Chemistry Reference
In-Depth Information
linear mechanism for micro electrodes. Spherical diffusion at the edge of
the micro electrode can be neglected; for ultramicro electrodes, it is the frac-
tion of linear diffusion that can be ignored. This has several advantages:
• Despite much smaller current being recorded, the current density at
ultramicro electrodes is much higher.
• Owing to the small current measured, Ohmic drop (also called IR-drop)
effects in solution are much less pronounced. This includes that ultra-
micro electrodes can be used in highly resistive media (including non-
aqueous solution) and/or without the use of supporting electrolyte.
•The small area of the electrode also reduces the double-layer capaci-
tance and RC-time constant, which allows experiments to be performed
at high scan rates (e.g. 100 000 V s
-1
).
•
Owing to spherical diffusion, a diffusion layer is obtained with constant
thickness, therefore resulting in time-independent limiting-current
plateaus. This implies that its current is (almost) not affected by con-
vection of the solution, therefore it can be used in flowing streams as
long as turbulent behaviour in solution is avoided.
•
Converting the small currents measured at ultramicro electrodes results
in higher current densities compared with the values obtained at micro
electrodes. This is also caused by the spherical diffusion path. This effect
improves on the sensitivity and the detection limit for analytical pur-
poses, and the small size of the electrodes in combination with applica-
tion of high scan rates allow measurement to be performed
in vivo
and
with a minimal (negligible) disturbance of the conditions of the system
to be analysed.
The current obtained at ultramicro disc electrodes is described in
Equation 1.17
47
:
12
Ê
Á
ˆ
˜
1
Ê
Ë
ˆ
¯
[1.17]
I
=
4
p
rn
FD
c r
+
1
p
D
t
where
I
is the measured current,
r
is the radius of the ultramicro electrode,
F is the Faraday constant, D is the diffusion coefficient,
c
is the concentra-
tion and
t
is the time that the potential is applied. This equation shows that
the current becomes independent for longer values of
t
, which means that
a time-independent limiting-current will be obtained. Note also that the
time needed to obtain a steady-state condition is dependent on the radius
of the electrode.
The main disadvantage of ultramicro electrodes is the need to use equip-
ment that is able to measure very small direct currents (nA) with high pre-
cision and low noise (pA-fA). This equipment is fairly expensive, and
special precautions should be taken when it is used in an industrial envi-
