Homogeneous reactions, scanning

The kinetics of homogeneous reaction of several reactive dyes of the vinylsulphone type with methyl-a-D-glucoside (7.9), selected as a soluble model for cellulose, were studied in aqueous dioxan solution. The relative reactivities of the various hydroxy groups in the model compound were compared by n.m.r. spectroscopy and the reaction products were separated by a t.l.c. double-scanning method [38]. The only sites of reaction with the vinylsulphone system were the hydroxy groups located at the C4 and C6 positions [39,40]. 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

The two previous examples dealt with gas phase catalytic reactions studied in a TS-PFR. Temperature scanning, however, is not limited to this type of reaction or reactor. It is a broadly applicable technique of experimentation, applicable to a variety of chemical reactions, in a variety of reactor types. It is rare, however, to find a reaction that can conveniently be carried out in a variety of reactors. One such reaction is the hydrolysis of acetic anhydride, a liquid phase reaction with particularly simple kinetics. This reaction can therefore be used to examine the consistency of data obtained from various reactors, as well as to provide an illustration of the application of the TSR technique to a homogeneous reaction in the liquid phase. 

Figure 2 Cyclic voltammograms for a one-electron oxidation of A followed by a first-order homogeneous reaction that consumes electrode-generated A. The numbers are rate constants (s ) and a voltage scan rate v =. OVs " is assumed. The decrease in cathodic current and the increasing kinetic potential shift are evident as the rate of the follow-up reaction increases.

As shown in Figure 2, the voltammetric response is significantly altered by the coupled chemical reaction and thus allows the energetics and dynamics of these homogeneous chemical reactions to be probed. This figure shows that as the scan rate is increased, the contribution from the homogeneous reaction becomes less pronounced and the voltammogram approaches the shape of that for an electrochemically reversible process. The fact that ascorbic acid is electroactive means that its concentration in fruit juices can be determined without interferences from the coloration of the sample. 

A DISP 2-type mechanism [11] was then considered and investigated by simulation [12] of the cyclic voltammograms at various concentrations and different scan rates. A close agreement with the experimental set of voltammograms was observed, indicating that the oxidation of Re > formed in the anodic process, is an homogeneous reaction performed by (eqs. 5-7, where B denotes a basic species). 

Ihus determine whether a species is formed or consumed, and whether the reaction is under kinetic or mass transport control. The potential scan rate sets the timescale of the experiment, e.g. with coupled homogeneous reactions, the value of v will determine whether intermediates are formed or consumed and to what extent. Typically v ranges from a few mV sec" to a few V sec", although this upper limit can easily be extended to a few kV s" with microelectrodes see Section 11.2.4. 

Linear scan voltammetry (LSV) and cyclic voltammetry (CV) (see Chapter 11) are among the most common electrochemical techniques employed in the laboratory. Despite their utility, however, they are not particularly well suited to careful measurements of diffusion coefficients when using electrodes of conventional size. We will briefly discuss techniques for measuring D with LSV and CV, but the reader should be cautioned that these measurements under conditions of planar diffusion (i.e., at conventional electrodes) are probably useful to only one significant digit, and then only for nemstian systems with no coupled homogeneous reactions and with no adsorption. For more reliable results with LSV and CV, UMEs should be used. 

Approach curve (SECM) — A current-distance curve recorded as an ultramicro electrode (tip) approaches a surface in scanning electrochemical microscopy. On a surface where the reverse of the tip reaction occurs on the substrate surface the current increases as the distance between the tip and substrate, d, decreases (positive feedback). On an insulator surface the tip current decreases with decreasing d (negative feedback). The approach curve is useful in determining the absolute magnitude of d and can provide information about the heterogeneous electron transfer kinetics on the substrate and the rates of homogeneous reactions that occur in the gap between tip and substrate. 

FIGURE 5.12 Kinetic parameter k as a function of the collection efficiency ( /s//x )- (A) E Cj mechanism K=k tPlD, solid line was computed from Equation 5.85, diamonds are simulated data taken from Ref. [5]. (B) EjCji mechanism k =c° k tP/aD, solid line was computed from Equation 5.88, squares are simulated data taken from Ref. [54]. (Reprinted with permission from Treichel, D.A., Mirkin, M.V., and Bard, A. J., Scanning electrochemical microscopy. 27. Application of a simplified treatment of an irreversible homogeneous reaction following electron transfer to the oxidative dimerization of 4-nitrophenolate in acetonitrile, J. Phys. Chem., 98, 5751-5757, 1994. Copyright 1994 American Chemical Society.)... 

Treichel, D. A., Mirkin, M. V., Bard, A. J. 1994. Scanning electrochemical microscopy. 27. Application of a simplified treatment of an irreversible homogeneous reaction following electron transfer to the oxidative dimerization of 4-nitrophenolate in acetonitrile. J Phys Chem 98 5751-5757. 

Scanning electrochemical microscopy (SECM) and related ultramicroelectrode (UME) methods have proven powerful for measuring the kinetics of homogeneous reactions coupled to heterogeneous electron transfer. For this type of investigation, the tip and substrate are both electrodes and one can usefully consider the tip/substrate electrode configuration as a variable gap ultrathin layer cell. In essence, the gap thickness determines the diffusional transit time of chemical species between the tip and substrate and hence the range of timescales that can be studied. 

The use of SECM to probe homogeneous reaction kinetics can be traced to the earliest applications of UMEs to profile concentration gradients at macroscopic (millimeter-sized) electrodes. There has been considerable progress subsequently, such that short-lived intermediates, in electrode reactions, can readily be identified by SECM under steady-state conditions, that would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (FSCV). 2... 

In Figure 7.26a, the current image reflects the expected hemispherical concentration profile of Fe(CN)g " that extends well beyond the limits of the scan size. The addition of amidopyrine to the solution diminishes the tip current because Fe(CN)g " is converted to Fe(CN)6 locally due to its reaction with amidopyrine in the solution. The images in Figure 7.26b and c clearly show that the homogeneous reaction compresses the concentration profile of Fe(CN)g closer to the UME surface, as expected for an EC process. With 60mM amidopyrine, the solution reaction is so fast that almost all of the Fe(CN)g " is depleted in the area of the scan and only a small current signal (pA levels) is detected at the tip. 

DSC Studies. DSC analyses of blends of 1 and 3a show a homogeneous reaction initially to a thermoset. The DSC scans to 400 C of the blends exhibit only one cure exotherm for each of the compositions studied (see Figures 1 and 2). For example, mole percent mixtures (10/90, 25/75 and 50/50) of 1 and 3a display exotherms (polymerization... 

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