Halide electrode potentials

Salaita G N, Lu F, Laguren-Davidson L and Flubbard A T 1987 Structure and composition of the Ag(111) surface as a function of electrode potential in aqueous halide solutions J. Electroanal. Chem. 229 1-17... 

Mercury cyanide, 5, 1062 Mercury electrodes potential range aqueous solution, 1, 480 Mercury fluoride, 5. 1059 Mercury fulminate, 2, 7, 12 5, 1063 Mercury halides, 5, 1049 Mercury iodate, 5,1068 Mercury iodide, 5. 1059 Mercury ions Hgf... 

Experimental tests of the theoretical predictions have involved the electrochemical reduction of alkyl and benzyl halides as well as their reduction by homogeneous electron donors.22,29-31 In the first case, AG° = E - rx r.+x=f where E is the electrode potential and rx r.+x=f is the standard potential of the RX/R + XT couple. In the homogeneous case, AG° = E q — rx r-+xt> where E Q is the standard potential of the outer-sphere electron donor or acceptor couple P/Q, and + stands for a reduction and — for an oxidation. 

The relative position of the electronic level eo to the Fermi level depends on the electrode potential. We perform estimates for the case where there is no drop in the outer potential between the adsorbate and the metal - usually this situation is not far from the pzc. In this case we obtain for an alkali ion eo — Ep — where is the work function of the metal, and I the ionization energy of the alkali atom. For a halide ion eo — Ep = electron affinity of the atom. 

Fig. 9 Electrochemical reductive cleavage of aryl halides in a poor H-atom donor solvent. Cyclic voltammetry as a function of the scan rate, v. E, Electrode potential i, current. Reduction (cathodic) currents are represented as being upwards.

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

Fig.1 Surface concentration of adsorbed ions versus rational electrode potential curves for the Cd(OOOl) electrode in aqueous solution with constant ionic strength O.lx M KA + 0.1 (1 - x) M KF, where A is the surface-active halide ion (Br curves 1-3) and (1 curves 4-6), and x is its mole fractions, x = 0.1 (curves 1,4) ...

Kerner and Pajkossy [73] have measured impedance spectra for Au(lll) electrode in perchlorate solutions additionally containing S04 , Cl , Br , and 1 at concentrations of about 10 M. Measurements were performed at adsorption potentials of these anions. Analysis of the impedance spectra led the authors to the conclusion that the adsorption rates of S04 and Cl are immeasurably high. For halide anions, the apparent rate coefficient changes in the order 1 < Br < Cl and decreases with the increasing electrode potential and coverage. 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

Equation (177) predicts that the amount of adsorbate at the interface depends logarithmically on the concentration in solution and linearly with the electrode potential such dependence has been found, for instance, for the specific adsorption of halides on gold [106]. 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

Methods. All solutions were prepared to be ImM Cytochrome c, 0.1mM DCIP, 0.10M alkali halide, and 0.10M phosphate buffer at pH 7.0 or pD 7.0. The DCIP served as a mediator-titrant for coupling the Cytochrome c with the electrode potential. E° values were measured using a previously described spectropotentiostatic technique using an optically transparent thin-layer electrode (OTTLE) (7,11,12). This method involved incrementally converting the cytochrome from its fully oxidized to fully reduced state by a series of applied potentials. For each potential a spectrum was recorded after equilibrium was attained. The formal redox potential was obtained from a Nernst plot. The n value... 

MCrX3 (M = Li—Cs X = halide) have been reviewed770 and mass spectrometric studies of the compositions of MCrCl3 (M = K or Cs) vapours have been reported and the thermodynamic characteristics for the evaporation of these compounds determined.776 Electrode potential data for Cr11 in KCl-LiCl melts have been reported.78 CsCrI3 has been obtained by melting Csl and Crl2 (1 1) in a sealed, evacuated quartz tube. The lattice parameters of this compound have been determined and compared with those of other compounds of this formula type.79... 

