Frans_M_Everaerts_Isotachophoresis_378342.pdf

COUNTER ELECTRODE COMPARTMENTS 21 5
generally filled with double-distilled water in order to decrease any interference from
impurities formed by electrode reactions. Moreover, a rapid change from one operational
system to another is possible because the membrane is not ‘soaked’ with different types
of electrolytes. If another operational system is chosen, less attention needs to be paid
to the membrane compartment, as explained briefly below.
Suppose one is interested in anion separations and for a complete separation three
different operational systems are needed. In general, in all systems chloride will be chosen
as the most mobile ion, because it is pure, stable and cheap. Even if another anion is
chosen as the leading ion, this will not affect the analysis because it migrates through the
membrane in the direction of the anode. Of course, the buffering counter ions move in
the opposite direction and in a different operational system another counter ion must
be taken. If double-distilled water is placed in the reservoir surrounding the anode, no
buffering counter ion coming from this reservoir will be present. Therefore, the membrane
is not saturated with the buffering counter ions. In most instances, simple rinsing of the
system is sufficient for cleaning the membrane. The disadvantage when double-distilled
water is used in the electrode compartment with the semipermeable membrane is that a
large potential drop is caused. Especially if a conductivity detector is applied, this
potential drop may cause an electric leak towards earth because the detector electrodes
may finally reach too high a voltage for which the insulation is not adequate.
The disadvantage of the cylindrical construction of the counter electrode compartment
is that sometimes small leakages may arise because the Araldite employed to fix the
membrane does not really fix it. Also, if experiments with methanol are performed, the
Araldite becomes brittle and electrolyte may flow from the reservoir of the terminating
electrolyte towards the counter electrode compartment. If these leakages are small, they
are hardly noticeable, but ultimately there may be a decrease in resolution. Because a
decrease in resolution may have many origins [e.g., electroendosmosis by adsorbed
material on the wall and electrode reactions (if the a.c. method is used for conductivity
determinations), due both to the driving current (polarization) and to electric leakages
to earth], the hydrodynamic flow cannot be directly localized in the initial phase often.
7.3.3. Counter electrode compartment with flat membrane
A more advanced counter electrode compartment is shown schematically in Fig.7.9 and
a photograph is shown in Fig.7.10. The electrode vessel is separated from the narrow-bore
tube in which the analysis is performed by a flat membrane made, for instance, of
cellulose polyacetate (0.2 mm thickness). This membrane is clamped by two screws and
an O-ring. The tap used is a common FTFElined Hamilton (IMMI) valve that provides
the connection with the reservoir of the leading electrolyte. This reservoir is generally an
ordinary polypropylene syringe with a volume of 20 ml. If the entire bystem has to be
rinsed or re-filled with fresh leading electrolyte, the liquid applied flows as well along the
membrane as along the septum constructed for the experiments with a counter flow of
electrolyte. Because the bore is relatively large compared with the inside diameter of the
narrow-bore tube (2 mm), the potential drop in the canals is small. Therefore, a normal
metal syringe can be inserted for the experiments with a counter flow of electrolyte
without the risk that gas will be produced owing to polarization (Fig.6.36). In addition to