Page 18 - Analytical Electrochemistry 2d Ed - Jospeh Wang

1-2 FARADAIC PROCESSES 3

Accordingly, the resulting current re¯ects the rate at which electrons move across the
electrode±solution interface. Potentiostatic techniques can thus measure any chemi-
cal species that is electroactive, in other words, that can be made to reduce or
oxidize. Knowledge of the reactivity of functional group in a given compound can be
used to predict its electroactivity. Nonelectroactive compounds may also be detected
in connection with indirect or derivatization procedures.
The advantages of controlled-potential techniques include high sensitivity,
selectivity towards electroactive species, a wide linear range, portable and low-
cost instrumentation, speciation capability, and a wide range of electrodes that allow
assays of unusual environments. Several properties of these techniques are summar-
ized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with
very small sample volumes (5±20 ml), thus allowing the determination of analyte
amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be
achieved via the coupling of controlled-potential schemes with chromatographic or
optical procedures.
This chapter attempts to give an overview of electrode processes, together with
discussion of electron transfer kinetics, mass transport, and the electrode±solution
interface.

1-2 FARADAIC PROCESSES

The objective of controlled-potential electroanalytical experiments is to obtain a
current response that is related to the concentration of the target analyte. This
objective is accomplished by monitoring the transfer of electron(s) during the redox
process of the analyte:

O ‡ ne „ R 1-1†

where O and R are the oxidized and reduced forms, respectively, of the redox couple.
Such a reaction will occur in a potential region that makes the electron transfer
thermodynamically or kinetically favorable. For systems controlled by the laws of
thermodynamics, the potential of the electrode can be used to establish the
concentration of the electroactive species at the surface [C 0; t† and C 0; t†]
R
O
according to the Nernst equation:
2:3RT C 0; t†
O
E ˆ E ‡ log 1-2†
nF C 0; t†
R
where E is the standard potential for the redox reaction, R is the universal gas

1
constant (8.314 J K 1 mol ), T is the Kelvin temperature, n is the number of
electrons transferred in the reaction, and F is the Faraday constant (96,487
coulombs). On the negative side of E , the oxidized form thus tends to be reduced,

and the forward reaction (i.e., reduction) is more favorable. The current resulting
from a change in oxidation state of the electroactive species is termed the faradaic