33
(1-26)
Similarly
(1-27)
For ideal dilute solutions, the mean ionic activity
coefficient will be equal to 1. If deviations from
ideality are caused entirely by electrical interactions
of point charges, it may be shown that, for aqueous
solutions at 25C (70)
log a- = -.509/Z + Z_/l1//2
(1-28)
where Z+ and Z_ are the charges of interacting species
and I is the total ionic strength of the system
2
m. Z7
1=1/2 E
(1-29)
. ill u
ill
Equation (1-28) is an expression of the Debye-Hiickel
limiting law for activity coefficients and is applicable
only to dilute solutions containing point charges. As the
ionic strength becomes large (e.g., >0.01m) and/or the
ions become less ideal (become larger, more polarizable,
etc.) the relationship becomes increasingly approximate.
Under these conditions, an extended Debye-Hiickel equation
may be invoked which introduces a correction for the finite
sizes of ions:
log =
-0.509/Z + Z Jl1/2
1/2
1 + Bdl
(1-30)