84 



The Sea-water and its Physical and Chemical Properties 



As well as the concentration of free carbon dioxide there are other factors also that 

 affect the solubility and, while they are not so important, they must still be taken into 

 consideration. The first of these is the concentration of Ca2+ derived not from the 

 dissolved calcium carbonate but from the calcium sulphate and calcium chloride, that 

 is, the excess of calcium ions above that corresponding to the combined carbon 

 dioxide. These calcium ions, by the law of mass action, reduce the solubility of the 

 calcium carbonate. An additional factor affecting the situation is the increase in the 

 solubility product due to the presence of neutral salts in the same way as for carbon 

 dioxide. Table 33 shows that the constant K^ is a hundred times greater in sea-water 

 than in pure water. 



The solubility constant depends not only on the salinity but also on the temperature 

 and, unlike most salts, decreases with increasing temperature. The pressure (at 

 constant carbon dioxide pressure) also has a considerable effect on the solubility of 

 calcium carbonate, but it is not yet certain how large this effect is. The factors affecting 

 the solubihty of calcium carbonate in sea- water thus fall into two groups: (1) those that 

 increase the solubility such as increasing carbon dioxide concentration, salinity and 

 hydrostatic pressure ; (2) those that decrease the solubility such as increasing tempera- 

 ture and calcium concentration. The values for solubility given in Table 34 show that 

 in sea-water these factors more or less compensate each other so that there are no 

 major differences from the solubility in pure water. 



Table 33. Dependence of the solubility product, K'^,for calcium carbonate on the salinity 



and temperature 

 (After Wattenberg, 1936) 



The calcium carbonate content can be found by determination of the alkahnity which 

 varies in direct proportion to the variations (in milliequivalents/litre) in calcium car- 

 bonate. Since the variations in calcium content are not very large it is necessary to 

 determine the alkalinity very carefully (Wattenberg, 1930). 



Table 34. Solubility of calcium carbonate 

 (CaCOg) in milligrams per litre in 

 sea-water (355'%o)/or different tempera- 

 tures and carbon dioxide pressures (in 

 10-4 atm) 

 (After Wattenberg, 1936) 



Pco2 X 10* 



