Frans_M_Everaerts_Isotachophoresis_378342.pdf

176 DETECTION SYSTEMS
f V
t C
I
t
I I
Fig.6.36. Polarization of the micro-sensing electrode. The potential gradient inside the narrow-bore
tube at the position where the microsensing electrode is mounted decreases if these measuring
electrodes change from an 'ideal' polarized electrode to a charge-transfer electrode, owing to the
neghgible resistance of the platinum electrode. As a result of this effect, the concentration of the
electrolyte, again inside the narrow-bore tube at the position where the measuring electrodes are
mounted (l), will decrease if the electrode changes its character in order to fulfil the isotachophoretic
conditions (2). L = position in the narrow-bore tube; V = increasing potential gradient; c = increasing
ionic concentration; Z = centre of the electrode.
obscure results are obtained. Although under normal conditions some hydrogen may be
produced at the beginning of the experiment, the evolution stops because the difference
in potential between the anodic side of the bipolar sensing electrode and the electrolyte,
which is surrounded by this electrode, is insufficiently great to start the evolution of
oxygen (provided that no anion is present that can be oxidized more easily than the
hydroxyl ion). If a sensing electrode changes, for any reason, into a charge-transfer
electrode, the zone length, as actually measured, of the ionic species present in that zone
is longer than can normally be expected according to the concentration in the sample. If
the micro-sensing electrodes are made of Pt-Ir, Pt, Pd or Au considerable amounts of
hydrogen and/or oxygen can be bound in the first two metallic layers of the electrodes.
If hydrogen is bound, the impedance of the electrolyte increases, but with oxygen the
contact of the electrolyte and the metallic electrode improves, which causes an apparently
lower impedance of the same electrolyte. If the overpotential against the formation of
oxygen is exceeded, the bipolar electrode always starts to produce both hydrogen and