The preponderance in seawater of the strongly basic ions, sodium, potassium and calcium, imparts a slight alkalinity and enables a considerable amount of carbon dioxide to be contained in solution. This is of great biological importance because carbon dioxide is a raw material for photosynthesis. Under natural conditions, plant growth in the sea is probably never limited by shortage of carbon dioxide.
Carbon dioxide is present in seawater mainly as bicarbonate ions, but there are also some dissolved CO2, undissociated H2CO3 and carbonate ions. At the surface, dissolved CO2 tends towards equilibrium with atmospheric CO2, the oceans acting as a regulator of the amount of CO2 in the atmosphere. The overall equilibrium can be represented as follows:
Dissolved CO2 J H2CO3 J H+ + HCO3~ J H+ + CO32"
The role of the oceans and especially of coral reefs in reducing the amount of atmospheric carbon dioxide and hence helping to control global warming is discussed in Chapter 10.
The pH of seawater normally lies within the range 7.5-8.4, the higher values occurring in the surface layer where CO2 is withdrawn by photosynthesis. The presence of strong bases together with the weak acids H2CO3 and H3BO3 confers an appreciable buffer capacity. Addition of acid to seawater depresses the dissociation of H2CO3 and H3BO3, and there is not much change of pH while reserves of CO32, HCO3 and H2CO3 ions remain. Addition of alkali increases the dissociation of H2CO3 and H3BO3, and the pH remains fairly stable so long as undissociated acid is still present.
The dissociation constants of the equilibrium are influenced by temperature, pressure and salinity. Increase of temperature or pressure causes a slight decrease of pH. At great depths the lowering of pH due to pressure may be sufficient to cause solution of some forms of calcium carbonate, which is not a conspicuous component of sediments below about 6000 m (Pytkowicz, 1968).
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