The oxidation of a compound involves the electron transfer from a molecule to the electrode surface. The two major advantages of most oxidative applications are (1) that oxygen is not electrochemically active, and (2) that solid electrodes can be used. Of course, both of these are definite advantages in the LCEC experiment as well. The importance in removing oxygen for reductive electrochemistry was discussed in the BASIC PRINCIPLES Section, and the LC system modifications required for these determinations will be discussed in the REDUCTIVE MODE Section.
The working electrode materials most commonly used are based on a carbon matrix, typically an anisotropic solid such as glassy carbon. Obviously, other materials exist and can be used for the LCEC determination. For example, gold, nickel, silver, platinum, and a thin mercury amalgam can all be used for specific measurements.
This section will primarily deal with typical functional groups that can be oxidized, but the list generated below is by no means exhaustive. The only conclusive way of knowing if a compound is electrochemically active is by a technique such as cyclic voltammetry. Even with our experience, we cannot always guess at how a specific compound may behave. A voltammetric assessment can provide information about the reversibility of the reaction as well, which can influence a decision on which mode of LCEC to use (e.g., Dual Series). In general, for electrochemical oxidation to take place, one can look for the same features and reactive centers as one would for a homogeneous oxidation reaction (e.g., delocalized electrons, stability of product, etc.). This does not mean that if a homogeneous chemical reaction does take place then this is directly applicable to the heterogeneous electrochemical reaction. The correlation is often thought to occur, and is many times an incorrect assumption.
The listing in Table 4.1 is not all-inclusive but only indicates some typical functional groups and compounds that are known to be electrochemically active. Later sections in this manual contain additional information based on cyclic voltammetry studies. If you have any questions regarding the electrochemical activity of your particular compound under a given set of conditions, it is best to do the voltammetry experiment.
Laboratories that are often investigating new compounds would find a BASi cyclic voltammetry instrument or the BASi 100B Electrochemical Analyzer a worthwhile investment.
Table 4.1. Functional Groups Suitable for Oxidative Electrochemical Detection.