Electroinactive Ions Research Articles

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br −, l −, SCN −, N 3−, Ti + and C(NH 2) 3+

The rate constant k f for CCl 3COO − electroreduction at constant applied potential, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q i due to specifically adsorbed supporting ions. Thus, in the presence of the adsorbed anions Br −, I −, SCN − and N 3 −, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with | q i|, albeit slightly. In the presence of the adsorbed cations Tl + and C(NH 2) 3 +, Φ increases with the positive charge density q i due to the cation at a rate which is 3 to 8 times greater than the corresponding rate of decrease in the presence of the adsorbed anions. This behaviour has been interpreted on the basis of the theoretical treatment of ref. 22 by considering that the time of adsorption of the electroactive anion CCl 3COO − is long enough to permit a perturbation in the distribution of the adsorbed supporting ions in the immediate vicinity of the adsorbed reactant.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I − and SCN −

An investigation of the d-effect in the absence of ionic specific adsorption shows that the overall charge of the reactants in dimethylglyoxime (DMG) electroreduction on mercury from aqueous solutions of pH≅2 equals+1, in agreement with a protonation reaction either preceding (DMG + H +DMGH ++e → products) or else simultaneous with (DMG + H + + e → products) the charge transfer. In the presence of the specifically adsorbed supporting anions I − and SCN − the logarithm Φ of the rate constant for DMG electroreduction corrected for diffuse-layer effects decreases linearly with an increase in the absolute value | q i| of the charge density at the inner Helmholtz plane due to the adsorbed anions. This behaviour is explained semiquantitatively by assuming that protonation takes place simultaneously with charge transfer, with the DMG molecule in the adsorbed state and the H 3O + ion in the non-adsorbed state. In this way the H 3O + ion, due to the screening effect of the surrounding diffuse-layer ions, escapes the accelerating attractive effect exerted by the adsorbed I − or SCN − ions; on the other hand these latter anions exert a retarding repulsive effect upon the electron transfer from the metal to the adsorbed DMG molecule.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part IV. Carbon Tetrachloride electroreduction in the presence of Cl −, Br −, SCN −, and N 3−

The rate constant for CCl 4 electroreduction on mercury, once corrected for diffuselayer effects according to Frumkin, still depends on the charge density q i due to specifically abdsorbed supporting ions. Thus, in the presence of the adsorbed anions Cl −, Br −, SCN − and N 3 −, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with | q i|. The slopes of the various Φ vs. q i plots are in fairly good agreement with the theoretical treatment of ref. 9, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring adsorbed electroinactive ions within the compact layer. An analogous behaviour is observed in the reduction of CBr 4.

A theory of the electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes with reactant adsorption

The electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes involving adsorbed reactants with a time of adsorption greater than the characteristic time of surface-diffusion jumps is considered. The validity of the most general form of the Frumkin correction term for double-layer effects, with ψ 1 equal to the local potential ‡ at the position occupied by the reactant in the transition state, is first examined on the basis of current molecular theories of electrode kinetics. Subsequently, approximate expressions of ‡ are derived for the case in which adsorbed reactant and adsorbed supporting ion are charged both of equal and of opposite sign. In both cases the logarithm Φ of the rate constant for the electrode reaction at constant applied potential, as corrected for diffuse-layer effects only, is expected to vary linearly with the charge density q i due to the adsorbed supporting ion. |ΔΦ/Δ q i | is, however, much greater in the case of electrostatic attraction between reactant and adsorbed supporting ion than in the case of electrostatic repulsion. The influence of reactant adsorption with partial charge transfer upon the magnitude of ΔΦ/Δ q i is considered.

A theory of the electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes with reactant adsorption

The electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes involving adsorbed reactants with a time of adsorption greater than the characteristic time of surface-diffusion jumps is considered. The validity of the most general form of the Frumkin correction term for double-layer effects, with ψ 1 equal to the local potential ‡ at the position occupied by the reactant in the transition state, is first examined on the basis of current molecular theories of electrode kinetics. Subsequently, approximate expressions of ‡ are derived for the case in which adsorbed reactant and adsorbed supporting ion are charged both of equal and of opposite sign. In both cases the logarithm Φ of the rate constant for the electrode reaction at constant applied potential, as corrected for diffuse-layer effects only, is expected to vary linearly with the charge density q i due to the adsorbed supporting ion. |ΔΦ/Δ q i| is, however, much greater in the case of electrostatic attraction between reactant and adsorbed supporting ion than in the case of electrostatic repulsion. The influence of reactant adsorption with partial charge transfer upon the magnitude of ΔΦ/Δ q i is considered.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes part III. Further considerations on perbromate electroreduction in the presence of halide and pseudohalide ions

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes part III. Further considerations on perbromate electroreduction in the presence of halide and pseudohalide ions

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part III. Further considerations on perbromate electroreduction in the presence of halide and pseudohalide ions

The effect of specifically adsorbed halides and pseudohalides upon the kinetics of BrO 4 − electroreduction, already investigated by the authors and Cozzi [7] and, more recently, by de Levie and Nemes [8], is further examined. The inadequacy of a simple diffuse-layer correction for the kinetics of BrO 4 − electroreduction and the “direct” effect of the charge density q i at the inner Helmholtz plane ( q i-effect) upon such kinetics are stressed. An alternative method for estimating the q i-effect, based on the use of corrected Tafel plots at constant q i, is proposed.

