77?^ Sea-water and its Physical and Chemical Properties 39 



show sometimes smaller variations from the above-mentioned general propor- 

 tionality (not more than 1%) which are due to biological processes (precipitation of 

 calcium carbonate), to the solution of calcium carbonate from sea bottom and in 

 coastal areas to the inflow of river water (containing calcium carbonate). 



The very constant proportions of the ions present in sea-water allow chlorine to be 

 used as a measure of the salinity of a sample of sea-water. This was done many 

 years ago by Forchhammer (1859, 1865) and later by Knudsen (1902), from a very 

 careful examination between 2-69 and 40-18%o, derived the simple equation 



S = 0-030 + 1-8050 CI, 



which is now used generally for the calculation of the salinity (S) from the chlorine 

 content. This salinity is that given in the definition above. It is a little smaller than the 

 actual salt content (by about 0-14%o) but since it is the differences in salinity that are 

 important this has very little significance. 



The most convenient method for the determination of salinity is that of Mohr 

 (1956) in which the sample is titrated with silver nitrate with a calcium chromate 

 solution as indicator; this is also suitable for use on board ship. This chemical method 

 gives a relatively fast and accurate determination of the chlorine in sea water, and the 

 salinity can be calculated from this value using the equation given above. This method 

 is the usual method used at the present time in practical oceanography (see especially 

 Meyer (1932) for the practical details of the titration and the necessary working rou- 

 tine). 



The chlorine titration is only a relative determination, and to find the absolute value 

 it is necessary to standardize the solution used for titration against the "Normal 

 water" introduced by Knudsen (1903, 1925); this standardization very largely elimi- 

 nates the effect of the subjective assessment of the colour of the indicator. Normal 

 water is sea-water kept in sealed glass tubes of which the chlorine content has been 

 very accurately determined, formerly by the central laboratory of the International 

 Hydrographic Institute in Copenhagen, and at the present time by the Woods Hole 

 Oceanographic Institution. The difference between the value obtained by titration 

 of the normal water and that marked on the tube gives the total error in the titration. 

 Knudsen (1901) has prepared hydrographic tables for the comparison of chlorine 

 determinations of sea-water with different salinities with the chlorine determination 

 made on normal water. 



If the average salinity of the ocean is taken as 35%o then calculation gives the total 

 amount of salt in the ocean as 4-84 x lO^*' tons; this corresponds to a volume of 21-8 

 miUion km^ which, spread evenly over the sea (361 million km-), would be a layer of 

 salt 60 m thick. 



In addition to the substances already mentioned, sea-water also contains traces of 

 a large number of elements which are of little importance for oceanography, though 

 they are probably important in the metabolism of marine organisms. The determination 

 of the concentrations of these elements presents very great analytical difiiculties and 

 the older determinations must be treated with great caution. Table 1 1 shows a more 

 recent list of the elements present in the sea according to Kalle (1945), which is 

 based on a similar one given earlier by Watterberg (1938). In many cases the figures 

 given represent only the order of magnitude of the concentration of an element. Of 



