Frans_M_Everaerts_Isotachophoresis_378342.pdf

EXPERIMENTAL 381
caused by the counter flow of electrolyte, than the photographic registration shown in
Fig.17.2. It should be noted that the zones shown in the isotachopherogram in Fig.17.3d
move more slowly than those in Fig.17.3a, that the speed of the recorder paper was the
same in both analyses, and that nevertheless the zones shown in Fig.17.3d are sharper.
Moreover, Fig.17.3d shows that an impurity is present that is difficult to see in Fig.17.3a.
This shows once more the risk involved when zone profiles are used for the quantitative
determination of small amounts of components (see section 10.5).
Both Figs.17.2 and 17.3 show that the thickness of the electrodes is sufficient, if the
disturbances of the zone boundaries are not suppressed, assuming that the electrode
reactions do not play an important role. Moreover, the suppression of the boundary
disturbance by a counter flow of electrolyte can only be used for specific boundaries,
because the consecutive boundaries all have their own curvature. From the photographs in
Fig.17.2 (both the experiments in which 2% of hydroxyethylcellulose was added and
those carried out in the free solution), the profile disturbances were photographically
enlarged. The results are given in Fig.1 7.4: at higher rates of counter flow of electrolyte,
the disturbance proved to be no longer a function of the viscosity.
Another series of experiments was carried out in order to investigate the disturbance
of the zone boundary profile when the difference in effective mobilities is small.
Fig.17.4. Disturbance of the zone profiles by a counter flow of electrolyte. (a) Experiments in a free
solution; (b) experiments with 2% of hydroxyethylcellulose added to the leading electrolyte (viscosity
= 100 cP). V, = electrophoretic velocity; V, = hydrodynamic velocity (counterflow of electrolyte).