92 Water 



Table 3. A larger amount of each of the common ions occurs in sea 

 water than in typical fresh water. Hard fresh water contains more 

 dissolved salts than soft fresh water, particularly with respect to the 

 calcium and the carbonate ions. A comparison of the relative abun- 

 dance of ions in each type of water may be made by recalculations in 

 which sodium is given a value of 100 in each case (Table 4). In sea 

 water chloride and sodium are the first and second most abundant 

 ions, respectively, whereas in typical hard fresh water carbonate is 

 the most abundant with calcium second. In typical soft fresh water 

 the calcium and carbonate ions are relatively less concentrated than 

 sodium and chloride. 



TABLE 3 



Composition of Some Typical Natural Waters 

 (After Baldwin, 1948) 



Total 



Na K Ca Mg CI SO4 CO3 (g/Iiter) 

 Soft fresh 



water 0.016 — 0.010 — 0.019 0.007 0.012 0.065 

 Hard fresh 



water 0.021 0.016 0.065 0.014 0.041 0.025 0.119 0.30 



Sea water 10.7 0.39 0.42 1.31 19.3 2.69 0.073 34.9 



TABLE 4 



Relative Abundance of Ions in Natural Waters 



Samples of sea water from different localities are surprisingly con- 

 stant in respect to the relative abundance of the major constituents. 

 So much is this the case that for many purposes it is not necessary to 

 measure the concentration of more than one ion. The standard pro- 

 cedure for determining the salinity of sea water is to find the amount 

 of chloride ion present by titration and then to calculate the total 

 salinity and the concentration of the other ions from the known ratios. 

 The individual salts occurring in fresh water by contrast, vary widely 

 —often with far-reaching consequences that will be considered later. 



At the moment our concern is with the maintenance of the proper 

 osmotic balance between the inside and the outside of the organism. 



