Carbonate species in seawater

Fig. 11-7 Distribution of dissolved carbon species in seawater as a function of pH. Average oceanic pH is about 8.2. The distribution is calculated for a temperature of 15°C and a salinity of 35%o. The equilibrium constants are from Mehrbach et al. (1973).

Neutral molecules have activity coefficients essentially equal to unity in solutions of less than 10 mM ionic strength. At higher salt concentrations, most neutral molecules are increasingly salted out of water that is, the activity coefficient > 1, so that a, /c, < 1 for molecules in higher ionic strength solutions. In our discussion of dilute aqueous acids and bases, we will assume ideal behavior of the neutral species. The importance of salting out of dissolved CO2 will be reflected in considering dissolved carbonic species in seawater (Chapter 4). 

The plot in Fig. 4.2 demonstrates the relative importance of the three carbonate species in seawater as a function of pH. At pH = pKj the concentrations of GO2 and HGO3 are equal and at pH = pK the concentrations of HGO3 and GOg" are equal. Since we know that the pH of surface waters is about 8.2, it is clear that the dominant carbonate species is HGO3. What has been done so far, however, does... 

Although most of the reactions between carbonate species in seawater are nearly instantaneous, the hydration of CO2... 

Appendix 4A1.1 describes the equations necessary for determining the concentrations of carbonate species in seawater for the three different definitions of alkalinity given in the text. Appendix 4A1.2 is a listing of the Matlab program for determining carbonate buffer species by using the equations for the case where Aj = Ac b-... 

Given any two of the four quantities SC, Aik, pH, Pcoj/ the other two can always be calculated provided appropriate equilibrium constants are available (the equilibrium constants depend on temperature, salinity, and pressure). The standard analytical technique for the carbonate species in seawater measures alkalinity and total carbon simultaneously in an acid titration. Hydrogen ion concentration can then be determined with the equation ... 

Radium, like most other group II metals, is soluble in seawater. Formation of Ra and Ra by decay of Th in marine sediments leads to release of these nuclides from the sediment into the deep ocean. Lead, in contrast, is insoluble. It is found as a carbonate or dichloride species in seawater (Byrne 1981) and adheres to settling particles to be removed to the seafloor. 

Carbon dissolved in seawater takes part in fast chemical reactions involving the species dissolved carbon dioxide H2CQ3, bicarbonate ions... 

In seawater, the major chemical species of copper are Cu(OH)Cl and Cu(OH)2 and these account for about 65% of the total copper in seawater (Boyle 1979). The levels of copper hydroxide (Cu(OH)2) increase from about 18% of the total copper at pH 7.0 to 90% at pH 8.6 copper carbonate (CuC03) dropped from 30% at pH 7.0 to less than 0.1% at pH 8.6 (USEPA 1980). The dominant copper species in seawater over the entire ambient pH range are copper hydroxide, copper carbonate, and cupric ion (USEPA 1980). Bioavailability and toxicity of copper in marine ecosystems is promoted by oxine and other lipid soluble synthetic organic chelators (Bryan and Langston 1992). 

Figure 4.6. Carbonate species of seawater (20°C) in equilibrium with the atmosphere. A comparison with Figure 4.5 shows the influence of the salt concentrations on the equilibrium distribution.

Table 4.8. 1 The degree of approximation involved in calculations using Eq. (4.38) (1) Distribution of carbonate species in surface seawater at chemical equilibrium (25 °G, S = 35). (2) After the addition of 20 jmol kg of CO2 (a) guess using Eq. (4.38), (b) assuming Ax=Ac only, (c) carbonate equihbriiun equations assmning Ax=Ac b. Note differences in the changes in HGO f and GO. (3) The same as (2) except for dissolution of the equivalent of 20 ijmolkg GaG03. All concentrations are in [jmolkg except Ac b and Ac, which are peqkg". ...

In seawater, important amounts of sulfate, bicarbonate, and carbonate are ion-paired with calcium, magne.sium and sodium ions (cf. Carrels and Christ l%5 Stumm and Morgan 1981). Percentage distributions of free and ion-paired species in seawater, according to Pytkowicz and Hawley (1974), are given here, where L denotes the respective anion in the ion pair. 

The predominant carbonate species in normal seawater is bicarbonate (HCO3 ), close to 70 % of the dissolved inorganic carbon (DIC). About 10 % of the total calcium in normal sea-water exists as CaS04 complex. This complex is rather insignifi-... 

