Concentration from electrode potential

Fig. 6.104. Surface concentration vs. electrode potential for the adsorption of (a) HSO4,4x1Cf3M, and (b) cr, 1CT3 M ions on platinum electrodes. The calculated values are represented by the filled circles, while the open circles represent the experimental points. (Reprinted from J. O M. Bockris, M. Gamboa-Aldeco, and M. SzkJarczyk, J. Electroanal.

The vast majority of small-amplitude methods are based on small-amplitude potential excitations with potential control of the surface concentrations. In earlier chapters, the relationship between surface concentration and electrode potential was explored and the concept of concentration profiles was presented. Whenever there is a flux of electrons at the electrode surface, the concentration profiles of at least two species will exhibit nonzero slopes at the electrode surface, as the electrochemical conversion of one member of a couple into another takes place and mass transport processes act to reestablish a uniform concentration distribution. These processes occur irrespective of whether the current flux arises from a potential or current excitation of the cell. In either case, they result in a perturbation from the previously existing concentration profile. The initial surface concentrations (which existed prior to the application of the new perturbation) are often termed the dc surface concentrations. It is useful to note that at any time, the distance integral of the concentration excess or defect is directly proportional to the charge passed due to that per-... 

Unfortunately, the products predicted from electrode potentials are not always the products that form. For gases such as H2(g) and 02(g) to form at metal electrodes, an additional voltage is required. This increment above the expected required voltage is called the overvoltage. The phenomenon of overvoltage has major practical significance in the chlor-alkali process for the industrial production of chlorine and several other chemicals, which is based on the electrolytic oxidation of Cl ion from concentrated aqueous NaCl solutions. Chlorine ranks among the top 10 chemicals produced in the United States. 

In contrast, increasing aqueous Fe(III) and Cu(II) concentrations from 10 to 10 M increased magnetite electrode potentials by several hundred mV (Figure 6B and 6C). At very low aqueous concentrations (<10 M), the potentials are influenced only by differences in pH. At moderate concentrations, the electrode potentials increased with both decreasing pH and increasing aqueous concentrations. At concentrations >10 M, the potentials are independent of pH and follow a log-linear relationship predicted by the Nemst equation. However, the slopes generated by the... 

It was first pointed out by Jaques (6), in the appendix to his book that appeared in 1914, that dissociation constants of the various complexes can in principle be calculated from electrode potential measurements. The total concentration of the metal ion is the sum of the concentrations of each species. Assuming the formation of N mononuclear complexes only, this sum is... 

Weaver calculated the open circuit potentials of these and other possible reactions that might occur under open circuit conditions, finding agreement between measured potentials and the potentials calculated from thermodynamic tables (Weaver et al, 1979). Hemmes and Cassir (2004) recalculated the cell open circuit potentials. They determined the equilibrium concentrations and electrode potentials in a system comprised of carbon, carbonate, CO2, CO, O ", and electrons, using the phase rule modified for electrochemical systems by Coleman and White (1996). Hemmes expressed the half-cell potentials of the anode reactions (3) and (4) referenced to an idealized cathode reaction (unit oxygen and CO2 partial pressures) ... 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Figure C3.6.4(a) shows an experimental chaotic attractor reconstmcted from tire Br electrode potential, i.e. tire logaritlim of tire Br ion concentration, in tlie BZ reaction [F7]. Such reconstmction is defined, in principle, for continuous time t. However, in practice, data are recorded as a discrete time series of measurements (A (tj) / = 1,...

It is stated that in time the acidity (up to 2,5 units) of 0,1-1,0 M HMTA aqueous solutions changes maximally at 1°C, in comparatively to other temperatures (11, 16, 21°C). When the temperature arises the change of HMTA aqueous solutions pH values decreases in time. Formaldehyde and ammonium ions (end products of HMTA hydrolysis) have been fixed only in more diluted solutions (0,10 and 0,25M). The concentration of NH in them in some times is higher than H2C=0 concentration that is caused by oxidation of the last one to a formic acid, being accompanied by the change of the system platinum electrode potential. It is stated that concentration NH in solutions does not exceed 5% from HMTA general content. The conclusion the mechanism of HMTA destruction in H,0 to depend essentially on its concentration and temperature has been made. 

As may be seen from the diagram, silver in highly alkaline solution corrodes only within a narrow region of potential, provided complexants are absent. It is widely employed to handle aqueous solutions of sodium or potassium hydroxides at all concentrations it is also unaffected by fused alkalis, but is rapidly attacked by fused peroxides, which are powerful oxidising agents and result in the formation of the AgO ion Table 6.6 gives the standard electrode potentials of silver systems. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

The equation obtained can be used when the electrode potential can be varied independent of solution composition (i.e., when the electrode is ideally polarizable). For practical calculations we must change from the Galvani potentials, which cannot be determined experimentally, to the values of electrode potential that can be measured E = ( q + const (where the constant depends on the reference electrode chosen and on the diffusion potential between the working solution and the solution of the reference electrode). When a constant reference electrode is used and the working solutions are sufficiently dilute so that the diffusion potential will remain practically constant when their concentration is varied, dE (i(po and... 

When, after the attainment of zero surface concentration, a constant current density is maintained artificially from outside, the electrode potential will shift to a value such that a new electrochemical reaction involving other solution components can start (e.g., in aqueous solution, the evolution of hydrogen or oxygen). It follows from Eq. (11.9) that at a given concentration Cy the product is constant and is... 

Electrochemical reactions differ fundamentally from chemical reactions in that the kinetic parameters are not constant (i.e., they are not rate constants ) but depend on the electrode potential. In the typical case this dependence is described by Eq. (6.33). This dependence has an important consequence At given arbitrary values of the concentrations d c, an equilibrium potential Eq exists in the case of electrochemical reactions which is the potential at which substances A and D are in equilibrium with each other. At this point (Eq) the intermediate B is in common equilibrium with substances A and D. For this equilibrium concentration we obtain from Eqs. (13.9) and (13.11),... 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

It is the basic task of electrochemical kinetics to establish the functional relations between the rate of an electrochemical reaction at a given electrode and the various external control parameters the electrode potential, the reactant concentrations, the temperature, and so on. From an analysis of these relations, certain conclusions are drawn as to the reaction mechanism prevailing at a given electrode (the reaction pathway and the nature of the slow step). 

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