The Sea-water ami its Physical and Chemical Properties 



65 



If the solution process is limited to purely physical absorption the absorbed gas 

 does not enter into chemical combination with the water; the situation is then fairly 

 simple. It is, however, possible for the gas to combine chemically with the liquid. 

 Both possibilities occur in the atmosphere-ocean system. Oxygen, nitrogen and the 

 rare gases obey the pure physical absorption ; carbon dioxide, on the other hand, fol- 

 lows the second possibility since it reacts both with the water itself and in part also 

 with the salts dissolved in it. (For chemistry of sea waters see especially Harvey, 1955.) 



(a) Oxygen, Nitrogen and Hydrogen Sulphide Contents of Sea-water 



The composition of the air absorbed by pure water can be calculated from the 

 absorption coefficients of the gases present in the atmosphere and is shown in Table 24 

 for 0° and 30°C. It is different from that of atmospheric air since the absorption coeffi- 

 cient of the individual gases is very different. In atmospheric air the ratio of oxygen 

 to nitrogen is 21 : 78 or about 1 : 4, but in the air dissolved in water at 0°C it is 35 : 62, 

 and at 30°C 33 : 64 or about 1 : 2. The air dissolved in water is thus twice as rich in 

 oxygen as atmospheric air, but it should not be forgotten that while a litre of air con- 

 tains 210 ml of oxygen, a litre of water saturated with air contains only about 10 ml. 



Table 24. Distribution of atmospheric gases at saturation dissolved 



in sea-water 



The solubility of gases in water is very strongly dependent on the temperature and 

 falls off rapidly as the temperature rises. The sea-water as a dilute salt solution shows 

 also a dependence on the salinity and the absorption coefficients fall with increasing 

 salinity. Fox (1905, 1907, 1909) has carried out extensive research on this subject, and 

 Rakestraw and Emmel (1937, 1938) have made further investigations. Table 25 

 shows saturation volumes at different temperatures and salinities for oxygen and 

 nitrogen. The weights present in mg can be obtained by multiplying the figures for 

 oxygen by 1-4292 and those for nitrogen by 1-2542. If oxygen-nitrogen ratios Oa/Ng 

 for different temperatures and salinities are worked out, it can be seen that there is 

 little variation; the dependence on salinity is small; with temperature it falls off 

 slightly. 



For chemical methods of determining the oxygen and nitrogen contents of a sample 

 of sea-water see Report of "'Meteor' Expedition, 3 or "Oceanographic Instrumenta- 

 tion. Chemical Measurements" (Carrit, Nat. Acad. Sci. Nat. Res. Coun., no. 309, 

 pp. 166-85, 1952). 



At the surface of the sea, in contact with the atmosphere, there is ample oxygen and 

 nitrogen available and it would be expected that the upper layers of the ocean were 

 saturated with both gases. This is generally the case, especially for nitrogen which is 



