Electrochemistry of organic redox liquids. Reduction of 4-cyanopyridine

Abstract

Electrochemical studies of liquid 4-cyanopyridine (4-CNPy; melting point = 78 °C) are reported. In solutions containing only tetra-fl-butylammonium perchlorate (TBAP) as supporting electrolyte, 9.6 M 4-CNPy can be reduced at a steady-state mass-transport rate (10 A/cm1 2 34) yielding a highly concentrated solution layer of the product, TBA+-4-CNPy'", adjacent to the electrode surface. The transient voltammetric response is distorted by the slow coupled diffusion/migration of the supporting electrolyte cation (TBA+) necessary to maintain electroneutrality within this layer. Transport-related phenomena unique to concentrated solutions are presented and discussed with regard to the observed electrochemical behavior of 4-CNPy. These include the effects of (1) counterion dilution and migration, (2) fluid convection induced by interdiffusion of reactant (4-CNPy) and product (4-CNPy'"), and (3) concentration-dependent diffusivities. General flux equations (diffusion, migration, and convection) are presented in terms of the partial molar volumes of each solution component for a constant density fluid.A theoretical analysis of the current-dependent ohmic solution resistance, Rfi), at a spherical electrode is also presented for the special case where the concentration of supporting electrolyte is significantly less than the redox active species. This current-dependent resistance generates a 200 V/cm electrical potential gradient, d<p(r)/dr, associated with the diffusion-limited reduction of undiluted 4-CNPy.

Voltammetry of Electroactive Oil Droplets. Part II: Comparison of Experimental and Simulation Data for Coupled Ion and Electron Insertion Processes and Evidence for Microscale Convection

“…Evidence for this kind of process has recently been reported by Anson and co-workers [11,12] for the case of continuous thin ®lm deposits of organic solvents. This model is also relevant to the case of electrolysis of pure redox reagents [13,14] studied extensively by White et al In model B the droplet deposit is assumed to act as an insulator except for the oil±aqueous electrolyte interface, which is assumed to rapidly conduct electrons [15]. The electrochemical conversion of the deposit in this case commences from the oil±aqueous electrolyte interface (see Figure 1) and diffusion of cation±anion pairs is the dominant transport mechanism.…”

Digital simulations to solve electrochemical processes involving a diffusion coefficient varying linearly with the concentration

“…Prior papers for theory [4][5][6][7][8][9][10][11][12][13] and experiments [14][15][16][17][18][19][20] on concentration-dependent diffusion coefficient in electrochemistry have been published. In general, the diffusion coefficient may be a function of concentration of the diffusing species, or a function of the positional coordinates [16].…”

“…Redox reactions in the absence of a solvent have shown a diffusion coefficient for the electroactive reactant varying as a function of the distance across the depletion layer [18][19][20]. By considering the diffusion of linear polymers, the diffusion coefficient varies linearly with the concentration for sufficiently low concentrations [7].…”

Slow Formation of an Extra-Viscous Ionic Layer at Platinum Microelectrodes During Voltammetric Oxidation of Undiluted Dimethylformamide

“…These compounds include aniline and pyrrole [1], acetonitrile [2], simple alcohols [3], 4-cyanopyridine [4], dimethylsulfoxide [5], and nitrobenzene [6][7]. Various interesting physicochemical aspects related to voltammetry of undiluted liquids [4,[6][7][8] were reported. New analytical possibilities [3,5] are connected with the total electrolysis at the microelectrode surface, and with in situ estimation of water level.…”

“…This equation is valid only if the diffusion coefficient of the substrate remains constant throughout the solution, and the partial molar volumes of all species involved in the electrode reaction are approximately equal [4]. Apparently, these conditions can rarely be met in experimental situations.…”