Reactions of Carbon Dioxide in Water 263 



ions (HCO3-) and in the carbonate ions (C03=) is called the com- 

 bined CO2. If strong acid is added, the combined CO2 will be con- 

 verted to the free form. The amount of acid required to accomplish 

 this is a measure of the alkalinity, that is, of the amount of anions of 

 weak acids (chiefly HCO3" and COg^) in the water, and also of the 

 cations balanced against them. The alkahnity of sea water bears a 

 fairly constant relation to the chlorinity and hence to the salinity. 

 The relation of the equilibria of CO2 to the alkalinity of sea water is 

 discussed in further detail by Rakestraw (1950); general accounts of 

 the reactions of COo are given by Harvey (1945) for sea water and 

 by Ruttner ( 1953 ) for fresh water. 



When H2CO3 dissociates, it releases hydrogen ions (or more prop- 

 erly, hydronium ions, H3O+). These affect the pH, which is the 

 negative logarithm of the hydronium ion activity. The pH is a meas- 

 ure of the degree to which the water is acid ( pH below 7 ) or alkaline 

 (pH above 7). If the equilibrium which is established in the water 

 is near neutrality (pH = 7), most of the CO2 will be present as 

 HCOa" ion. At high pH values more CO2 is present as CO3" ion, 

 and at low pH values more is present in the free condition. Thus 

 pH and the relative distribution of CO2 in its three components are 

 mutually interdependent, as indicated in Fig. 7.9. Addition or re- 



l 5 6 7 8 9 10 11 12 



Fig. 7.9. Percentage of total carbon dioxide in each of its three forms in water as 

 a function of hydrogen ion concentration. (Emerson and Green, 1938;) 



moval of CO2 will affect pH; conversely, any other factor changing 

 the pH will affect the CO, equilibrium (Dye, 1952). 



The bicarbonate and carbonate ions and other anions of weak acids 

 form a buffer system which tends to resist changes in pH. The buffer 

 capacity of the water is determined by the abundance of these anions, 

 and it is therefore also directly related to the alkalinity. Sea water 

 and hard fresh water are relatively highly buffered natural media. 



