Heterogeneous electron transfer kinetics for reduction of Fe(H 2 O) 6 3+ at Pt electrodes in the presence of equimolarFe(H 2 O) 6 2+ in noncoordinating 1.0 M HClO 4 have been studied during irradiation with high-intensity ultrasound at 20 kHz both in the presence and in the absence of small solid particles. Increases in heterogeneous rate constants have been measured during sonication in the presence of 1-µm particles of Al 2 O 3 . During sonication, particles collide with the electrode surface, generating heat and enhancing kinetic rates relative to those determined for sonication in the absence of particles. Control experiments (in the absence of ultrasound) were conducted with a rotating disk electrode (RDE). Rate constants from charge transfer resistance measurements within (0.025 V of the equilibrium potential obtained by RDE, sonication, and sonication with 16.7 g of Al 2 O 3 per liter of solution were (3.4 ( 0.1) × 10 -4 , (4.2 ( 0.5) × 10 -4 , and (20 ( 2) × 10 -4 cm/s, respectively. By comparison, Tafel plots corrected for mass transport effects gave rate constants (RDE, sonication, and sonication with Al 2 O 3 ) of (4.4 ( 0.2) × 10 -4 , (6 ( 1) × 10 -4 , and (15 ( 5) × 10 -4 cm/s, respectively. Additional studies indicated that no increases in electrode area or rate constants were measurable postsonication; thus, rate enhancements are attributed to thermal effects (cavitation and collisions). Similar enhancements were also found for Fe(CN) 6 3-reduction in the presence of equimolar Fe(CN) 6 4-. Nonsonication, variable-temperature studies of Fe(H 2 O) 6 2+/3+ gave activation energies for the kinematic viscosity, Fe(H 2 O) 6 3+ diffusion coefficient, and standard heterogeneous rate constant of 15.8, 14.9, and 12.1 kJ/mol, respectively. From the Arrhenius relationship, effective temperatures at the electrode surface during sonication of a solution held at 273 K were found to be (282 ( 5) K without Al 2 O 3 and (410 ( 10) K with 16.7 g of Al 2 O 3 per liter of solution. The addition of larger quantities of Al 2 O 3 produced even greater effects.
A new procedure for preparing arrays of microelectrodes based on sonochemical ablation of thin insulating polymer films on electrode surfaces is described. Films are formed through the oxidative electropolymerization of o-phenylenediamine and micron-diameter holes subsequently burned through films by interfacial acoustic cavitation. Electrodes prepared in this way exhibit steady-state currents in quiescent solution and yield lower detection limits than bare electrodes. This preparation procedure has the potential advantage of being scalable to prepare arrays of arbitrary size.
Report to The Electrochemical Society for the 1996 Joseph W. Richards Summer Research Fellowship FIG. 1. (a) Clnonocoulograms in 10.0 mM K3Fe(CN)6 (or (A) bare electrode, and (or electrodes (allowing electrodeposition (or 60 5 o(poly(o-phenylenediamine) (rom (B) quiescent solution and (C) sonicated solution. (b) Chronocoulograms in 10.0 mM K 3 Fe(CN)6 (or (A) bare electrode, and (or electrodes (allowing electrodeposition (or 10.\ o( phenol (rom (B) quiescent solution and (C) sonicated solution.
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