Electrostatic effect of specifically adsorbed electro-inactive ions upon electrode processes II. Perbromate electroreduction in the presence of Cl−, Br−, SCN−, and N3−

The reduction rate kf of BrO4− in the presence of halides and pseudohalides at sufficiently negative potential with respect to the point of zero charge satisfies the well-known Butler-Volmer equation corrected for diffuse-layer effects according to Frumkin. On the other hand at less cathodic potentials, where halide and pseudohalide adsorption becomes appreciable, the rate constant kf at constant applied potential E, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density qi due to the specifically adsorbed halide and pseudohalide ions. Thus in the presence of Cl−, Br−, SCN−, and N3− the logarithm of kf,d=0 (where kf,d=0 denotes the rate constant at constant E corrected for diffuse-layer effects) decreases linearly with |qi, at least for |qi|>∼10 μC cm−2. The slopes of the various log kf,d=0vs. qi plots are in satisfactory agreement with a theoretical treatment proposed by two of the authors1,2, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring specifically adsorbed ions within the compact layer.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes I. Tetrathionate electroreduction in the presence of Br−, I−, SCN−, N3− and TI+

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes I. Tetrathionate electroreduction in the presence of Br−, I−, SCN−, N3− and TI+

A comparison between some recent theories on the effect of ionic specific adsorption upon electrode kinetics

A theoretical expression for the rate constant of an electrode process as a function of the charge density q i of specifically adsorbed electroinactive ions, derived by Guidelli and Foresti 1 , is modified taking discreteness-of-charge effects into account, for the particular case in which the activated complex is specifically adsorbed. The modified equation is compared with analogous theoretical expressions derived by Parsons 4 and by Fawcett and Levine 5 , so as to point out both the analogies and the main distinguishing features.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: II. Perbromate electroreduction in the presence of Cl −, Br −, SCN −, and N 3−

Summary The reduction rate kf of BrO4− in the presence of halides and pseudohalides at sufficiently negative potential with respect to the point of zero charge satisfies the well-known Butler-Volmer equation corrected for diffuse-layer effects according to Frumkin. On the other hand at less cathodic potentials, where halide and pseudohalide adsorption becomes appreciable, the rate constant kf at constant applied potential E, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density qi due to the specifically adsorbed halide and pseudohalide ions. Thus in the presence of Cl−, Br−, SCN−, and N3− the logarithm of kf,d=0 (where kf,d=0 denotes the rate constant at constant E corrected for diffuse-layer effects) decreases linearly with |qi, at least for |qi|>∼10 μC cm−2. The slopes of the various log kf,d=0vs. qi plots are in satisfactory agreement with a theoretical treatment proposed by two of the authors1,2, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring specifically adsorbed ions within the compact layer.

Electrostatic effect of specifically adsorbed electro-inactive ions upon electrode processes: I. Tetrathionate electroreduction in the presence of Br −, I −, SCN −, N 3− and Tl +

Summary The reduction rate k f of S 4 O 6 2− in the presence of halides and pseudohalides at sufficiently negative potentials with respect to the point of zero charge satisfies the well-known Butler-Volmer equation corrected for diffuse-layer effects according to Frumkin. On the other hand at less cathodic potentials, where halide and pseudohalide adsorption becomes appreciable, the rate constant k f at constant applied potential E , once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q i due to the specifically adsorbed halide and pseudohalide ions. Thus in the presence of Br − , I − , SCN − , and N 3 − , the logarithm of k f, d =0 (where k f, d =0 denotes the rate constant at constant E corrected for diffuse-layer effects) decreases linearly with | q i |, at least for | q i |>∼10 μC cm −2 . Analogously, in the presence of the specifically adsorbed Tl + ion, log k f, d =0 increases linearly with the positive charge density q i due to this cation. The slopes of the various log k f, d =0 vs . q i plots are in satisfactory agreement with a theoretical treatment proposed by the authors 5,7 , which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring specifically adsorbed electroinactive ions within the compact layer.

A comparison between some recent theories on the effect of ionic specific adsorption upon electrode kinetics

A theoretical expression for the rate constant of an electrode process as a function of the charge density qi of specifically adsorbed electroinactive ions, derived by Guidelli and Foresti1, is modified taking discreteness-of-charge effects into account, for the particular case in which the activated complex is specifically adsorbed. The modified equation is compared with analogous theoretical expressions derived by Parsons4 and by Fawcett and Levine5, so as to point out both the analogies and the main distinguishing features.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes

The electrostatic effect exerted by specifically adsorbed electroinactive ions upon electrode processes is accounted for by introducing into the Frumkin equation the average potential at the position occupied by the reacting particle in the transition state. The value of this potential is obtained from a simple model of the double layer in the presence of specific ionic adsorption. It is thus shown that the logarithm of the rate constant k f at constant applied potential is expected to vary linearly with the charge density q i at the inner Helmholtz plane. Some examples in support to the above theoretical prediction are reported.

Speculation on Some Effects of Complexation and Anion‐Induced Adsorption on the Polarography of Indium in Acidic Halide and Thiocyanate Media

Some phenomena found in the polarographic reduction of indium (III) in acidic halide and thiocyanate media have been studied by a‐c polarography. Speculation on the role of complex formation and anion‐induced adsorption on the polarographic reduction at potentials near the half‐wave potential are given. At low ligand concentrations the a‐c peak potential is believed to be influenced by anion‐induced adsorption while at high ligand concentrations the value is the thermodynamic value predicted from bulk complexation equilibria. The unusual d‐c minimum found at more negative potentials and the above phenomena are discussed in terms of the coupled occurrence of electroinactive ions at potentials where the complexed ion is electroactive and anion‐induced adsorption. The reduction mechanism at the half‐wave potential and at more negative potentials are concluded to be similar. The irreversible reduction of the uncomplexed indium ion is attributed to the slow rate of exchange of co‐ordinated water, in agreement with other workers.