Probing electrode reactions

A particularly interesting approach for probing electrode reactions at the micro-level has arisen from the combination of microelectrodes and the accurate control instrumentation associated with scanning probe microscopies. 

Typical theoretical concentration profiles, observed at a probe electrode, for the consumption of a receptor phase species in a first-order interfacial reaction are shown in Fig. 16. The simulation involved solving Eq. (30) with appropriate boundary conditions. 

The reaction in water at pH 7.4 has been much studied since the discovery of the importance of nitric oxide. The products are as for the thermal and photochemical reactions, except that the final product is nitrite ion. This is to be expected since nitric oxide in aerated water at pH 7.4 also yields quantitatively nitrite ion25, by it is believed the series of equations 7-9, which involves oxidation to nitrogen dioxide, further reaction to give dinitrogen trioxide which, in mildly alkaline solution, is hydrolysed to nitrite ion. Under anaerobic conditions it is possible to detect nitric oxide directly from the decomposition of nitrosothiols using a NO-probe electrode system26. Solutions of nitrosothiols both in... 

The aforementioned methodology has been applied to measure the kinetics of a series of monovalent ions by using the oxidation of LBPC [26-29], As the redox probe LBPC is oxidized to the stable hydrophobic cation LBPC+, and the electrode reaction is accompanied by either anion ingress from the aqueous phase (4.12) or cation expulsion from the organic phase (4.13), which depends on the type of ions and their relative affinity for both liquid phases. 

CV investigations of 6-mercaptopurine and 8-mercaptoquinoline SAMs on pc-Au electrodes have been presented by Madueno et al. [186] and He etal. [187], respectively. Several model electrode reactions involving various redox probes were studied using such modified electrodes. Baunach and Kolb etal. [188] have deposited copper on disordered benzyl mercaptan film on Au(lll) surfaces. They have also studied the behavior of benzyl mercaptan SAM on Au(lll) in H2SO4 solution using CV and STM. Structural and electrical properties of SAMs based on tetrathiafulvalene derivatives on Au(lll) were investigated. These mono-layers were disordered, or at least loosely... 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

Perhaps the most useful aspect of CV is its application to the qualitative diagnosis of electrode reactions that are coupled to homogeneous chemical reactions [37,41-44]. The forte of CV is its ability to generate a species during one scan and then probe its fate with subsequent scans. Examples of CV usage in unraveling electrode mechanisms are presented in Chapters 21 and 23. 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Thus, when a substituent of interest is incorporated into an olefinic substance and the resulting compound allowed to react with the electrode surface, the substituent becomes connected to the surface.. .. By this means, ionic species have been tethered within the double layer region in order to probe the mechanisms of electrode reactions involving platinum complexes.. . . Alternatively, the electrochemical reactant itself can be connected to the electrode surface, allowing its reactivity to be observed as a function of charge, orientation, and structure, as described here. 

Let us deal with the electrode reaction method controled by mass transport rate and its probes. 

Figure 4.13 Velocity-measuring probes based on electrode reaction controlled by mass transfer rate.

The usual objective of scanning probe microscopy techniques [41] is to provide images of a solid surface—normally topographic information— with up to atomic resolution. However, they can also be used to probe local solution composition and electrode reactions, as will be described. 

For the investigation of electrode reaction parameters and chemistry at these dimensions, another approach is necessary, in order to make the system species-selective through monitoring the electrochemistry. This involves making tip and substrate into independent electrodes the tip is thus a microelectrode. The microelectrode tip is scanned over the surface this is known as scanning electrochemical microscopy (SECM) [43] and due to its local probe nature can be used to probe microvolumes. 

Bipotentiostat — An instrument that can control the potential of two independent -> working electrodes. A - reference electrode and an -> auxiliary electrode are also needed therefore the cell is of the four-electrode type. Bipotentiostats are most often employed in electrochemical work with rotating ring-disk electrodes and scanning electrochemical microscopes. They are also needed for monitoring the electrode-reaction products with probe electrodes that are independently polarized. All major producers of electrochemical equipment offer this type of potentiostat. The instruments that can control the potential of more than two working electrodes are called multipotentiostats. 

In this form of voltammetry, the concentration distributions of each species in the electrode reaction mechanism are temporally invariant at each applied potential. This condition applies to a good approximation despite various processes still occurring such as mass transport (e.g. diffusion), heterogeneous electron transfer and homogeneous chemical processes. Theoretically it takes an infinite time to reach the steady state. Thus, in a practical sense steady-state voltammetric experiments are conducted under conditions that approach sufficiently close to the true steady state that the experimental uncertainty of the steady-state value of the parameter being probed (e.g. electrode current) is greater than that associated with not fully reaching the steady state. The... 

The oxidation of cobalt (III) tris(dithiocarbamate) (C0L3) in acetonitrile provides an interesting example of an electrode reaction that had to be probed using a combination of cyclic and channel electrode voltammetry over a wide range of experimental conditions (i.e. C0L3 concentration, flow rate, scan rate, etc.) in order to confirm unambiguously that the mechanism was EC2C (89) in nature (Alden et al., 1998). 

---