Frans_M_Everaerts_Isotachophoresis_378342.pdf

DETERMINATION OF IONIC MOBILITIES 37
TABLE 3.1
TIIEORETICAL EFFECTIVE MOBILITIIiS OF MONO- AND DIVALENT CATIONS IN WATER
AND METHANOL (9570, w/w)
In both instances the counter ions are monovalent.
Water Methanol
105 meff.-1O5 m,.105 ineff; lo5
Univalent cations and anions 50 46 50 31.5
Divalent cations and univalent anions 50 43 50 25
For water as solvent at 25°C:
For methanol as solvent at 25°C:
(3.38)
2s
(Y * = 1.1 5 ___ - (z' z- 1 A8 + 55.3 (IZ’ I + 1z-I) (3.39)
1 +&
The effects discussed above are even stronger for solvents with lower dielectric
constants and for cations with higher charges.
3.5. DETERMINATION OF IONIC MOBILITIES
As already mentioned in preceding sections, ionic mobilities can be calculated from
equivalent conductivities and corrections can be made for the influence of concentration,
relaxation and retardation effects. As the exact data for many ionic species are unknown,
many workers have sought correlations between ionic mobilities and parame ten such as
the radius of the molecule, ionic volume and the entropy of the ions. Some of these
approaches are considered here and may be useful in estimating unknown mobilities.
3.5.1. Relationship between volume and ionic mobility
In general, it is said that for a 'steady flow' of molecules, Stokes' law can bc applied in
order to calculate the resistance force (assuming a spherical particle in an infinite fluid
[3]). If the ionic radius is not too small, the following equations can be deduced [4-61 :
v * = (qE+F,,, +Fre,.)/fc
(see eqn. 3.5) so that at infinite dilution
vd =4Elfc
mi = v d/E= q/fc =z'e/6nqr
For smaller particles (3-5 A) [7, 81, a modified expression can be used, viz. :
(3.40)
(3.41)
mi= z'el [5nvrCf/fo)] (3.42)