Frans_M_Everaerts_Isotachophoresis_378342.pdf

180 DETECTION SYSTEMS
R t
1 -
t
Fig.6.38. Detection of zone boundaries in isotachophoretic analyses performed by the a.c. method
(b) and the d.c. method (a) simultaneously. At the point marked with an asterisk, the micro-sensing
electrodes change their behaviour from polarized to charge-transfer electrodes. In this particular
instance, gas was produoed. ?he coating of the electrode is more visible in the stepcurves of the d.c.
method. The difference in inclination in the trace of the d.c. method in each zone should be noted.
1 = Chloride; 2 = oxalate; 3 = citrate; 4 = acetate; 5 = glutamate.
If stable coatings are obtained for any reason, the electrodes must be cleaned by
rinsing with aqua regia for about 10 sec.
The effect of the increment in the d.c. trace in Fig.6.38 must be ascribed largely to the
evolution of gas. The small layer of gas influences the d.c. method much more than the
a.c. method of conductivity determination.
6.6.4. Additives
Components that inhbit electrode reactions are characterized by strong adsorption on
the electrode surface. The electrode reactions may be inhibited in two ways: (a) the
transport of ions involved in the electrode reactions towards the electrode decreases as a
result of an increase in viscosity in the vicinity of these electrodes; (b) the inhibitor, as it
is adsorbed, inhibits the reaction by its presence. In general, adsorption on the electrode
surface is caused by the interaction of free-electron pairs (e.g., in oxygen, nitrogen and
sulphur compounds) or by n-electrons (e.g., in aromatic compounds). Again, two groups
can be distinguished; (1) surface-active compounds, including detergents; (2) organic
nitrogen or sulphur compounds, commonly used as corrosion inhibitors.