Frans_M_Everaerts_Isotachophoresis_378342.pdf

EFFECTIVE IONIC MOBILITY 31
obtain A*" and A*- for the calculations of the ionic mobilities at concentrations other
than infinite dilution cannot be properly used, because the law of independent
migration of the ions is invalid and the conductance is really a property of the electrolyte
rather than of the individual ions of the electrolyte. This means that the ionic conduc-
tivities (and hence ionic mobilities) of a chloride ion in 1 N calcium chloride solution
and in 1 N sodium chloride solution are different.
In such instances a correction must be made for the influence of relaxation and
retardation effects and for incomplete dissociation (ion pair formation). Also, for
"weak" electrolytes it is sometimes very difficult to obtain correct values for the
equivalent ionic conductances at zero concentration (infinite dilution). For such
solutions, we can calculate the correct values from the ionic contributions of strong
electrolytes at infinite dilution. For example:
A,*(HAc) = A,*(NaAc) + A: (Ha)- A,* (NaCl)
because the right-hand side can be interpreted as
AX+ (N2) + A:- (Ac- ) + Ag"(H) + A:- (GI-) -Ax' (Na') -A,*- (a- )
= Ar(H+) + A:- (Ac-) = A;f' (HAc)
(3.14)
(3.15)
This procedure is not valid at concentrations other than zero, but in practice it can be
used in order to obtain conductivities and mobilities at concentrations other than zero.
In fact, corrections for the differences in relaxation and retardation effects and ion pair
formation in electrolytes are neglected and it can be used only as a rough approximation.
3.4. EFFECTIVE IONIC MOBILITY
The absolute ionic mobility, m:, is defined as the average velocity of an ion per unit
of electric field strength at infinite dilution. This absolute ionic mobility is a characteristic
constant for every ionic species in a certain solvent and is proportional to the equivalent
conductance at zero concentration:
A,*=AE'fX:- =(m,'+m;)F (3.16)
In practice, we are not working at infinite dilution and the influence of other ionic species
present in an electrolyte solution cannot be neglected. The effective mobility of an ionic
species is related to the absolute mobility. Corrections have to be made for influences
such as the electrophoretic retardation and the relaxation effect, as described by Onsager
(see ref. 1). By using the Onsager equation, a correction can be made for ion-ion
interactions. Another influence is the effect of partial dissociation. Tiselius [2] pointed
out that the effective mobility is the sum of all products of the degree of dissociation and
the ionic mobilities:
meff.= 7 aimi (3.17)
where meff. is the effective mobility, ai is the degree of dissociation and mi is the ionic
mobility.