The Sea-water and its Physical and Chemical Properties 



37 



of this property all the water on the earth is more or less impure, that is it contains in 

 addition to chemically Hnked hydrogen and oxygen (HgO) a number of other substances 

 in varying amounts. If the salt content, the salinity, were defined as the weight of all 

 the salts dissolved in a kg of sea-water this would provide to be the simplest numerical 

 specification of the amount of dissolved salts in the water. Unfortunately it is rather 

 difficult to measure this definite quantity since, when sea-water is evaporated to dry- 

 ness and heated to red heat to remove the last traces of water, some hydrogen chloride, 

 carbon dioxide and a small amount of hydrogen bromide are also lost. This loss is not 

 easily compensated with sufficient accuracy by adding a corresponding correction. 

 At the suggestion of Forch, Sorensen and Knudsen (1902) the salinity has been de- 

 fined as the total amount of solid material in grammes contained in 1 kg of sea-water 

 when all the bromine and iodine have been replaced by the equivalent amount of 

 chlorine, all the carbonate converted to oxide and all organic matter has been com- 

 pletely oxidized. The salinity defined in this way can be determined with great accuracy 

 and can thus serve as a basis for the investigation of the relationship between any 

 single component and the total salinity. 



Sea-water is a dilute solution of a mixture of salts; in such an aqueous solution salts, 

 acids and bases are more or less completely electrolytically dissociated (Arrhenius 

 and van't Hoff). The chemical compounds precipitated on evaporation of such solu- 

 tion are in solution split into atoms or groups of atoms with an electric charge, either 

 positive (cations) or negative (anions). The electrical charges balance exactly so that 

 the solution remains electrically neutral. The constituents of this mixture of salts 

 are therefore listed as their ions. Table 8 shows the composition of a typical sample of 

 sea-water with a salinity of 34-40%o. 



Table 8. The principal constituents of sea-water 

 (34-40 /oo salinity) 



Cations 



Sodium 



Potassium 



Magnesium 



Calcium 



Strontium 



g/kg 1 mmole/kg 



10-47 

 0-38 

 1-28 

 0-41 

 0-013 



455-0 



9-7 



52-5 



10-2 



0-15 



percent- 

 age of S 



30-4 

 1-1 

 3-7 

 1-2 

 0-05 



Anions 



Chloride 



Bromide 



Sulphate 



Bicarbonate 



Borate 



g/kg 



18-97 

 0065 

 2-65 

 0-14 

 0027 



mmole/kg 



5351 

 0-81 



27-6 

 2-35 

 0-44 



percent- 

 age of S 



55-2 

 0-2 

 7-7 

 0-4 

 0-08 



It was formerly customary to give the constituents of sea-water in terms of the com- 

 pounds that were precipitated on evaporation. Dittmar (1884) has given the figures 

 shown in Table 9 as the mean of seventy-seven very complete analyses of sea-water 

 samples made by the "Challenger" Expedition; they have been calculated on the basis 

 of a salinity of 35 g of salts in 1 kg of sea-water. 



In the open ocean the total concentration of salinity varies between moderate 

 limits, usually between about 33 and 38%o depending in the first place on the climate 

 (precipitation, evaporation and in polar regions ice melting). In coastal areas where 

 there is a considerable inflow of fresh water from rivers and from ground water the 

 salinity may have a considerably lower value. Especially in the almost closed adjacent 

 seas of higher latitudes (such as the Baltic) with low evaporation, a considerable 



