Bicarbonate in seawater

In order to investigate the dependence of a on dissolved CO2 and pH, we prepared solutions of simpler composition in which the concentrations of several important free ions and ion-pairs were maximized. In order to do this, we calculated the chemical speciation of carbon-ate/bicarbonate in seawater and in solutions of KCl, NaCl, NgCl2 end mixtures of these salts as a function of pH in the range 6.0 to 8.9. Ion strengths were kept equivalent to those of natural seawater (y 0.7 at S = 35 / <). For the seawater calculations, we assumed the concentrations of major cations given by Culkin (5) and a total CO2 concentration of 2.333 x 10 N. We used an iterative con uter program to solve a series of simultaneous equations based on thermodynamic association constants and estimated activity coefficients at ionic strengths of solutions for free carbonate and bicarbonate, and for the carbonate/bicarbonate ion-pairs with Mg, Ca and Na (6). Our calculations did not consider the reduction in available cations... 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Calcium ions in seawater muds can be controlled and removed by forming insoluble precipitates accomplished by adding alkalis such as caustic soda, lime, or barium hydroxide. Soda ash or sodium bicarbonate is of no value in controlling the total hardness of sea water. 

Various workers have discussed the determination of total alkalinity and carbonate [ 10-12], and the carbonate bicarbonate ratio [ 12] in seawater. A typical method utilises an autoanalyser. Total alkalinity (T milliequivelents per litre) is found by adding a known (excess) amount of hydrochloric acid and back titrating with sodium hydroxide solution a pH meter records directly and after differentiation is used to indicate the end-point. Total carbon dioxide (C milliequivelents per litre of HCO3 per litre) is determined by mixing the sample with dilute sulfuric acid and segmenting it with carbon dioxide-free air, so that the carbon dioxide in the sample is expelled into the air segments. The air... 

Sulphate has been determined in seawater by photometric titration with hydrochloric acid in dimethyl sulfoxide [223]. The sample (5 ml) is slowly added to dimethyl sulfoxide (230 ml) and titrated with 0.02 M hydrochloric acid (standardised against sulfate) with bromo-cresol green as indicator. Since borate, carbonate, and bicarbonate interfere, a separate determination of alkalinity is necessary. 

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

In Chapter 5. we noted that COj readily dissolves in seawater to form several inorganic carbon species. TDIC is defined as the sum of the concentration of those species, i.e., the sum of the carbonate, bicarbonate, carbonic acid, and carbon dioxide concentrations. 

Most of the titratable charge in seawater is supplied by bicarbonate because its concentration is much greater than that of carbonate or any of the other weak bases in seawater, such as B(OH). A typical acid titration curve for a seawater sample is shown in Figure 15.7. If the titration is performed in an open container, initial addition of acid does not cause much of a drop in pH. During this phase of the titration, is readily consumed, first by carbonate (Eq. 5.57) and then by bicarbonate (Eq. 5.56). Most of the buffering is provided by bicarbonate because of its high concentration. Once most of the bicarbonate has been consumed, further addition of acid causes a rapid decline in pH. 

In practice, the alkalinity of seawater is determined by titrating a sample until the pH drops to 3, well below the bicarbonate equivalence point (pH 4.5). Although most of the titratable negative charge is contributed by bicarbonate and carbonate, the other weak bases present in seawater do consume some acid above and below the bicarbonate... 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

Reverse weathering is thought to occur during halmyrolysis, diagenesis, and catagenesis as clay minerals react with the major ions, bicarbonate, and DSi in seawater. The nature of these reactions is not well known. Their general form is thought to be... 

Carbonate alkalinity The concentration of negative charge in seawater contributed by bicarbonate (HCO3) and carbonate (COj ). It is usually reported in units of meq/L. 

Magnesium hydroxide is commonly produced from seawater, which is rich in Mg2+ ion. The average concentration of Mg2+ in seawater is about 1,300 mg/L. The first step of the process involves removal of interfering substances from seawater, the most notable being the water-soluble calcium bicarbonate. Bicarbonate removal is crucial, as it can form insoluble calcium carbonate, a side product that cannot be separated from magnesium hydroxide readily. Acidification of seawater converts bicarbonate into carbon dioxide, which is degassed by heating. Alternatively, seawater is treated with bme to convert calcium bicarbonate to carbonate ... 

Depending on the relative nucleophilicities, [Nu]50% ranges from micromolar to molar concentrations (Table 13.5). Although these values represent only order-of-magnitude estimates, they allow some important conclusions. First, in uncontaminated freshwa-ters (where bicarbonate typically occurs at about 10"3 M, chloride and sulfate occur at about 10 4 M, and hydroxide is micromolar or less, Stumm and Morgan, 1996), the concentrations of nucleophiles are usually too small to compete successfully with water in SN2 reactions involving aliphatic halides. Hence the major reaction will be the displacement of the halide by water molecules. In salty or contaminated waters, however, nucleophilic substitution reactions other than hydrolysis may occur Zafiriou (1975), for example, has demonstrated that in seawater ([CL] 0.5 M) an important sink for methyl iodide is transformation to methyl chloride ... 

Berner, R. A. Activity coefficients of bicarbonate, carbonate and calcium ions in seawater. Geochim. Cosmochim. Acta 29, 947-965 (1965). 

Pytkowicz, R.M. Activity coefficients of bicarbonates and carbonates in seawater, Limnol. Oceanogr. 20, 971-975 (1975). 

An alternative pathway to bicarbonate is possible due to reaction of CO2 and OH in water but is normally less important (Skirrow, 1975). The proton concentration is also inbuenced by total alkalinity and water dissociation, which, in turn, will inbuence the details of the simplibed chemical process depicted above. The relative concentrations of the DIC species in Equation (15) are mainly a function of pH, temperature, and salinity. In seawater, bicarbonate is the dominant species (Skirrow, 1975), and bicarbonate and carbonate are the main components of alkalinity (Broecker and Peng, 1974). The chemical equilibrium for each of the steps in the DIC system in Equation (15) is determined from the specibc temperature sensitive reaction constants (Aix). Note that in the complex system of seawater, empirical Kj values, rather than thermodynamic theoretical values, are usually adopted. Such reacbon constants are of course sensitive to the effects of molecular mass, with molecules containing heavy isotopes favoring slower reactions rates, and are therefore associated with temperature-sensitive isotopic fractionations. Typical equilibrium fractionation values for the carbonate system in dilute solution at 25 °C are (Deuser and Degens, 1967 Mook, 1986 Mook et al., 1974) ... 

A shift in community composition may also be important to the biological pump if it is from a calcareous to a noncalcareous phytoplankton, as precipitation of CaCOs diminishes the ocean s ability to hold dissolved CO2. DIC (XCO2) is present in seawater as several species, dissolved CO2, carbonic acid, and the dissociated forms, bicarbonate ion, and carbonate ion ... 

As discussed above, the chemistry of carbon in seawater is such that less than 1% of the carbon exists as dissolved CO2. More than 99% of the DlC exists as bicarbonate and carbonate anions (Table 3). The chemical equilibrium among these three forms of DlC is responsible for the high solubility of CO2 in the oceans. It also sets up a buffer for changes in oceanic carbon. The buffer factor (or Revelle factor), is dehned as follows ... 

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