Ca2q 2HCOaq CaCO3aq CO H2Oeqn

We have already noted the importance of this reaction in buffering the pH of continental waters (see eqn. 5.12), and it behaves in exactly the same way in seawater. The Le Chatelier Principle (see Box 3.2) predicts that any process that decreases the concentration of HCO- in equation 6.4, will encourage dissolution of CaCO3 to restore the amount of HCO- lost (Fig. 6.8). The oceans contain an effectively infinite amount of CaCO3 particles suspended in surface waters and in bed sediments (see below). This CaCO3 helps maintain (buffer) the pH of the oceans at values between 7.9 and 8.1, in much the same way as pH in continental waters is buffered by reacting with limestone (see Section 5.3.1 & Fig. 5.5). If, for example, more atmospheric CO2 is taken up by the oceans as a result of global warming (see Sections 7.2.2 & 7.2.4), the acidity added to the oceans is largely neutralized by dissolution of CaCO3, such that the HCO- ion concentration is unchanged (Fig. 6.8), maintaining the pH around 8.

Seawater is a concentrated and complex solution in which the ions are close together, compared with those in a more dilute solution. Electrostatic interaction occurs between closely neighbouring ions and this renders some of these ions 'inactive'. We are interested in the available or 'active' ions and we correct for this effect, using activity coefficients, denoted g (see Section 2.6).

Activity coefficients are notoriously difficult to measure in complex solutions like seawater, but are thought to be around 0.26 for Ca2+ and around 0.20 for car-

Fig. 6.8 (a) Schematic diagram to illustrate the buffering effect of CaCO3 particles (suspended in the water column) and bottom sediments on surface seawater HCO- concentrations (after Baird 1995, with permission from W.H. Freeman and Company.). (b) A sample of the seawater in (a) will have a pH very close to 8 because of the relative proportions of CO2, HCO3- and CO32-, which in seawater is dominated by the HCO 3- species. Increased CO2 concentrations in the atmosphere from anthropogenic sources could induce greater dissolution of CaCO3 sediments including coral reefs.

Fig. 6.8 (a) Schematic diagram to illustrate the buffering effect of CaCO3 particles (suspended in the water column) and bottom sediments on surface seawater HCO- concentrations (after Baird 1995, with permission from W.H. Freeman and Company.). (b) A sample of the seawater in (a) will have a pH very close to 8 because of the relative proportions of CO2, HCO3- and CO32-, which in seawater is dominated by the HCO 3- species. Increased CO2 concentrations in the atmosphere from anthropogenic sources could induce greater dissolution of CaCO3 sediments including coral reefs.

bonate (CO2-). Measured Ca2+ and CO2- ocean surface concentrations are 0.01 and 0.000 29 moll-1 respectively and thus the ion activity product (IAP) (see Box 4.12) can be calculated:

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