Aqueous processing potential/limitations

Fast scan cyclic voltammetry also provides only limited chemical resolution. The redox potential (E°) of a substance is insufficiently unique for molecular identification. In addition, to distinguish between chemical species that are involved in diffusion-controlled one-electron electrolysis processes, their E° s need to differ by at least 0.118 V. in aqueous solution, the potential limits are less than 2.0 V and hence, even under optimum conditions, less than 15 compounds could be resolved[45]... 

The role of the pH of the medium in the electrode reactions of organic compounds in aqueous solutions is well understood and has been recently reviewed in detail (Zuman, 1969). In particular, our understanding of this parameter is due to the large number of polarographic investigations where it has been found that the half-wave potential, the limiting current and the shape of the wave for an oxidation or reduction process may all be dependent on the acidity of the medium. 

In initial ET rate measurements, both the NB and aqueous phases contained 0.1 M TEAP, enabling measurements to be made with a constant Galvani potential difference across the liquid junction. In these early studies, the concentration of Fc used in the organic phase (phase 2) was at least 50 times the concentration of the electroactive mediator in the aqueous phase which contained the probe UME (phase 1). This ensured that the interfacial process was not limited by mass transport in the organic phase, and that the simple constant-composition model, described briefly in Section IV, could be used. 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

The results indicate that persistence of organosilicone surfactants in the parent molecule form will be limited on typical soil media and in aqueous environments. Reduced recovery was considered to be a result of abiotic degradation and/or strong sorption processes. Losses were most significant on solid media exhibiting extreme pH values and were also enhanced in the presence of clay substrates. Studies on clays indicated that pH, potential for intercalation and surface charges are important factors in the removal process. 

---