44 



The Sea-water and its Physical and Chemical Properties 



oceanographic vessel "Carnegie". Experience with this "saline tester" was not very 

 encouraging and the accuracy attained was, in spite of the greatest precautions, not 

 entirely satisfactory. 



5. Vapour Pressure, Freezing Point, Boiling Point and Osmotic Pressure of Sea -water 



Sea-water is a "dilute" solution and has the properties of such a solution. Due to the 

 low concentration of the dissolved material these will in several respects approach 

 those of the pure solvent, i.e. of pure water. It was shown quite early that the vapour 

 pressure p of a dilute solution is always less than the vapour pressure p^ of the pure 

 solvent and that the elevation of the boiling point is accompanied by a depression of 

 freezing point. As shown by Raoult and van't Hoff the relative lowering in vapour 

 pressure is independent of the nature of the material in solution and of the temperature 

 of the solution, and is proportional to the amount of dissolved material in solution in 

 the solvent. For a solution of « moles of a substance in Nq moles of a solvent: 



Po 



Po 



n 



No 



Figure 22 shows the different phase states for pure water and for sea-water; it illus- 

 trates more clearly the relationship between the three well-known properties of dilute 

 solutions mentioned above. The curve G'S' showing the lowering of vapour pressure is 

 always lower than the vapour-pressure curve for pure water by the amount of the 



/*(?■ tc Temperature fs h' 

 0°C IOO°C 



Fig. 22. Phase states for pure water and for sea-water (schematic). 



depression p^ — /;. Since for a given concentration (po — pMPq is constant, this de- 

 pression increases with increasing pressure and therefore also with increasing tem- 

 perature. The Po-curve for pure water cuts the line for a pressure of 760 mm Hg at 

 the point 5; this is the boiling point of pure water for which the corresponding tem- 

 perature ts = 100°C. The vapour-pressure curve for sea-water cuts this isobar first 

 at S' and this boiling point corresponds to a temperature z^- which is higher than /,. 

 The elevation of boiling point /1/s of sea-water of a given concentration is given by 



At, = /v - r,. 



