Frans_M_Everaerts_Isotachophoresis_378342.pdf

CHECK OF THE ISOTACHOPHORETIC MODEL 77
true value, but there is a linear relationship between dTth and the real difference in
temperature [22] (see Chapter 6). As a linear relationship between the conductivity of a
zone and the temperature inside the capillary tube can be expected over a limited traject,
a linear relationship can also be expected between the conductivity of the zones and the temperature
detected by means of a thermocouple. This relationship is used to check the theory.
In this section, some calculations of the parameters of the different zones are made
and the results are compared with those obtained in some experiments. Calculations
were made both for anions and cations, correcting for the influence of activity
coefficients, relaxation and electrophoretic effects and different temperatures in the
zones. The temperatures in the zones were estimated from the thermocouple signals
and the temperatures in the capillary tube [22] .
Calculations were made for the cations BaZ+, Caz+, Mg2+, Fez+, k?, Ag and Na+. These
cations were chosen because the slope of the function A. = KJc* agrees reasonably
well with the expected slope according to the Onsager relationship. If other influences
such as complex formation occur, the decreasing effect on the mobility should be greater
and the calculations would not be valid as the computer program does not deal with
effects such as complex formation. For the anionic calculations, acids were chosen for
which data such as ionic mobilities and pK values were readily available [ 13, 19,23,24] .
The concentrations, pH values, step heights and zone resistances are given in
Table 4.6 for cations in the system WKAC (see Table 11.3) and in the Tables 4.7 and 4.8
for anions in the systems histidine hydrochloride and imidazole hydrochloride (see Tables
12.1 and 12.2) respectively.
Firstly, calculations were made with no corrections. The relationship between the
experimentally measured step heights and the uncorrected calculated conductivities of
the zones are given in Figs.4.l5a, 4.16a and 4.17a. Although one continuous relationship
would be expected, two distinguishable curves are obtained for these relationships.
This can be understood easily, as follows. If the zone resistances are computed without
applying corrections for the Onsager relationship, there will be deviations from the real
electrical resistances actually present. The zone resistances calculated will be smaller
than the actual resistances because relaxation and electrophoretic effects, which decrease
TABLE 4.6
SOME EXPERIMENTAL AND CALCULATED VALUES FOR CATIONS IN THE OPERATIONAL
SYSTEM AT pH 5.4 (SEE TABLE 11.3)
A values are given in C’ cm-I.
Cation
K+
&+
Na+
BaZ+
Ca ’+
Mg"
Fe 2+
l/h. los
without
correc-
tions
0.874
1.029
1.272
1.013
1.077
1.210
1.188
i/h 10'
with
correc-
tions
0.8930
1.0440
1.2825
1.1152
1.1818
1.3215
1.2969
-
Calculated
concentration of
the ionized part
(mole/ 1)
0.0100
0.0094
0.0086
0.0048
0.0046
0.0044
0.0045
PH Step height
(mm)
5.39 220
5.36 260
5.32 302
5.36 264
5.35 284
5.33 3 14
5.33 3 12