Bulk electrochemical methods

Although there are only three principal sources for the analytical signal—potential, current, and charge—a wide variety of experimental designs are possible too many, in fact, to cover adequately in an introductory textbook. The simplest division is between bulk methods, which measure properties of the whole solution, and interfacial methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode. The measurement of a solution s conductivity, which is proportional to the total concentration of dissolved ions, is one example of a bulk electrochemical method. A determination of pH using a pH electrode is one example of an interfacial electrochemical method. Only interfacial electrochemical methods receive further consideration in this text. 

The classical electrochemical methods are based on the simultaneous measurement of current and electrode potential. In simple cases the measured current is proportional to the rate of an electrochemical reaction. However, generally the concentrations of the reacting species at the interface are different from those in the bulk, since they are depleted or accumulated during the course of the reaction. So one must determine the interfacial concentrations. There axe two principal ways of doing this. In the first class of methods one of the two variables, either the potential or the current, is kept constant or varied in a simple manner, the other variable is measured, and the surface concentrations are calculated by solving the transport equations under the conditions applied. In the simplest variant the overpotential or the current is stepped from zero to a constant value the transient of the other variable is recorded and extrapolated back to the time at which the step was applied, when the interfacial concentrations were not yet depleted. In the other class of method the transport of the reacting species is enhanced by convection. If the geometry of the system is sufficiently simple, the mass transport equations can be solved, and the surface concentrations calculated. 

The method relies on the relationship between the surface concentration of the electroactive species O (which yields R according to O + ne = R), Co 0,t), having a bulk concentration Co, and the current, i. Under semi-infinite linear diffusion conditions and independently of the particular electrochemical method employed, Co(0, t) can be expressed by (22)... 

Electrochemical Synthesis of Bimetallic Particles. Most chemical methods for the preparation of metal nanoparticles are based at first on the reduction of the corresponding metal ions with chemical reagents to form metal atoms and then on the controlled aggregation of the obtained metal atoms. Instead of chemical reduction, an electrochemical process can be used to create metal atoms from bulk metal. Reetz and Hclbig proposed an electrochemical method including both oxidation of bulk... 

Bard, A.J., and L.R. Faulkner. 1980. Bulk electrolysis methods, in Electrochemical Methods Fundamentals and Applications, AJ. Bard and L.R. Faulkner (eds.), John Wiley Sons, Inc., New York, pp. 370-428. 

The book includes several chapters on vapor and trace detection chemiluminescence, mass spectrometry, ion mobility spectrometry, electrochemical methods, and micro mechanical sensors, such as microcantilevers. Other chapters deal with bulk detection techniques neutron techniques, nuclear quadrupole resonance, X-ray diffraction imaging, millimeter-wave imaging, terahertz imaging, and laser techniques. Special chapters are devoted to personnel portals and to biological detection. 

Edge diffusion — The diffusion edge is the fictional boundary between the diffusion layer and the bulk. It is a convenient concept when an extending diffusion later reaches an object to react, e.g., at a pair electrode and a -> scanning electrochemical microscope [i]. Ref. [i] Bard AJ, Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York, p 669... 

Several oxides with perovskite related stmctures can also be intercalated with oxygen ions by an electrochemical method. The oxide Sr2Fe20s with the brownmillerite stmcture has been electrochemically oxidized to SrFeOs. The reaction was carried out by controlled potential electrolysis at a potential below that for oxygen evolution in 1 M aqueous KOH at room temperature. Bulk oxidation was confirmed by Mossbauer spectroscopy and X-ray difflaction. Similar results have been obtained for electrochemical oxidation... 

Spectroelectrochemistry has become a valued technique coupling spectroscopy and electrochemistry. Spectroelectrochemistry is a bulk electrochemical technique and as such many of the cell requirements discussed above that pertain to BE apply for spectroelectrochemistry. Often concentrations for spectroelectrochemistry are much lower than most electrochemical techniques due to the spectroscopic absorbance requirements. The bulk solution must still be oxi-dized/reduced in spectroelectrochemistry. Large surface area working and auxiliary electrodes are employed as in the bulk methods described above. Cells designed with optically transparent electrodes like thin films of Sn02 or In203 or optically transparent mesh electrodes are employed, otherwise the electrode must be manually removed to record spectra. Optically transparent electrodes can be constructed such that the solution volume to electrode surface area ratio is very small making the BE occm rapidly. 

