Seawater ionic constituents

Table 12.1 lists elements in seawater that have concentrations greater than 0.1/tmol kg-1. The first seven elements in Table 12.1 (Cl, Na, Mg, S, Ca, K and C) have concentrations greater than or equal to 0.002 mol kg 1 and are generally treated as the major ionic constituents of seawater. The ion pairing constants of these relatively weakly interacting elements have been studied as a function of temperature, pressure and ionic strength. An equilibrium speciation...

Table 3.1 Major ionic constituents of seawater (after Harvey 1982 and other sources)...

The oceans of the world by no means have uniform physical or chemical properties. There are major differences in near-surface salinity and temperature gradients among the major oceans, which depend on local meteorological conditions, ocean current regimes, and nearness to land or ice masses. However, a combination of tides, currents, waves, and upwellings tend to keep the waters of the oceans more or less well mixed, so that on the whole the ratios of the major dissolved ionic constituents of seawater are nearly constant (Table 1.8). To a crude approximation, seawater is a solution 0.5 M in NaCl and 0.04 M in MgS04. The mean pH of the ocean is about 8.2. 

TABLE 18.5 Ionic Constituents of Seawater Present in Concentrations Greater Than 0.001 g/kg (1 ppm)... 

Bromate is a disinfection by-product that is produced from the ozonation of source water that contains naturally occurring bromide, whereas chlorite and chlorate are produced as a result of using chlorine dioxide as a disinfectant. Recently, bromate has become the most important inorganic oxyhalide by-product, and its concentration in drinking water has to be controUed. Another chaUenge is seawater, which represents a vay difficult matrix for the analysis of trace ionic constituents, because chloride, sulfate, and sodium are the primary ions and they are presort at extremely high concentrations. " ... 

Many other ions are present in much lower concentrations. Estimate the osmotic pressure of seawater due to its ionic constituents. 

The oceans contain vast quantities of ionic calcium,, to the extent of 400 mg/L of seawater (3). Calcium is present ia living organisms as a constituent of bones, teeth, shell, and coral. It is essential to plant as well as animal life. 

Solutions of substances that are good conductors of electricity are called electrolytes. Sodium chloride, the major constituent of seawater, is a strong electrolyte. Most salts, as well as strong acids and bases, are strong electrolytes because they remain in solution primarily in ionic (charged) forms. Weak acids and bases are weak electrolytes because they tend to remain in nonionic forms. Pure water is a nonconductor of electricity. 

One advantage of the ion interaction theory is that it can be applied to solutions of different salinities, that is, to brines, seawaters with different salinities, and estuarine waters. While the ionic medium method provides a very simple solution to many problems, especially for the speciation of constituents in the open ocean, it cannot readily be applied to solutions of different salinity that is, brines, seawater, and estuarine waters must be treated as separate solvents (Pabalan and Pitzer, 1988). 

Solvents are those chemicals that make up the bulk of a solution in which a solute has been dissolved by molecular or ionic forces. Solutions include gases in liquids liquids in liquids or gases, liquids, and solids in solids (mixtures of gases are said to be miscible). Common solutions include seawater, steel, carbonated beverages, and paints. Solvents work according to complex principles resulting in a solution which is more stable than its constituents. They fall into two main categories ... 

A difference between seawater and most other natural aquatic sources is its high ionic strength. This often complicates direct CE analysis. Another serious problem is due to large differences in concentrations between matrix components, such as sodium or chloride, and minor cationic and anionic constituents. One way to solve seawater matrix effects on separation and detection is sample dilution, but compounds present at lower concentrations can be diluted below the detection limits. 

Only 11 elements can be considered major components of seasalt the cations sodium, potassium, magnesium, calcium and strontium, and the anions chloride, sulphate, bromide, hydrogen carbonate (carbonate), borate (borid acid) and fluoride. These major dissolved constituents (concentrations > 1 mg/kg in ocean waters) make up > 99 % of the soluble ionic species of seawater. The elemental ratios are relatively constant throughout the world ocean, and their concentrations change due to the addition or substruction of water only (concept of conservatism ). Therefore, it is possible to characterize the composition by determining only one constituent that is easy to measure and is conservative in its behaviour. An example is chlorinity (Cl, as defined in Section 11.2.4). 

Early in the Cold War Era a second important distinction was realized regarding the fate of different radionuclides as fallout is being formed. Certain elements, notably Cs and I, partition strongly into the vapor phase at significantly lower temperatures than most other elements. These elements tend to be highly mobile and may even reside primarily as dissolved constituents in water droplets in clouds. Alternatively, elements such as the rare earths and Pu are not readily vaporized and tend to associated with refractory solid particles. These differences can lead to significant partitioning of the different radioisotopes found in fallout. Thus, for example in seawater, 70% of the fallout Cs and 87% of the fallout Sr are in an ionic form while the ionic fractions of radioactive Ce and Ru are 2% and 0% relatively. Instead, they reside primarily in the colloidal and suspended solid materials (Sereda, 1966). 

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