Specific Features of the Initial Stages of Oxidation of a Silver Electrode in Weakly Acidified Solutions of Sodium Fluoride

Abstract

The behavior of a silver electrode, which is renewed in situ by mechanical cutting, in weakly acidified sodium fluoride solutions is studied with the aid of an impedance method and a method of cyclic voltammetry. The application of said procedures makes it possible to record time effects, which reflect the processes of electrochemical adsorption of oxygen at the interface between the silver electrode and the solution, in the potential region extending from -0.7 to 0.2 V (SCE). Approximate calculations of the effect of potential and the time of the electrode's exposure (after its renewal) in contact with the electrolyte at given values of potential on the amount of adsorbed oxygen are performed on the basis of an analysis of the obtained experimental data. A comparison of the obtained results with the literature data makes it possible to put forth a substantiated opinion as to the reasons for the substantial difference in the intervals of potentials of ideal polarizability of the silver electrode/solution of a surface-inactive electrolyte system, which are presented in works of various authors.

Initial oxidation of silver electrode in weakly acidic sodium fluoride solutions

“…These dependences of the potential shift on the exposure time τ correlate with the surface relaxation effects on renewable Ag electrodes at the potentials more positive than −0.7 V, which were revealed by capacitance and voltammetric measurements [26] and associated with the beginning of oxidation of the metal surface (the appearance of upd oxygen).…”

“…Despite the θ values were obtained in experiments with the radically different pretreatment of the electrode surface (mechanical renewal), the results for different potentials are approximated by a single straight line that nearly passes through the origin. Generally speaking, tangents of [CdE/dT] vs. θ dependences, i.e., [dσ 2 (E)/dT − dσ 1 (E)/dT] values, should not be equal at dif- These results allow us to suggest that the time effects observed in electrochemical [26] and photoelectrochemical experiments are of the mutual origin and that the model (5) is applicable for the system under consideration.…”

“…2 and 4) and the model of two parallel capacitors. We assume that θ = 0 at τ = 0, i.e., immediately after the potential was reswitched from −1.3 V (a potential at which the Ag surface is completely reduced [26]) to a potential at which oxygen chemisorbs. Then, we can rewrite (6) as…”

“…To assess θ at the specified τ, we used the results of a study [26]. In the latter study, based on the linear voltammetry data, the amount of chemisorbed oxygen was assessed for different times of the exposure of a freshly renewed Ag electrode at the potentials E ≥ −0.7 V. Under an assumption that a monolayer of oxygen adatoms on the Ag surface corresponds to a charge of 210 C/cm 2 (i.e., at the complete coverage, one oxygen atom occupies two silver atoms), these dependences of the amount of chemisorbed oxygen (in electric units) on the exposure time were recalculated into θ vs. τ dependences.…”

“…In our study [26], this problem received special attention. As was revealed by measurements on the electrode surface renewed by mechanical cutting, the capacitance curves of silver in the potential range corresponding to the surface charges more positive than ∼7-8 C/cm 2 demonstrated pronounced time effects, i.e., the electric double layer capacitance decreased with the increase in the time (τ) the renewed surface stayed in contact with the solution.…”