All artificial seawater and salt solutions contained 8xl0 N KBr. a Rate term based upon the concentration of the relevant carbonate species in pH 8.1 seawater containing 2.333xl0 3 H total C02. 

Thus, in summary, most of the fast exponential decay, a, of Br2 observed in seawater is due to an interaction with carbonate species in which 003, NaC03 and HgCOy are all important reactants. The reaction product is unknown and the significance (if any) of the term C is also unclear. 

It is interesting to note that in fresh water OH reacts with the carbonate/bicarbonate system directly (H). In seawater, OH initially gives rise to Br2, which then reacts with carbonate species in an unknown reaction. Efforts are underway to study this reaction in more detail using pulse radiolysis, since the products are currently unknown, though C03 seems likely. If C03 is the product, the principal effect of Br in seawater is just to act as an intermediate in converting OH to C03. ... 

Thorium generally exists as a neutral hydroxide species in the oceans and is highly insoluble. Its behavior is dominated by a tendency to become incorporated in colloids and/or adhere to the surfaces of existing particles (Cochran 1992). Because ocean particles settle from the water column on the timescale of years, Th isotopes are removed rapidly and have an average residence time of = 20 years (Fig. 1). This insoluble behavior has led to the common assertion that Th is always immobile in aqueous conditions. While this is generally true in seawater, there are examples of Th being complexed as a carbonate (e.g.. Mono Lake waters, Anderson et al. 1982 Simpson et al. 1982) in which form it is soluble. 

The basic constituent of seashells is calcium carbonate, an insoluble compound formed from calcium ions secreted from the cells of the shellfish and carbonate ions present in seawater. But calcium carbonate is a white solid. The colors of seashells often arise from impurities and metabolic waste products captured in the solid shell as it is formed. Coloration is dictated by both diet and water habitat. For example, some cowries that live and feed on soft corals take on the hue of the coral species. Yellow and red colors often arise from carotenoid pigments such as //-carotene. Light refraction often generates the iridescent mother-of-pearl hues. 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Copper may exist in particulate, colloidal, and dissolved forms in seawater. In the absence of organic ligands, or particulate and colloidal species, carbonate and hydroxide complexes account for more than 98% of the inorganic copper in seawater [285,286]. The Cu2+ concentration can be calculated if pH, ionic strength, and the necessary stability constants are known [215,265-267]. In most natural systems, the presence of organic materials and sorptive surfaces... 

Laughlin et al. [122] analysed chloroform extracts of tributyltin dissolved in seawater using nuclear magnetic resonance spectroscopy. It was shown that an equilibrium mixture occurs which contains tributyltin chloride, tributyl tin hydroxide, the aquo complex, and a tributyltin carbonate species. Fluorometry has been used to determine triphenyltin compounds in seawater [123]. Triph-enyltin compounds in water at concentrations of 0.004-2 pmg/1 are readily extracted into toluene and can be determined by spectrofluorometric measurements of the triphenyltin-3-hydroxyflavone complex. 

Fig. 6-6. The evolution of the lagoon s waters in response to oscillations in biological productivity. The results show the adjustment of the system from an initial composition equal to that of seawater. This figure shows dissolved carbon species, the saturation index, and the precipitation rate.

Many of the analytes of interest for solid phase chemical reference materials are the same as those in seawater, but the need for and the preparation of reference materials for suspended particulate matter and sediments is quite different. The low concentrations of many seawater species and the presence of the salt matrix create particular difficulties for seawater analyses. However while sediments frequently have higher component concentrations than seawater, they also have more complicated matrices that may require unique analytical methods. A number of particulate inorganic and organic materials are employed as paleoceano-graphic proxies, tracers of terrestrial and marine input to the sea, measures of carbon export from the surface waters to the deep sea, and tracers of food-web processes. Some of the most important analytes are discussed below as they relate to important oceanographic research questions. 

Carbonate rocks and foraminifera tests (a sample of mixed species) are consistently lower in 5 Mg than Mg from seawater by several per mil. In addition. Mg in calcite is consistently lower in 5 Mg than Mg in dolomite by approximately 2%o (Fig. 1). These data together with the samples of coeval speleothem calcite and waters show that the heavy isotopes of Mg partition to water relative to carbonate minerals. In this respect the Mg isotopes behave much like the isotopes of Ca (Gussone et al. 2003 Schmitt et al. 2003). There is not yet sufficient data to assess with confidence the temperature dependence of the fractionation of Mg isotopes between carbonates and waters, although Galy et al. (2002) concluded that the evidence so far is that temperature effects are below detection in the range 4-18°C. 

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