion pairs are formed at 1= 0.7 in the experiments with individual electrolytes used to calculate activity coefficients in the MSM. This is true mainly for salts of potassium and chlorine, which are the ones primarily used in the MSM calculations. Because any ions that join to form ion pairs in a mixed electrolyte solution are effectively removed from solution with respect to reaction potential, an accurate determination of the extent of ion pairing in addition to the free ion activity coefficient must he made to determine the total activity coefficient in seawater. 

In a series of papers, Harvie and Weare (1980), Harvie el al. (1980), and Eugster et al (1980) attacked this problem by presenting a virial method for computing activity coefficients in complex solutions (see Chapter 8) and applying it to construct a reaction model of seawater evaporation. Their calculations provided the first quantitative description of this process that accounted for all of the abundant components in seawater. 

Computational speciation can be compared to analytical speciation for some species. There is always the problem that analytical methods also suffer from operational definitions, interferences, limits of detection, and associated assumptions. Nevertheless, there is no better method of determining accuracy of speciation than by comparing analytical results with computational results (Nordstrom, 1996). In the few instances where this has been done, the comparison ranges from excellent to poor. Examples of studies of this type can be found in Leppard (1983), Batley (1989), and Nordstrom (1996, 2001). Sometimes comparison of two analytical methods for the same speciation can give spurious results. In Table 3, measured and calculated ionic activity coefficients for seawater at 25 °C and 35%o salinity are compared, after adjusting to a reference value of yci = 0.666 (Millero, 2001). These values would indicate that for a complex saline solution such as seawater, the activity coefficients can be... 

By using medium-bound constants, we have bypassed activity coefficients in stoichiometric calculations. We can only do so if equilibrium constants valid for seawater or the medium of interest have been determined. If such constants are not known, we are forced to use so-called thermodynamic constants and estimated activity coefficients. ... 

Activity coefficients in most concentrated solutions reflect deviation from ideal behavior because of (1) the general electric field of the ions, (2) solute-water interactions, and (3) specific ionic interactions (association by ion pair and complex formation). None of the major cations of seawater appears to interact significantly with chloride to form ion pairs hence activity coefficients in these solutions appear to depend primarily on the ionic strength modified by the extensive hydration of ions. Thus synthetic solutions of these chlorides provide reference solutions in obtaining activity coefficients of the cations. The single activity coefficients for free ions in seawater can be obtained from mean activity coefficient data in chloride solutions at the corresponding ionic strength by various ways. 

Among the possible analytical methods for alkalinity determination, Gran-type potentiometric titration [2] combined with a curve-fitting algorithm is considered a suitable method in seawaters because it does not require a priori knowledge of thermodynamic parameters such as activity coefficients and dissociation constants, which must be known when other analytical methods for alkalinity determination are applied [3-6],... 

Table 6.4. Calculated molalities (m), activity coefficients (y), and log activities (a) of the most abundant species in seawater...

The resulting species distribution (Table 6.7), as would be expected, differs sharply from that in seawater (Table 6.4). Species approach millimolal instead of molal concentrations and activity coefficients differ less from unity. In the Amazon River water, the most abundant cation and anion are Ca++ and HCOJ in seawater, in contrast, Na+ and Cl- predominate. Seawater, clearly, is not simply concentrated river water. 

Calcite mole fraction X, solid state activity coefficients X, and Yp, the solute mole fractions of calcium at equilibrium in seawater. 

Despite the additional complexity, all the equations in Table 5.3 are functionally equivalent. That is, the activity coefficients approach a value of 1 as the ionic strength of the solution is decreased to 0 m. Thus, in dilute solutions, w,. In other words, the effective concentration of an ion decreases with increasing ionic strength. In contrast, the activity coefficients of uncharged solutes can be greater than 1 in solutions of high ionic strength, such as seawater. 

The individual activity coefficients calculated from (4.12), suitable for calibration of ISEs for chloride ions, the alkali metal and alkaline earth ions, are given in tables 4.1 and 4.2. Ion activity scales have also been proposed for KF [141], choline chloride [98], for mixtures of electrolytes simulating the composition of the serum and other biological fluids (at 37 °C) [106,107], for alkali metal chlorides in solutions of bovine serum albumine [132] and for mixtures of electrolytes analogous to seawater [140]. 

Gordon JE, Thome RL (1967b) Salt effect on the activity coefficient of naphthalene in mixed aqueous electrolyte solutions salts 2. Artificial and natural seawater. Geochim Cosmochim Acta 31 2433-2443... 

In summary, we can conclude that at moderate salt concentrations typical for seawater ( 0.5 M), salinity will affect aqueous solubility (or the aqueous activity coefficient) by a factor of between less than 1.5 (small and/or polar compounds) and about 3 (large, nonpolar compounds, n-alkanals). Hence, in marine environments for many compounds, salting-out will not be a major factor in determining their partitioning behavior. Note, however, that in environments exhibiting much higher salt concentrations [e.g., in the Dead Sea (5 M) or in subsurface brines near oil fields], because of the exponential relationship (Eq. 5-28), salting-out will be substantial (see also Illustrative Example 5.4). 

Estimate the solubility and the activity coefficient of phenanthrene in (a) seawater at 25°C and 30%o salinity, and (b) a salt solution containing 117 g NaCl per liter water. 

Explain in words how environmentally relevant inorganic salts affect the aqueous solubility of a (a) liquid, (b) solid, and (c) gaseous compound. Is it true that the effect is linearly related to the concentration of a given salt What is the magnitude of the effect of salt on the aqueous activity coefficient of organic compounds in typical seawater ... 

P 5.6 Evaluating the Effect of Temperature on the Solubility and/or the Activity Coefficient of a Gaseous Compound (Freon 12) in Freshwater and in Seawater... 

For an assessment of the global distribution of persistent volatile halogenated hydrocarbons, the solubility and activity coefficients of such compounds in natural waters need to be known. Warner and Weiss (1985) have determined the solubilities of dichlorodifluoromethane (Freon 12) at 1 bar partial pressure at various temperatures in freshwater and in seawater (35.8%o salinity) ... 

A colleague of yours who works in oceanography bets you that both the solubility as well as the activity coefficient of naphthalene are larger in seawater (35%o salinity) at 25°C than in distilled water at 5°C. Is this not a contradiction How much money do you bet Estimate C and for naphthalene in seawater at 25°C and in distilled water at 5°C. Discuss the result. Assume that the average enthalpy of solution (A wsHh Fig. 5.1) of naphthalene is about 30 kJmol-1 over the ambient temperature range. All other data can be found in Tables 5.3 and 5.7 and in Appendix C. 

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