179
Ionic Interactions
The importance of using these mixing terms to estimate the physical properties of a mixture is demonstrated in Table 4.15. The estimated densities are in better agreement with
the measured values when the excess mixing terms are considered (ΔV EX in this case, the
volume of mixing the major sea salts).
For most physical properties in dilute solutions, the estimates can be made without the
ΔP EX term. The apparent molal property, like the volume, can be considered to be made up
of two major components: an ion–water interaction term and an ion–ion interaction term.
The physical property of seawater is given by
P = P o + Σ ion- water + Σ ion- ion
(4.102)
y
0.0
0.2
0.4
0.6
0.8
1.0
∆V
mix
–0.2
0.0
0.2
0.4
0.6
y
0.0
0.2
0.4
0.6
0.8
1.0
–0.2
0.0
0.2
0.4
0.6
MgCl 2 -Na 2 SO 4
NaCl-MgSO 4
NaCl-Na 2 SO 4
MgSO 4 -MgCl 2
Na 2 SO 4 -MgSO 4
MgCl 2 -NaCl
Figure 4.37
Values for the volume of mixing the major sea salts versus the ionic strength fraction Y 3 .
Table 4.15
Comparison of the Measured and Calculated
Densities (g cm –3 ) of Red Sea Brines Diluted
with Water at 25°C
I
Measured (ρ – ρ 0 )10 3
Δρ, 10 6
0.470
18.360
–18
1.218
46.136
81
1.752
64.986
134
2.614
93.984
199
3.561
123.974
211
4.234
144.207
235
5.333
175.610
297
6.124
197.281
444