PHYSICAL PROPERTIES OF SEA WATER 9 



total quantity of dissolved solids in a given sample. Furthermore, it is 

 impossible to obtain reproducible results by evaporating sea water to 

 dryness and weighing the residue, because certain of the material present, 

 chiefly chloride, is lost in the last stages of drying. These difficulties can 

 be avoided by following a technique yielding reproducible results which, 

 although they do not represent the total quantity of dissolved solids, do 

 represent a quantity of slightly smaller numerical value that is closely 

 related and by definition is called the salinity of the water. This tech- 

 nique was established in 1902 by an International Commission, according 

 to which the salinity is defined as the total amount of solid material in 

 grams contained in one kilogram of sea water when all the carbonate 

 has been converted to oxide, the bromine and iodine replaced by chlorine, 

 and all organic matter completely oxidized. 



The determination of salinity by the method of the International 

 Commission is rarely if ever carried out at the present time because it is 

 too difficult and slow, but, inasmuch as it has been established that the 

 dissolved solids are present in constant ratios, the determination of any of 

 the elements occurring in relatively large quantities can be used as a 

 measure of the other elements and of the salinity. Chloride ions make 

 up approximately 55 per cent of the dissolved solids and can be deter- 

 mined with ease and accuracy by titration with silver nitrate, using 

 potassium chromate as indicator. The empirical relationship between 

 salinity and chlorinity, as established by the International Commission, is 



Salinity = 0.03 + 1.805 X Chlorinity. 



The chlorinity that appears in this equation is also a defined quantity and 

 does not represent the actual amount of chlorine in a sample of sea water. 

 Both salinity and chlorinity are always expressed in grams per kilogram 

 of sea water — that is, in parts per thousand, or per mille, for which the 

 symbol °/oo is used. In practice, salinity is determined with an accuracy 

 of +0.027oo. 



The salinity of a water sample can also be found by determining the 

 density of the sample at a known temperature, because empirical relation- 

 ships have been established by which the density can be represented as a 

 function of salinity (or chlorinity) and temperature. Other indirect 

 methods for determining salinity are based on measurements of the 

 electrical conductivity or the refractive index at a known temperature. 

 In order to obtain the needed accuracy, the difference is measured in 

 electrical conductivity or refractive index between the sample and a 

 sample of known salinity that lies fairly close to that of the unknown 

 sample. 



The salinity in the oceans is generally between 33°/oo and 37°/oo- 

 In regions of high rainfall or dilution by rivers the surface salinity may be 

 considerably less, and in certain semi-enclosed areas, such as the Gulf of 



