Ionic strength of seawater

The field of chemical oceanography/marine chemistry considers many processes and concepts that are not normally included in a traditional chemical curriculum. While this fact makes the application of chemistry to the study of the oceans difficult, it does not mean that fundamental chemical principles cannot be applied. The chapters included in this book provide examples of important chemical oceanographic processes, all taking place within the basic framework of fundamental chemistry. There are three principal concepts that establish many of the chemical distributions and processes and make the ocean a unique place to practice the art of chemistry (1) the high ionic strength of seawater, (2) the presence of a complex mixture of organic compounds, and (3) the sheer size of the oceans. 

It has been reported for both anionic [14,20,28] and non-ionic surfactants [2,15,23] that sorption increases with the number of carbon atoms in the hydrophobic chain (Table 5.4.5). The sorption coefficient of LAS in activated sludges [22] increases by 2.8 times with each methylene group, and a similar variation has been observed for river and marine sediments (Table 5.4.5). The partition coefficients obtained for the marine medium are slightly higher than those for river sediment, as a consequence of the higher ionic strength of seawater [14]. 

Seawater has high concentrations of solutes and, hence, does not exhibit ideal solution behavior. Most of this nonideal behavior is a consequence of the major and minor ions in seawater exerting forces on each other, on water, and on the reactants and products in the chemical reaction of interest. Since most of the nonideal behavior is caused by electrostatic interactions, it is largely a function of the total charge concentration, or ionic strength of the solution. Thus, the effect of nonideal behavior can be accoimted for in the equilibrium model by adding terms that reflect the ionic strength of seawater as described later. 

At the pH and ionic strength of seawater, the dominant dissolved species of silicon is orthosilicic acid [H4Si04(aq) or Si(OH)4(aq)]. The speciation of silicic acid is shown in Figure 5.19. At the pH of seawater, a minor amount of dissociation occurs, such that about 5% of the dissolved silicon is in the form of HjSiO faq). Dissolved organic complexes of silicon do not occur naturally. 

The lA method using Equation (13) for major ions in natural water compares well with other more precise methods (see below) up to an ionic strength of seawater (0.7) but not much higher. 

Table A6.2 includes historical information on the comparison of ion activity coefficients at the effective ionic strength of seawater. Activity coefficients measured in single salt solutions are compared with those measured in seawater and those calculated from an association model. We have to distinguish between total activity coefficients (cf. equations 3 and 4)...

The effect of a pressure increase to 1000 bars on some dissociation reactions is shown in Table 1.2. Increased pressure favors dissociation in every case. The effect is proportional to the size of-AV°. The solubility of calcite increases with increasing pressure as indicated by Table 1.3. The effect is reduced at the ionic strength of seawater. 

Clearly, the chief contribution to 7hco., electrostatic effects as described by the DH term. At the relatively low ionic strength of seawater the other effects combined change ynco, l s than 10%. 

Correia dos Santos, M. M., and M. L. S. Simoes Goncalves (1986), Electroanalytical Chemistry of Copper, Lead, and Zinc Complexes of Amino Acids at the Ionic Strength of Seawater (0.70 M NaClOJ, J. Electroanal. Chem. 208, 137-152. 

The concentrations of the major ionic components in seawater of any salinity can be calculated by using the data in Table III.3.9 because their concentration ratios are essentially constant. However, minor variations ( 10%) in Ca, Sr, and HCOJ concentrations are possible. The density of seawater is a complex function of the salinity and temperature. The ionic strength of seawater / = 0.5 X depends on the density and the salinity ... 

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