Electrochemical reactions during ohmic heating.

摘要

Electrochemical reactions, chemical reactions at electrode/solution interfaces induced by current, are undesirable during ohmic heating. These reactions may be avoidable or suppressible through an understanding of electrochemical behavior of ohmic heaters. Though numerous studies have dealt with the applications of ohmic heating, little is known regarding electrochemical aspects.; Electrochemical behavior of four types of electrode materials: titanium, stainless steel, platinized-titanium, and graphite, was studied at (initial) pH 3.5, 5.0, and 6.5 using 60 Hz sinusoidal alternating current. Concentrations of metal ions and elemental carbon migrated into the heating media were determined by inductively coupled plasma (ICP)---mass, and---emission spectrometers. Hydrogen gas accumulation in the headspace of the ohmic heater, and pH changes of the heating media were also measured. Stainless steel was found to be the most electrochemically active electrode material, whereas platinized-titanium exhibited relatively inert electrochemical behavior at all the pH values. The potential use of platinized-titanium electrodes for ohmic heating operations was further demonstrated on a pilot scale.; Effects of frequency, pulse width, and delay time of a pulsed ohmic heating technique on electrochemical reactions were studied, in comparison with conventional (60 Hz, sine wave) ohmic heating. Analyses of electrode corrosion, hydrogen generation, and pH measurements suggest that the pulsed ohmic heating is capable of significantly reducing the electrochemical reactions of titanium, stainless steel, and platinized-titanium electrodes. The delay time was found to be a critical factor.; Electrochemical and secondary chemical reactions during 60 Hz ohmic heating of ascorbic acid in citrate-phosphate buffer with stainless steel electrodes were characterized by a number of analytical methods. Electrode corrosion showed marked effects on the heating buffer medium forming metal-phosphates and metal-citrate complexes. Effects of reactions on pH, buffer capacity, and ascorbic acid degradation are discussed.; Free radical generation was investigated by spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and employing electron spin resonance (ESR) spectroscopy. The frequency range of 1--8 kHz is recommended to suppress free radical generation with platinized-titanium electrodes. Ohmic heating operated at 60 Hz (sine wave) and 10 kHz (pulses) indicated the generation of •OH radicals.