Solvents half-wave potential

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

TABLE 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic Compounds at 25°C The solvent systems in this table are listed below ... 

Physical Properties of Solvents and Half-Wave Potential E1/2 of Bis-biphenylchromium (BBCr), in 0.1 M NaClO. Solution... 

Fig. 10. Half wave potentials (at a rotating platinum electrode) vs. d-electron configuration for Et2dtc complexes. The E1/2 values depend upon solvent and reference electrode used (see text), but this is a minor effect as compared with the influence of the d-electron configuration.

Gibbs transfer energy of an ion i from phase a to p AG g Gibbs energy for ion-solvent interaction in phase a A log P partition coefficient difference between two solvent systems A 0 Galvani potential difference between a and p phases Ag(pi/2 half-wave potential... 

In the course of our polarographic studies on organic cations we determined the half-wave potentials, 1/2, for various arylmethylium ions [1-11]. The aim of the present work is to extract from these values some new information concerning the relative magnitude of their solvation enthalpies in three very different solvents. A comparison of our results [obtained in methanesulphonic acid (MSA) and dichloromethane (DCM)] with those of Volz and Lotsch [12] [obtained in cyanomethane (CM) solutions] yields some useful conclusions. 

If one writes equation (6) for two ions, A and B, in the same solvent and subtracts one from the other, one obtains for a"bAE1/2, the difference between the half-wave potentials of the two ions, Equation (7) ... 

Since (b) and (c) are independent of the nature of the solvent, the half-wave potential for a reversible reduction is ameasure of the amount of interaction of the metal ion with the solvent molecules according to the reaction... 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

In the absence of data for the solvation enthalpies in nonaqueous media, an attempt has been made to plot the half-wave potentials for a given ion in different solvents (expressed in the bis(biphenyl)chromium(I) scale) vs the donicity of the solvent molecules. The Fig. 22 to 26 reveal a relationship between Et /2 and DN, i.e., the half-wave potential becomes more negative with increasing donicity of the solvent. 

In this plot a characteristic curve is found for each ion. The half-wave potentials of Na+, K+, Rb+ and Cs+ (Fig. 22) are similar in each of the solvents. The curve in the j2-DNdiagram reveals that in strong donor solvents E, j2 remains nearly constant at increasing donicity 120 121F This observation suggests that these ions cannot utilize the strong donor properties of such solvents and that solvation is mainly due to electrostatic forces between ion and solvent dipoles. 

For Ni2, Co2 and Tl increasing donicity of the solvent has a more pronounced effect on the half-wave potential (Fig. 26) the solvate bonds become increasingly covalent with strong donor molecules. 

The half-wave potentials of K+, Tl+ and Ca2+ in water are slightly more negative and thosefor Zn2+, Cd2+, Mn2+, Ni2+ and Co2+ considerably more negative than is expected according to the donicity rule. It has been shown in the previous sections that water is a rather unique solvent. The effect in question may be interpreted by the so-called Katzin-effect according to which water forms a royal core of coordinated water molecules which are hooked together by hydrogen bonds 70,71>122,1231. 

In most other cases the relationship will allow the approximate prediction of the half-wave potentials of a given ion in a solvent of given donicity by interpolation. It may be expected that E jy2 for a certain metal ion in tetramethylene sulfone (DN = 14.8) will be similar to that in PDC (DN = 15.1), benzylcyanide (DN = 15.1) or ethylene sulfite (DN = 15.3). Likewise, the half-wave potentials are expected to be similar in nitrobenzene (DN = 4.4) and nitromethane (DN = 2.7). In an analogous manner the half-wave potentials may be predicted in methyl acetate, diethylether, pyridine, and various other solvents. 

The presence of water may have an appreciable effect onE j, since water is a fairly strong donor. It is known that it is extremely difficult to remove the last traces of water from any solvent and it is therefore of interest to know the influence of water. It is apparent that in solution of a strong donor such as DMF, DMA, DMSO or HMPA the presence of small amounts of water is not reflected in a shift of the half-wave potential. On the other hand, the half-wave potential is shifted to negative potential values by the presence of water in a weak donor solvent. 

Rather surprisingly, the differences in half-wave potentials of hydrocarbons from one solvent to another are very small. This constancy in energy values as well as slopes of correlation lines in widely varying solvents and supporting electrolytes implies that solvation energies, provided they are not small, change in the same way from system to system. 

Conjugation with an electron-withdrawing group substantially lowers the energy of the lowest unoccupied molecular n-orbital, which results in less negative reduction potentials for the alkene system. The class of compounds is referred to as activated alkenes, Polarographic half-wave potentials for some activated alkenes in aprotic solvents are listed in Table 3.3... 

Polarographic half-wave potentials for activated alkenes in aprotic solvents... 

The half-wave potentials of Cd(II), Zn(II), and Pb(II) ions electroreduction in 22 nonaqueous solvents were used in the analysis of solvent effect on electrode potential [68]. 

Moreover, Brand and Snedden were also able to correlate the mass and the number of halogen substituents in the solvent with spectral changes. Even better correlation was found between the wavelength of the 307 mu band and the half-wave potential of the halide—a measure of its tendency to act as an electron acceptor. 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

Ion exchange and solvent extraction techniques have been used extensively as the basis for radiochemical generators exploiting the differences in absorption behavior between the parent nuclide and its useful daughter nuclide. Many parent/daughter pairs of nuclides have sufficiently different polarographic half wave potentials so that their electrochemical behavior may be exploited for rapid separation of the daughter from the parent with minimal contamination of the product with the parent isotope. ... 

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