Seawater carbonate alkalinity

A pressure- and temperature-independent property of seawater that determines in part the carbon content of seawater. Carbonate alkalinity is the sum of the concentration of bicarbonate plus two times the concentration of the... 

All of these measures of saturation are very useful in predicting the geographic distribution of sedimentary carbonates. For example, sediments lying below waters that are undersaturated with respect to calcite should be devoid of calcareous oozes. Since direct measurement of [COj ] observed difficult, its concentration is usually computed from two more easily measured parameters, the carbonate alkalinity and pH of a seawater sample. 

THE EFFECT OF POC AND PIC FORMATION AND DEGRADATION ON THE pH, CARBONATE ALKALINITY, AND XCO2 OF SEAWATER... 

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. 

Figure 8.35 shows the redox state and acidity of the main types of seawaters. The redox state of normal oceanic waters is almost neutral, but they are slightly alkaline in terms of pH. The redox state increases in aerated surface waters. Seawaters of euxinic basins and those rich in nutrients (eutrophic) often exhibit Eh-pH values below the sulfide-sulfate transition and below carbonate stability limits (zone of organic carbon and methane cf figure 8.21). We have already seen (section 8.10.1) that the pH of normal oceanic waters is buffered by carbonate equilibria. At the normal pH of seawater (pH = 8.2), carbonate alkalinity is 2.47 mEq per kg of solution. 

Carbonate alkalinity is not a directly analyzable quantity, but it can be derived from the titration alkalinity (A ) by a relationship which is temperature and pressure dependent. pH is also variable with pressure and temperature. In addition, pH can be defined several ways depending on which buffer system is used, whether liquid junction potentials are considered, and what definition of ionic strength is used. The values of the apparent dissociation constants and calcium carbonate solubility constants are dependent on exactly what definition of seawater pH is used and what standardization technique is used (15, 16, 17). 

Lueker T. J., Dickson A. G., and Keeling C. D. (2000) Ocean PCO2 calculated from dissolved inorganic carbon, alkalinity, and equations for Ki and K-i. validation based on laboratory measurements of CO2 in gas and seawater at equilibrium. Mar Chem. 70, 105-119. 

Another practical consideration when dealing with the seawater carbonic acid system is that in addition to carbonate alkalinity, H and OH , a number of other components can contribute to the total alkalinity (TA). The seawater constituent that is usually most important is boric acid. Under most conditions, boric acid contributes — 0.1 mmol alkalinity it is usually taken into consideration when making calculations. Nutrient compounds, such as ammonium, phosphate, and silica, whose concentrations in seawater are highly variable, can also influence alkalinity. They must be taken into account for very precise work. In anoxic pore waters a number of compounds, such as hydrogen sulfide and dissolved organic matter, can be significant contributors to alkalinity (e.g., see Berner et al, 1970). 

Example 7.8. Calcite in Seawater Compare the composition of a CaC03(s) (calcite)-C02-H20 seawater model system, made by adding calcite to pure H2O containing the seawater electrolytes (but incipiently no Ca and no carbonates and, for simplicity, no borate) and by equilibrating this solution at 25°C and 1 atm total pressure with the atmosphere (pcoi = 3.55 X 10 atm), with the composition of a real surface seawater whose carbonate alkalinity, Ca(II) concentration, and pH have been determined as 2.4 x 10 eq liter", 1.06 X 10 M, and 8.2, respectively. Estimate the extent of oversaturation of this seawater with respect to calcite. The solubility of calcite at 25 °C is taken as "K q = [Ca/-] [CO3 ] = 5.94 X 10 , where [Ca ] and [CO37] are the concentration of total soluble Ca(II) ([Ca ] plus concentration of Ca complexes with medium ions) and of total soluble carbonate ([CO "] and concentration of carbonate complexes with medium ions), respectively. The other constants needed, Henry s law constant and the acidity constant of H2CO, are taken as ... 

Several factors must be taken into account. Firstly, there is the chemistry of seawater itself. Compared with distilled water or even a solution of sodium chloride (NaCl) of equivalent ionic strength (see Box 5.1) to the oceans, seawater has a significantly greater ability to take up excess C02. This comes about from the existence in seawater of alkalinity (see Sections 5.3.1, 6.4.4 Box 5.2) in the form of carbonate ions (COf ), which can react with C02 molecules to form bicarbonate ions (HCO ) ... 

Bicarbonate ion is usually the chief anion in freshwaters. In and on silicate rocks, the HCOj concentration is usually 50 to 200 mg/L, whereas in groundwaters that contact a few percent carbonate materials up to pure limestone and dolomite, bicarbonate levels are usually in the range of 200 to 400 ppm. Seawater contains 140 mg/L HCOj. Carbonate alkalinity (CO3 ) rarely exceeds 10 mg/L. Why The presence of caustic alkalinity (free OH ) at pH s above 10 usually indicates artificial contamination of a water by, for example, Ca(OH)2 (portlandite) from the setting of concrete at newly completed wells. Cg concentrations can reach 1000 ppm as HCO3 in sodium carbonate-bicarbonate brines found in evaporative, closed basin lakes. 

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 ... 

Removal of dissolved C does not affect the amount of cations in seawater, so alkalinity (A) remains constant and Eqn 3.13 indicates that the concentration of carbonate ions will increase. Dissolved C02 levels decrease as both salinity and temperature increase (Weiss 1974) and, ultimately, seawater can become supersaturated with respect to calcium carbonate. Removal of CaC03 affects both alkalinity (because of the decrease in Ca2+ ions) and dissolved C concentrations. 

In pure water, therefore, the operation of the above mechanisms would lead smoothly to conversion of ozone to molecular oxygen. However, actual waters almost always contain substrates which can compete efficiently for either ozone or the intermediate free radicals such as OH that are formed during its decomposition. In particular, in seawater molecular ozone reacts most rapidly with dissolved T, despite its low concentration of about 10" A/(Haag and Hoigne, 1983) in moderately alkaline freshwater with a typical degree of hardness (roughly millimolar in carbonate alkalinity), it is usually the carbonate or (with lower effectiveness) bicarbonate anion that reacts with the ozone-derived HO ... 

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. 

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... 

A computer program has been used to calculate the magnitude of systematic errors incurred in the evaluation of equivalence points in hydrochloric acid titrations of total alkalinity and carbonate in seawater by means of Gran plots. Hansson [13] devised a modification of the Gran procedure that gives improved accuracy and precision. The procedure requires approximate knowledge of all stability constants in the titration. 

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. 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

The ocean is chemically homogeneous and, in addition to carbonates, contains Ca2+ plus enough of inert ions to adjust alkalinity to the values observed in modem seawater ( 2.1 x 10 3eqkg 1). The inert ions, assigned as Na+ and Cl- for convenience, are assumed to be time-invariant in the ocean. 

Figure 7.17 Same as Figure 7.16 for pCOl, seawater alkalinity A, runoff [Ca2+] and the fraction F of precipitated calcite which is preserved on the ocean floor. It takes a little less than 10000 years for runoff calcium to neutralize the excess dissolved COz, but calcite precipitation takes much longer to eliminate Ca and carbonate excess. 

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