Frans_M_Everaerts_Isotachophoresis_378342.pdf

76 MATHEMATICAL MODEL FOR ISOTACHOPHORESIS
the separation of anionic species, show a strong temperature dependence. Therefore, it
is to be expected that for the separations of anions of very low effective mobility this
influence cannot be neglected. In the computer program, different mobilities and pK
values can be entered for the different zones and corrections for this effect can be made.
4.4.4. Influence of activity coefficients
#en an equilibrium
is considered, the pH and pK values are defined as
pH = -log aH
and
pK=-log K= pH +log (uAH/uA)
and the activities are defined as
-
‘A - ‘A TA
where a,is the activity, cAis the molar concentration and yAis the activity coefficient
of component A. These activity coefficients can be calculated from the Debye-Huckel
limiting law as
log yA= Kz’fl
where K is a constant, z is the valence and Z is the ionic strength of the solution. Although
it is possible to compute all activity coefficients and to develop a computer program that
includes these coefficients, they are neglected in our isotachophoretic model. This means
that the following definitions are used:
pH = - log [H+]
and
PK= PH + log ([A] / [AH1 )
Interpreting all pH values as -log [W] and all pK values as p&, correct computations
can be carried out, The p& values can be calculated from the p& values by correction
for the activity coefficients and repeated calculations give the exact values.
4.5. CHECK OF THE ISOTACHOPHORETIC MODEL
When working with a stabilized electric current, the conductivity of a zone determines
the characteristic potential gradient over the zone. The heat produced per unit volume
corresponds to ZE and determines the temperature of the zone in the steady state.
For a check of the theory, the difference between the temperature inside the
capillary tube and the temperature of the air (air cooling) should be known. The
difference in temperature measured with a thermocouple (dT,,) is different from the