increases, activity of water decreases, the non-ideality 
increasing with concentration. The derivative of activity 
with salinity increases and hence we expect BPE to have an 
increasing slope with increase in concentration. This 
increase in non-ideality is brought about by the increase 
in short range interactions. Theoretical predictions of 
activity coefficients have not been successful, except in 
the very dilute cases. Empirical extensions of the Debye- 
Hiickel theory are available, which are reasonable, in the 
concentration range of interest. 
The variation of BPE with temperature follows the 
variation of activity with temperature and pressure (since 
the pressure also changes with temperature to maintain 
equilibrium). The variations with pressure are negligible, 
since the compressibilities of the solution are small. 
The variation with temperature is governed by the heats of 
waporizationsandvsolution.«/This deviates alittle from 
linear as seen in Figure 18. 
The osmotic and activity coefficients are strong 
functions of salinity. In the low concentration regions, 
where Debye-Htickel theory is valid, the long range ion-ion 
interactions tend to lower the activity coefficients, 
according to the Debye-Htickel limiting law. For a mixture 
like sea water, where there is a large amount of 2-2 and 
higher valence electrolytes, the concentration range where 
Debye-Htickel limiting law holds is rather low (less than 
10-4 molar solutions). The short range forces, such as 
ion-solvent interactions, start dominating, even at a low 
concentration. As the concentration increases, the dielec- 
tric constant of the medium is decreased. Also there are 
structural changes in the solution. These short range 
forces tend to increase the osmotic and activity coeffi- 
GENES. 
Thus, a minimum is expected in the osmotic coefficient. 
This is not very clear in the present work since the 
concentrations considered are far too high, i.e. the 
interactions neglected in Debye-Htickel theory dominate in 
even the lowest concentrations considered here, and all the 
values observed are in the increasing osmotic coefficient 
region. 
Further data at lower concentrations (less than 0.3% 
salinity) should show a minimum in the osmotic’ cocfficient — 
with the osmotic coefficient of pure water equal to 1.0. 
65 
