Page 235 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 171

The terms n and n are numbers of nonelectrolyte molecules per cubic
NE NE,b
centimeter; N n = S and N n = S ; these terms were defined in Section 2.20.2. It
A NE A NE,b 0
follows that

This equation clearly shows the effect of the secondary solvation. It turns out that
the orientation polarizabilities and depend on the square of the permanent
dipole moments of the molecules. Water has a higher dipole moment than most
nonelectrolytes. When is less than and there is salting out. HCN is an
example of a substance the dipole moment of which is greater than that of water. (It
masquerades as a nonelectrolyte because it is little dissociated in aqueous solution.)
Appropriately, HCN is often salted in.

2.20.4. Net Effect on Solubility of Influences from Primary and
Secondary Solvation

Equation (2.137) can be written as

Equation (2.151) can be written as

The treatment of the effect of secondary solvation has assumed that the primary
solvational effects do not exist. In fact, the secondary solvational effects work on the
diminished concentration of nonelectrolyte which arises because of the primary
solvation. Hence,

S as written in Eq. (2.154) has taken into account the primary and secondary
solvation and can be identified with the solubility of the nonelectrolyte after addition
of ions to the solution. Hence,