In these clusters tantalum atoms are bound to other tantalum atoms and are also edge bridged via halide. As our deposit was completely amorphous without any XRD peak we concluded that it did not consist of crystalline tantalum but rather of such clusters. We varied the electrode potential for deposition and tried deposition with very low constant current densities, but in no case was crystalline tantalum obtained. Thus, the electrochemical window of our liquid was surely wide enough, but for some reason the electrodeposition stopped before Ta(0) was obtained. When we studied the literature dealing with metal clusters we found that the cluster chemistry with fluoride seems to be less comprehensive. Consequently... 

Metal ion and halide impurities are an issue in ionic liquids with discrete anions. As we have demonstrated in Chapter 11.5 Li+ (and K+) are common cationic impurities, especially in the bis(trifluoromethylsulfonyl)amides which typically contain 100 ppm of these ions from the metathesis reaction. Although Li and K are only electrodeposited in the bulk phase at electrode potentials close to the decomposition potential of the pyrrolidinium ions, there is evidence for the underpotential deposition of Li and K on gold and on other rather noble metals. For a technical process to deposit nickel or cobalt from ionic liquids the codeposition of Li and/or K, even in the underpotential deposition regime, has to be expected. 

A streaming cell made of Perspex, holding a small amount of material (2 to 4 cc.), was used. The details of its construction have been published (34). The cell was fitted with two platinized platinum electrodes and two Ag/Ag halide electrodes and is shown in Figure 1. This cell was coupled with two Keithley electrometers Model 610A (one electrometer for each pair of electrodes), whose outputs were fed into a Varian double-channel recorder. Streaming potentials could, thus, be measured simultaneously using two different kinds of electrodes for comparison purposes. 

Preparation of Electrodes. The Ag/AgBr and Ag/AgI electrodes were prepared by anodic electrolysis of the appropriate sodium salt solution (IN) at 6.5 ma. per sq. cm. for 5 minutes. The two electrodes to be coated were connected and dipped into the solution a wire of pure silver was used as the cathode. In the case of the Ag/AgCl electrode IN HC1 was used. Just after preparation the electrodes were immersed in the appropriate halide solution (10-3 M). The Ag/Ag halide electrodes prepared in this manner showed a difference of potential of 0.3 to 0.5 mv. in the presence of 10 5 M solution of sodium halide. This potential will be referred to as rest or nonflow potential. 

The flow effects were more pronounced the older the Ag/Ag halide electrodes. Even in 10-3 M KCl a flow potential was observed for the Ag/AgCl electrodes this effect disappeared, however, after the electrodes were freshly electroplated. 

The behavior of the platinized platinum electrodes is entirely different from that of the Ag/Ag halide electrodes, both under flow conditions and at rest. Whereas the nonflow potential for the nonpolarizable electrodes remained constant with time, the rest potential for two platinized platinum electrodes increased with time. Also, the flow of liquid produced a more pronounced effect on the platinum electrodes than on the Ag/Ag halide electrodes. The erratic behavior of the platinum electrodes appears to be due to polarization effects which are difficult to eliminate. In Figure 3 the effect of flow on both Ag/AgCl and platinized electrodes can be assessed. The effect of flow on the platinum electrodes was known to Helmholtz (19) and still appears to remain unexplained. 

Strong adsorption of Cl, Br, or I occurred when Ag(lll) was immersed in aqueous KC1, KBr, or KI solutions, respectively, at open circuit and throughout most of the useful range of electrode potentials. As for Pt surfaces, halide adsorption on Ag(lll) was a redox process. 

Halogen Electrodes.—The determination of the standard potentials of the halogens is simple in principle it involves measurement of the potential of a platinum electrode, coated with a thin layer of platinum or iridium black, dipping in a solution of the halogen acid or a halide, and surrounded by the free halogen. The uncertainty due to liquid junction can be avoided by employing the appropriate silver-silver halide or mercury-mercurous halide electrode as reference electrode. In practice, however, difficulties arise because of the possibility of the reactions... 

---