IV. SALINITY AND DENSITY MEASUREMENTS 



Murray H. Schefer 



A. INTRODUC TION 



The salinity of sea water is defined as the total amount of dissolved 

 material in grams, excluding organic matter, contained in one kilogram 

 of sea water. The term density, generally used in oceanography, is 

 really specific gravity which is the ratio of the mass per unit volume 

 of a sea water sample to the mass of an equal volume of distilled water 

 at 4°C and standard atmospheric pressure. In the c.g.s. system the 

 density of distilled water at 4°C and atmospheric pressure is equal to 

 unity, and therefore specific gravity is numerically identical with den- 

 sity. 



Salinity concentration is expressed in parts per thousand, or per 

 mille, for which the symbol %o is used. It has been found that regardless 

 of the absolute concentration, the relative proportions of the different 

 major constituents are virtually constant, except in regions of high 

 dilution (low salinity) where minor deviations may occur. Density is 

 expressed as a pure number. 



In oceanography, salinity is of importance because it is one of the 

 three variables which affect the density of sea water. The other two 

 variables are temperature and pressure. The density of sea water, and 

 especially changes in density, affect a number of oceanographic factors. 

 Some are: propagation of underwater sound, propagation of light under 

 water, buoyancy of submarines, vertical mixing, generation of ocean 

 currents, and stratification. 



Owing to the nearly constant composition of the dissolved solids, 

 the determination of a single element present in relatively large 

 quantity can be used as a measure of the total salinity. Chloride ions 

 make up approximately 55 percent of the dissolved solids and can be 

 determined with reliability and accuracy. The empirical relation- 

 ship between salinity and chlorinity is: Salinity = 0.03 + 1.805 x 

 Chlorinity. Thus, the traditional method of determining total salinity 

 has been to raise uncontaminated sea water samples to the surface 

 and to determine quantitatively the concentration of chloride ions and 

 apply the formula given above. The concentration of chloride ions is 

 determined by titrating a sample of sea water with a known solution of 

 silver nitrate. This technique as practiced today is known as the Knud- 

 sen Method. The end point of the titration is determined by the use of 

 an indicator that changes color. Titrations can be performed so that 



IV- 1 



