Frans_M_Everaerts_Isotachophoresis_378342.pdf

25 8 PRACTICAL ASPECTS
concentration of H' in the leading zone (normally about 0.01 N hydrochloric acid) is
rather high. No disturbances can be expected. However, if a leading electrolyte that con-
sists of a metal chloride, e.g., potassium chloride, is used, hardly any H+ is present in the
cathode compartment and it can therefore be expected that OH- will be formed in the
cathode compartment according to the equation
2Hz0 + 2e + Hz + 2 OH-
The OH- ions formed will migrate in the direction of the anode but will not meet H+ for
neutralization to occur (the leading electrolyte is potassium chloride) and a flow of OH-
through the whole capillary tube, and all zones, will be the result. Of course, this distur-
bance will be visible if the concentration of the OH- formed is fairly high and if the time
of analysis is sufficient for OH- to reach the detector.
For a leading electrolyte of lithium chloride, an increase in pH from 6.7 to 10.7 in the
cathode compartment could be measured after some experiments. In such a case, distur-
bances can be expected and renewal of the cathode compartment is necessary. An example
of such a disturbance is shown in Fig.9.4.
The isotachopherogram shows the disturbances of a flow of OH- from the cathode
compartment, moving in an opposite direction to the migration of the sample zones. The
* rim.
Fig.9.4. Disturbance due to the presence of OH- formed in the cathode compartment in cationic
separations. The leading electrolyte is KCl and the terminator is LiCl (unbuffered system). Thermo-
couple (2) is mounted closer to the cathode compartment and therefore records the disturbance by
OH- first (marked with an arrow). The amplification of the signal of thermocouple (1) is twice that
of thermocouple (2). T = increasing temperature.