The Sea-water and its Physical and Chemical Properties 



71 



(b) The Carbon Dioxide Cycle in the Ocean and its Relationship to the Atmosphere 



Unlike nitrogen and oxygen, the carbon dioxide in the sea is present not only in 

 solution but also in considerably larger amounts chemically combined as salts. 

 Conditions are thus much more complicated, and the situation has only been clarified 

 in recent times by Buch and McClendon using modern dissociation theory. Funda- 

 mentally one realized by this that the free and the combined carbon dioxide in solu- 

 tion are not independent of each other, but according to the law of mass action are in 

 chemical equilibrium with each other. The combinations occurring can be repre- 

 sented by the following equations : 



CO2 (in the air) ^ CO2 (in solution )+ HgO ^ H2CO3 (carbonic acid). 



The carbonic acid splits partially into its ions according to: 



H2CO3 ^ H^ 



which can dissociate further by: 



HCO3 



(bicarbonation), 



HCO3 ^ H+ + COg^ (carbonation). 



All these forms derived from carbon dioxide are present in sea-water principally as 

 carbonate and bicarbonate ions, and only to a lesser extent in the free state. Equili- 

 brium exists between these forms, the carbonate and the bicarbonate ions, free carbon 

 dioxide and the hydrogen ion, and it will be discussed later. The reasons are now 

 understood for the long time needed in oceanographic research to obtain suitable 

 accuracy in the determination of the carbon dioxide in solution in sea-water. 



Free carbon dioxide and carbon dioxide pressure. The solubility of carbon dioxide 

 in sea-water is relatively large, almost thirty times that of nitrogen. Fox investigated its 

 dependence on the temperature and on the chlorine content of NaCl solutions, and 

 corresponding measurements have been made by Krogh for sea-water. On this basis 

 of these investigations Buch and collaborators (1932) Wattenberg, (1936) prepared 

 tables showing the dependence of the solubility of carbon dioxide in sodium chloride 

 (NaCl) solutions on the temperature and the salinity. Table 28 shows a condensed 

 extract from these tables. The solubility of carbon dioxide decreases considerably 

 with increasing temperature and salinity. One litre of sea-water at 0° and 35T9%o S, 

 when in equilibrium with the atmosphere (partial pressure of carbon dioxide 0-0003 



Table 28. Solubility of carbon dioxide in sodium chloride (NaCl) solution in 

 millilitres per litre at a carbon dioxide pressure of 1 atm. 



10 



15 



20 



25 



30 



40 



1540 



1406 



1167 



990 



850 



742 



648 



572 



