Electrochemistry of dilute solutions

Chemical equilibrium in homogeneous systems—Dilute solutions (continued)— Outlines of the electrochemistry of dilute solutions... 

Electrochemistry of Dilute Solutions 1 Hernsl s Theory of the Solution Pressure of an Electrode. 

The measurement of solution conductivity is a long-established field of physical electrochemistry and, as discussed in the introduction, has been used to develop the ideas of solution structure. The interest in this section is in the practical values displayed by concentrated solutions and in the values of dilute solutions for the light it sheds upon the ion-ion association. These topics will be taken up in turn. For guidance in measuring electrical conductance, see Evans and Matesich [19] or Coetzee and Ritchie [20], For guidance in measuring the related transference numbers, see Kay [21] or Spiro [22],... 

The chemical potentials of dilute solutions may be expressed in terms of molality (moles of solute per kilogram of solvent) or molarities Ck (moles of solute per liter of solution) instead of mole fractions X/t. In electrochemistry it is more common to use molality ntk. For dilute solutions, since Xk = (A jfc/A soivent)> we have the following conversion formulas for the different units... 

Photoemission phenomena are of great value for a number of areas in electrochemistry. In particnlar, they can be used to study the kinetics and mechanism of electrochemical processes involving free radicals as intermediates. Photoemission measurements can be also used to study electric double-layer structure at electrode surfaces. For instance by measuring the photoemission current in dilute solution and under identical conditions in concentrated solutions (where we know that / = 0), we can find the value of / in the dilute solution by simple calculations using Eq. (29.9). 

Walsh FC, Reade GW (1993) Electrochemical techniques for the treatment of dilute metalion solutions, in CAS. Sequiera (ed), Environmentally oriented electrochemistry, Elsevier, Amsterdam... 

In all other solutions the so called degree of dissociation, as determined from the measurement of some colligative property, merely indicates the magnitude of interionic forces, it cannot, however, be taken as a measure of the quantity of dissociated and undissociated molecules of the solute. A complete theory of strong electrolytes, at least of their diluted solutions, has been developed by Debye and Hiickel, this theory is the basis of modern electrochemistry. 

The original form of the Debye-Hiickel equation permits the calculation of the mean activity coefficients of strong electrolytes in solutions defined by their molarity c. Should the value of this coefficient be expressed by molality, whioh is more advantageous in electrochemistry, it will be possible in the case of a sufficiently diluted solution to substitute into the equation (V-58) for = y m (see V-41e) and for molarities of all ions the product of their molalities and the density of the solvent s wqp°, so that ... 

These relations are summed up in some of the classical equations of electrochemistry, which were derived by consideration of dilute aqueous solutions in which complete dissociation into independently moving ions could be assumed. Although these solutions present a rather different physical situation from that of solvent-free ionic liquids, the laws developed for their description remain very relevant to the description of the ionic liquid properties. The main difference is that the notion of dissociation is more obscure. In ionic liquids the state of dissociation must be decided by operational criteria, as we outline below. 

In solution, all six possible fulleride anions can be prepared and have been studied by various methods. The results are summarized in the review by Reed and Bolskar [18]. The most common reduction methods are the reaction with alkali metals or electrochemistry. In these cases, marked solvent dependence is observed indicating that the effect of the environment is not negligible even in dilute solutions. 

The electrochemical properties of many functional groups have been described in reviews by Steckhan, Degner (industrial uses of electrochemistry), Kariv-Miller,543 and Feoktistov. The synthetic applications of anodic electrochemistry has also been reviewed. There are interesting differences between dissolving metal reductions (secs. 4.9.B-G) and electrochemical reactions. Cyclohexanone, for example, can be reduced to cyclohexanol (sec. 4.9.B) or converted to the 1,2-diol (556) via pinacol coupling by controlling the reduction potential, the nature of the electrode and the reaction medium. 46 Presumably, the more concentrated conditions favor formation of cyclohexanol via reduction of the carbanion. More dilute solutions appear to favor the radical with reductive dimerization to 556. More important to this process, however, is the difference in reduction potential (-2.95 vs. -2.700 V) and the transfer of two Faradays per mole in the former reaction and four Faradays per mole in the latter. 

Emf measurements yield reliable standard potentials only when data analysis uses well-founded extrapolation methods which take into luxoimt association of the electrolyte compounds A knowledge of reliable standard potentials is important for electrochemistry in non-aqueous solutions, especially for solvation studies and technological investigations. A comprehensive survey of these questions is in preparation. Data analysis with the help of Eqs. (28) gives and R values which are compatible with those from other methcKls. Table V illustrates the satisfactory agreement of activity coefficients from emf measurements, y (emO, and heats of dilution, y (4>H ), both evaluated by appropriate methods. 

Diffractive spectroelectrochemistry is a technique which is very much in its infancy, but it is clear that it has the potential to improve on the performance of previous methods. The low sensitivity of previous spectroelectrochemical methods limited their applicability to strong chromophores with millimolar concentrations. In the diffractive approach, the path length is not constrained by the diffusion process, so the sensitivity is much higher, and dilute solutions of weak chromophores can be examined. A wide variety of both analytical and kinetic experiments are possible with diffractive spectroelectrochemistry with its potentially increased generality over previous methods. In addition, a spatially sensitive probe of the thin layer of solution near an electrode permits many new questions about mass transport and chemical reactions to be addressed. Diffractive spectroelectrochemistry should evolve into an excellent probe of the diffusion layer, a region of utmost importance to electrochemistry, but one which is exceedingly difficult to probe with conventional methods. 

Before deciding to start the electrochemical research, the writer made a very thorough search of Chemical Abstracts and all treatises on electrochemistry, including chapters in books on microchemistry. No previous studies of very dilute solutions were found under titles such as electrochemistry, Nernst Equation, or electrodeposition. So our work began. 

While molten salt electrochemistry dates back to the time of Sir Humphry Davy and Michael Faraday, electroanalytical measurements in such systems are of much more recent origin. In such measurements we are primarily interested in solutes in dilute solutions rather than in properties such as activity coefficients or conductivities of major components. Molten salt solvents are almost always mixtures rather than pure compounds and therefore there is literally an infinite array of possible solvents. 

Walsh, F.C. and Reade, G.W. (1994) Electrochemical techniques for the treatment of dilute metal ion solutions, in Environmental Oriented Electrochemistry (ed. C A.C. Sequeira), Elsevier. 

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