Page 144 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4-5 WORKING ELECTRODES 129

Microelectrodes exhibit several other attractive and important properties that have
expanded the possibilities of electrochemistry:

Because of the very small total currents at microelectrodes, it is possible to
work in highly resistive solutions that would develop large ohmic (iR) drops
with conventional electrodes. The decreased ohmic distortions allow electro-
chemical measurements to be made in new and unique chemical environments
that are not amenable to macroscopic electrodes. Microelectrode experiments
have thus been reported in low-dielectric solvents (e.g., benzene, toluene),
frozen acetonitrile, low-temperature glasses, gaseous and solid phases, super-
critical carbon dioxide, ionically conductive polymers, oil-based lubricants,
and milk. In addition, more traditional systems can be studied with little or no
deliberately added supporting electrolyte, and with two-electrode systems. The
use of electrolyte-free organic media can greatly extend the electrochemical
potential window, thus allowing studies of species with very high oxidation
potentials. Acetonitrile, for example, can be used to about 4 V (versus a silver
reference electrode), making possible studies of short-chain alkanes.
The greatly reduced double-layer capacitance of microelectrodes, associated
with their small area, results in electrochemical cells with small RC time
constants. For example, for a microdisk the RC time constant is proportional to
the radius of the electrode. The small RC constants allow high-speed voltam-
metric experiments to be performed on the microsecond time scale (scan rates
1
6
higher than 10 Vs ) and hence to probe the kinetics of very fast electron-
transfer and coupling-chemical reactions (83), or the dynamic of processes
such as exocytosis (e.g., Figure 4-23). More discussion of such high-speed
experiments is given in Section 2-1.
Enhanced rates of mass transport of electroactive species accrue from the radial
(nonplanar) diffusion to the edges of microelectrodes. Such ``edge effects''
contribute signi®cantly to the overall diffusion current. The rate of mass
transport to and from the electrode (and hence the current density) increases
as the electrode size decreases. As a consequence of the increase in mass-
transport rates and the reduced charging currents, microelectrodes exhibit
excellent signal-to-background characteristics in comparison to their larger
counterparts. In addition, steady-state or quasi-steady-state currents are rapidly
attained, and the contribution of convective transport is negligible. The fact that
redox reactions that are limited by mass transport at macroscopic electrodes
become limited by the rate of electron transfer can also bene®t many kinetic
studies.

4-5.4.1 Diffusion at Microelectrodes The total diffusion-limited current is
composed of the planar ¯ux and radial ¯ux diffusion components:

i total ˆ i planar ‡ i radial 4-14†