Frans_M_Everaerts_Isotachophoresis_378342.pdf

32 CONCEPT OF MOBILITY
To summarize, we can state that the effective mobility of an ionic species depends
on several factors such as the ionic radius, solvation, dielectric constant and viscosity of
the solvent, shape and charge of the ion, pH, degree of dissociation and temperature. It is
very difficult to give a precise mathematical expression for the effective mobility. When
speaking about effective mobilities, we shall use the expression
meff. = C cui rimi
i
where cti is the degree of dissociation, yi is a correction factor for the influence of
relaxation and retardation effects and mi is the absolute ionic mobility. The correction
factors c+ and yi will be described in more detail.
3.4.1. Partial dissociation
(3.18)
If an ion does not exist in the free form, but is in an equilibrium with the undissociated
form, its effective mobility is smaller than its ionic mobility. For example, acetate, in
water, is always in equilibrium with acetic acid according to the equation
HAc + Hz 0 * H3 O+ + Ac-
and the equilibrium constant is
As the degree of dissociation is defined as
cu=
[Ac-]
[HAc] + [Ac-]
then, during time t, the ionic species exists in the form of acetate during time cut.
Therefore, the migration distance in time f is
(3.19)
(3.20)
(3.2 1)
s=vat = cumEt (3.22)
Normally, the migration distance of an ion is
s =v t = m Et (3.23)
From eqns. 3.22 and 3.23, it can be concluded that the effective mobility can be
calculated as
meR. =am (a < 1)
Tiselius [2] pointed out that a substance consisting of several forms with different
mobilities in equilibrium with each other will generally migrate as a uniform substance
with an effective mobility of
meff.= F aimi
(see eqn. 3.17) provided that the time of existance of each ionic species is small in