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



45 



The depression of freezing point by a dissolved substance can also be inferred from 

 this diagram. The intersection G of the solid and liquid phases (the triple point) 

 corresponds to a temperature of 0-0075 °C. At 760 mm Hg the freezing point to of 

 pure water is 0°C and is fixed by the position of the intersection of the melting-point 

 curve Grwith the 760 mm isobar. It is the temperature at which the two phases (water 

 and ice) have the same vapour pressure, and therefore are in equilibrium with each 

 other. On the other hand, the freezing point of sea-water is at the intersection G' of 

 the vapour pressure curve for sea-water and that for ice; at this point the vapour 

 pressures over sea-water and over ice are the same. This corresponds at 760 mm Hg 

 to the freezing point of sea-water to' which is lower than to- The freezing-point de- 

 pression for sea-water is given by J/c = to' — to- 



From this diagram it can immediately be deduced that both quantities Ate and At^ 

 are larger the larger the value oi p^— p of the relative lowering of vapour pressure 

 ApJ p, that is the larger the concentration of the solution of the salinity. Quanti- 

 tatively it has been shown experimentally and theoretically that for low concentrations 

 the elevation of the boiling point and the depression of the boiling point are both 

 proportional to the concentration. In dilute solutions of substances termed in physical 

 chemistry "strong electrolytes", amongst which sea-water is included, it is found that 

 the electrolytic dissociation of the molecules is equivalent to an apparently larger 

 molecular concentration so that the simple proportionality no longer holds. The 

 accurate determination of saturated vapour pressures and of boiling points is experi- 

 mentally difficult and has been described in detail. The freezing point has been de- 

 termined by Hansen on eleven samples of sea-water, and by Knudsen (1903), using 

 determination of the constants, and the following empirical equation has been found 



to 



-0-0086 - 0-064633 a^ - 0000 1055 al. 



This gives freezing temperatures correct to ±0003°. 



Table 12. Freezing point and osmotic pressure of sea-water 



Table 12 shows related values of salinities, freezing point t„ the density of sea- water 

 at this temperature and also the osmotic pressure (see later, p. 48). For the relative 

 lowering of vapour pressure Witting (1908) has given the equation 



Aplp^ = 0-538 X 10-3 5. 



the elevation of boiling point can as a first approximation be obtained from 



At, = 0-01585. 



Table 13 gives related values for the elevation of boiling point and the lowering of 

 vapour pressure at boiling point (at 760 mm Hg). 



