Potential windows

An ITIES is, by definition, the interface between two immiscible electrolyte solutions. As for an electrode-electrolyte interface, we can distinguish polarizable and polarized interfaces. A polarizable interface usually separates a very 

FIGURE 1.3 Potential window for a system comprising LiS04 in water and TBATPB, that is, tetrabntylammoninm tetraphenylborate in 1,2-DCE limited, respectively, by the transfer of TBA+ and TPB from the organic to the aqueous phase as illustrated and BTTPATFPFB, that is, bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate as drawn. In that case, the potential window is limited, respectively, by the transfer of the aqueous ions sulfate and lithium to the organic phase as illustrated. 

A system is said to be polarizable if one can change the Galvani potential difference or, in other words, the difference of inner potentials between the two adjacent phases without changing noticeably the chemical composition of the respective phases, that is, without noticeable electrochemical reactions taking place at the interface. A system is said to be polarized if the distribution of the different charges and redox species between the two phases determine the Galvani potential difference. 

In the two cases, it is interesting to know how the electric potential varies from one phase to the next and therefore what the charge distribution is on either side of the interface. At present, molecular dynamics is not powerful enough to treat a system containing enough ions to evaluate the electric potential variation. Theoretical approaches have to more rely on classical models such as the modified Poisson-Boltzmann equation. 

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Ionic liquids possess a variety of properties that make them desirable as solvents for investigation of electrochemical processes. They often have wide electrochemical potential windows, they have reasonably good electrical conductivity and solvent transport properties, they have wide liquid ranges, and they are able to solvate a wide variety of inorganic, organic, and organometallic species. The liquid ranges of ionic liquids have been discussed in Section 3.1 and their solubility and solvation in... 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

Table 3.6-1 The room-temperature electrochemical potential windows for non-haloaluminate...

As shown in Figure 3.6-1, GC and Pt exhibit anodic and cathodic potential limits that differ by several tenths of volts. However, somewhat fortuitously, the electrochemical potential windows for both electrodes in this ionic liquid come out to be 4.7 V. What is also apparent from Figure 3.6-1 is that the GC electrode exhibits no significant background currents until the anodic and cathodic potential limits are reached, while the Pt working electrode shows several significant electrochemical processes prior to the potential limits. This observed difference is most probably due to trace amounts of water in the ionic liquid, which is electrochemically active on Pt but not on GC (vide supra). 

Tables 3.6-1 and 3.6-2 contain electrochemical potential windows for a wide variety of ionic liquids. Only limited information concerning the purity of the ionic liquids listed in Tables 3.6-1 and 3.6-2 was available, so these electrochemical potential windows must be treated with caution, as it is likely that many of the ionic liquids would have had residual halides and water present.

FIGURE 4-5 Accessible potential window of platinum, mercury, and carbon electrodes in various supporting electrolytes. 

The limited anodic potential range of mercury electrodes has precluded their utility for monitoring oxidizable compounds. Accordingly, solid electrodes with extended anodic potential windows have attracted considerable analytical interest. Of the many different solid materials that can be used as working electrodes, the most often used are carbon, platinum, and gold. Silver, nickel, and copper can also be used for specific applications. A monograph by Adams (17) is highly recommended for a detailed description of solid-electrode electrochemistry. 

S.2.2 Carbon Electrodes Solid electrodes based on carbon are currently in widespread use in electroanalysis, primarily because of their broad potential window, low background current, rich surface chemistry, low cost, chemical inertness, and suitability for various sensing and detection applications. In contrast, electron-transfer rates observed at carbon surfaces are often slower than those observed at metal electrodes. The electron-transfer reactivity is strongly affected by the origin... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

While electroanalytical techniques are inherently quite sensitive, the resolution of a mixture of electroactive compounds is very difficult. Practical considerations limit the usable potential window to no more than 3 V and typically around 1.5 V. This is because at more extreme potentials the medium or the electrode itself begin to oxidize or reduce. In addition, the electrochemical response of compounds as a function of applied potential is fairly broad so that at least a 200-400 mV difference in half-wave potentials is required for adequate resolution. This typically limits electrochemical resolution of mixtures to no more than three or four electroactive compounds. 

Differential pulse voltammetry provides greater voltammetric resolution than simple linear sweep voltammetry. However, again, a longer analysis time results from the more sophisticated potential waveform. At scan rates faster than 50 mV/sec the improved resolution is lost. Because it takes longer to scan the same potential window than by linear sweep, an even longer relaxation time between scans is required for differential pulse voltammetry. 

Huber et al. [12] investigated the same model by Monte Carlo simulations however, they focused on a different aspect the dependence of the interfacial capacity on the nature of the ions, which in this model is characterized by the interaction constant u. Samec et al. [13] have observed the following experimental trend the wider the potential window in which no reactions take place, the lower the interfacial capacity. Since the width of the window is determined by the free energy of transfer of the ions, which is 2mu in this model, the capacity should be lower, the higher u. ... 

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

The ET reaction between aqueous Fe(CN)g and the neutral species, TCNQ, has been investigated extensively with SECM, in parallel with microelectrochemical measurements at expanding droplets (MEMED) [84], which are discussed in Chapter 13. In the SECM studies, a Pt UME in the aqueous phase generated Fe(CN)g by reduction of Fe(CN)g. TCNQ was selected as the organic electron acceptor, because the half-wave potential for TCNQ ion transfer from DCE to water is 0.2 V more positive than that for ET from Fe(CN)g to TCNQ [85]. This meant that the measured kinetics were not compromised by TCNQ transfer from DCE to the aqueous phase within the potential window of these experiments. 

Comparing curve 1 (VCTTM) with curves 2 and 3 (voltammograms at the Wl/LM and LM/W2 interfaces), it is obvious that (1) the potential window in curve 1 is about twice that in curve 2 or 3, (2) the potential regions where the positive and the negative peaks appear in curve 1 are different from those in curve 2, and (3) the slopes of the positive peak, negative peak, final rise, and final descent in curve 1 are much smaller than those in curves 2 and 3. 

On the basis of similar analysis, the negative wave in curve 1 is considered to be consisted of the transfer of from LM to W1 (the negative wave in curve 2) and that of TPhB from LM to W2 (the final descent in curve 3). The final rise in curve 1 involves the transfer of TPhB from LM to W1 (the final rise in curve 2) and that of CV+ from LM to W2 (the final rise in curve 3), and the final descent in curve 1 involves the transfer of CV+ from LM to W1 (the final descent in curve 2) and that of TPhB from LM to W2 (the final descent in curve 3). These coupled reactions are responsible for the wide potential window. 

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