Similarly, impervious yttria-stabilized zirconia membranes doped with titania have been prepared by the electrochemical vapor deposition method [Hazbun, 1988]. Zirconium, yttrium and titanium chlorides in vapor form react with oxygen on the heated surface of a porous support tube in a reaction chamber at 1,100 to 1,300 C under controlled conditions. Membranes with a thickness of 2 to 60 pm have been made this way. The dopant, titania, is added to increase electron How of the resultant membrane and can be tailored to achieve the desired balance between ionic and electronic conductivity. Brinkman and Burggraaf [1995] also used electrochemical vapor deposition to grow thin, dense layers of zirconia/yttria/terbia membranes on porous ceramic supports. Depending on the deposition temperature, the growth of the membrane layer is limited by the bulk electrochemical transport or pore diffusion. 

From an electrochemical point of view it is easily inferred that the solution in a cell near an electrode is separable into two parts a stagnant layer adjacent to the electrode in which no convective motions occur, and the remainder of the solution, which is homogeneous (bulk solution). Yet this is not a particularity of electrochemical methods since the same phenomena occur at any solid/liquid interface, as when metal particles (reductions by Zn or Na, for example) or any heterogeneous reagent is used in organic homogeneous chemistry, as well as in phase-transfer catalysis or related methods. 

A second important property of Eq. (149) is that it provides an estimate of the rate, in terms of a characteristic time 6, associated with mass transfer. Indeed, this is the time 9 needed for a molecule to reach the electrode, that is, to cover the space interval in which the molecular concentration differs from that in the bulk. In transient methods this time is identical to that elapsed since the beginning of the experiment, provided that it is lower than tmax = conv/2D. For steady-state methods, the length to be covered is (Sconv and thus from Eq. (149) it follows that 9 = 5conv/2D. The rate of mass transfer can be defined as 1 /9, since it is obviously equivalent to a first-order process (see Chapter 3 for a demonstration of this point). Yet in light of the previous discussion, it is preferable to think in terms of a characteristic time 9 associated with a given electrochemical method rather than in terms of mass transfer rate, although this intuitive latter notion was extremely worthwhile up to this point. ... 

Since we have not previously considered, what happens when current is present in an electrochemical cell, we begin with a discussion of this. Then bulk electrolysis methods are discussed in some detail. The voltammetric methods described in Chapter 23 also require a net current in the cell but use such small electrode areas that no appreciable changes in bulk concentrations occur. 

Part IV is devoted to electrochemical methods. After an introduction to electrochemistry in Chapter 18, Chapter 19 describes the many uses of electrode potentials. Oxidation/reduction titrations are the subject of Chapter 20, while Chapter 21 presents the use of potentiometric methods to obtain concentrations of molecular and ionic species. Chapter 22 considers the bulk electrolytic methods of electrogravimetry and coulometry, while Chapter 23 discusses voltammetric methods including linear sweep and cyclic voltammetry, anodic stripping voltammetry, and polarography. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

The bulk electrolytic methods described in Section 11.3.4 are especially useful for examining the effects of slower reactions coupled to the electron-transfer reaction. Since the time window of such methods is about 100 to 3000 s, reactions with first-order rate constants of the order of 10 to 10 " s can be studied. Moreover, by analysis of the solution following electrolysis (e.g., by spectroscopic, chromatographic, or electrochemical methods), the products of the reactions, and hence the overall reaction scheme, can be determined. The experiments are usually carried out at potentials corresponding to the limiting current plateau, so that the kinetics of the electron-transfer reactions do not enter the analysis of results. Finally, the theoretical treatments for this technique and the analysis of the experimental results are frequently much simpler than those for the voltammetric methods. 

Specific adsorption can have several effects. If an electroactive species is adsorbed, the theoretical treatment of a given electrochemical method must be modified to account for the presence of the reactive species at the electrode surface in a relative amount higher than the bulk concentration at the start of the experiment. In addition, specific adsorption can change the energetics of the reaction, for example, adsorbed O may be more difficult to reduce than dissolved O. The effects of specific adsorption in different electrochemical methods are discussed in Section 14.3. 

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