Carbon dioxide equilibrium seawater

The radiocarbon ratio also evolves very rapidly from its initial value of -50 to an average value of about -8 per mil. This evolution is not a consequence of evaporative concentration but, instead, of an approach to equilibrium with atmospheric carbon dioxide. Average surface seawater contains significantly less radiocarbon than does the atmosphere because its isotopic composition is affected by exchange with the deep ocean as... 

Figure 4.2. The variation of total carbon dioxide (ICO2) and the saturation state of seawater with respect to calcite (Qc) with temperature for seawater with a total alkalinity of 2400 peq kg- seawater and in equilibrium with atmospheric CO2...

Due to the dynamic equilibrium between the atmospheric carbon dioxide and the oceanic bicarbonate and carbonate anions, the greatest amount of soluble calcium cations is contained in the ocean. This mass is four orders of magnitude higher than the total mass of bound calcium in living and dead matter of both terrestrial and aquatic organisms. The average calcium content in the seawater is 408 mg/L, and the overall pool is 559 x lO tons. 

When equilibrium with the atmosphere is reached, approximately 87% of ionic carbonate is present as bicarbonate ion, the remainder being carbonate. In many places, especially close to the surface, seawater is saturated with respect to calcium carbonate, which will precipitate slowly from solution, thus regulating the amount of carbonate in solution. This process is perhaps the most important of all the geological systems since it regulates the amount of carbon dioxide in the atmosphere. 

Zeebe, R. E., Wolf-Gladrow, D. A. Jansen, H. 1999. On the time required to establish chemical and isotopic equilibrium in the carbon dioxide system in seawater. Marine Chemistry, 65, 135-153. 

Of course natural solutions, such as seawater, are not necessarily at equilibrium. In Figure 16.2 we see a river carrying dissolved material, including calcium and carbonate ions, entering the sea. Carbonate ions are already there, because the sea is in contact with the atmosphere, which contains carbon dioxide, and when CO2 dissolves it produces carbonate and bicarbonate ions. Because calcium and carbonate are being added, there may be a tendency for them to increase beyond the equilibrium value, and for calcite to precipitate as a result. The product of the calcium and carbonate ion activities which are actually present in a solution, regardless of any theory, is called the ion activity product (lAP) for that solution. It follows that when lAP > K, calcite will precipitate, and when lAP < calcite will dissolve. The lAP/ Tsp ratio is called O, and the logarithm of the ratio is called the saturation index (SI), so that when SI > 0 calcite precipitates, and when SI < 0 calcite dissolves (Table 16.1). 

Carbon dioxide and its carbonate minerals play an important role in environmental chemistry and atmospheric physics. In natural waters, atmospheric CO2 has a significant influence on pH, which varies from alkaline seawaters to acidic low mineral lakes, rivers, and soil water. In freshwaters and in oceans the equilibrium relationships between the carbon dioxide, the chemical and biological components, temperature, and the pH are complex functions. Chemical thermodynamics provide quantitative relationships between chemical energy, ionic reactions, solubilities, speciation, pH, and alkalinity. In natural systems these relationships are also complex functions of chemical and biological effects. 

In either case, the production of hydroxyl ions results in an increase in pH for the electrolyte adjacent to the metal surface. In other terms, an increase in OH is equivalent to a corresponding reduction in acidity or H+ ion concentration. This situation causes the production of a pH profile in the diffuse layer, where the equilibrium reactions can be quite different from those in the bulk seawater conditions. Temperature, relative electrolyte velocity, and electrolyte composition will all influence this pH profile. There is both analytical and experimental evidence that such a pH increase exists as a consequence of the application of a cathodic current. In seawater, pH is controlled by the carbon dioxide system described in Eqs. (2.18) through (2.20) ... 

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