24
Fig. 1.20. Types of ion-ion interactions
Fig. 1.21. Apparent molal properties of solutes in aqueous
solutions.
1. Cation - Anion
2. Cation - Cation
3. Anion - Anion
Q
+ n sea salt
f$!
F. J. Millero
88
88
88
---.
G
p
cp =t.Pln=(P-f$!)ln
cP is apparent molal property
t.P is change in property
trolyte solution is nearly equal to the weighted sum of the component electrolytes. This
additivity, called Young's rule (Young and Smith 1954), is given by:
cI>= I.EirfJi
(1.32)
where Ei = ni I nr (nr = I.ni) is the equivalent fraction of electrolyte i in the mixture
and rfJi is the molal property of i at the ionic strength of the mixture. In terms of the
ionic components of a mixed electrolyte solution, the equation becomes:
cI>= I.MI.xEMExrfJ(MX)
(1.33)
where EM and Ex are the equivalent fractions of cations (M) and anions (X) and rfJ(MX)
is the apparent property of electrolyte MX at the ionic strength of the mixture. For
the major components of sea water, this sum can be made three ways:
cI>(SW) = ENaEc1rfJ(NaCI) + EMgEs04rfJ(MgS04)
cI>(SW) = ENaEso4rfJ(NazS04) + EMgEclrfJ(MgClz)
cI>(SW) = EN.EClrfJ(NaCI) + EN.Es04rfJ(MgS04) + EMgEClrfJ(MgClz) + EMgEs04rfJ(MgS04)
Experimentally, it is found that the third summation works best because it considers the weighted sum of all the possible cation-anion interactions in the solution. These